Relation between speech intelligibility, temporal auditory perception, and auditory-evoked potentials

Transcription

1 Relation between speech intelligibility, temporal auditory perception, and auditory-evoked potentials Alexandra Papakonstantinou s Master s Thesis Project supervised by Torsten Dau Oliver Fobel October 2005 Centre for Applied Hearing Research Acoustic Technology, Ørsted DTU Technical University of Denmark (

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3 ABSTRACT This study investigates the relationship between psychophysical experiments, speech intelligibility in noise and evoked auditory brainstem responses in high-frequency hearingimpaired listeners and normal-hearing listeners. Psychophysical experiments related to temporal processing and the bandwidth of auditory filters were performed in the lowfrequency region of hearing-impaired listeners where they showed normal pure-tone thresholds. Speech intelligibility in noise was measured for the same hearing-impaired listeners using the DANTALE II Danish sentences test. The auditory brainstem responses (ABR) elicited by a click and a chirp that compensates for cochlear travel-time difference across frequency were recorded. The amplitudes of the chirp-evoked wave-v responses were compared to click-evoked responses for the hearing-impaired and normal-hearing listeners. The main hypothesis was that listeners with normal low-frequency and reduced high-frequency audibility showing reduced performance in temporal processing tasks have problems in speech perception in noise and show a reduced wave-v response amplitude for the chirp. The results of the current study are as follows: (i) Hearing-impaired listeners with reduced temporal processing abilities showed reduced speech intelligibility in noise. (ii) Hearingimpaired listeners showed smaller wave-v amplitudes for the click and chirp in comparison to normal-hearing listeners. (iii) The chirp-evoked wave-v amplitude was larger than the click-evoked wave-v amplitude among the normal-hearing and the hearing-impaired listeners. (iv) The chirp produced the largest responses, particular at moderate stimulation levels for the normal-hearing and hearing-impaired listeners with near-normal performance in the temporal processing experiments. (v) Click-evoked responses were similar between listeners with significant differences in speech intelligibility, while the chirp-evoked responses were severely reduced for the listeners with reduced temporal processing performance. The chirp might therefore be useful in clinical ABR diagnostics since it demonstrates the temporal processing abilities of the auditory system better than the click, particularly for high-frequency hearing-impaired listeners.

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5 PREFACE The following study is the result of the master s project Relation between speech intelligibility, temporal auditory perception, and auditory-evoked potentials conducted at the Centre for Applied Hearing Research (CAHR) at the Technical University of Denmark (DTU). Firstly, I wish to thank Torsten Dau for the opportunity to carry out the following project. Your guidance and support were a constant source of inspiration. Likewise I am grateful to Oliver Fobel for your comments and motivation. Thanks to Thomas Ulrich Christiansen for helping with the writing of the information letters in Danish and for dealing with the paper work. Also, I greatly appreciated Torben Poulsen s help with the speech intelligibility experiment and for supplying me with the relevant literature. Many thanks to Erik Schmidt for providing me with the contact information of the test subjects and to Karsten Raahauge Bonke for being the operator during the speech intelligibility test. Also a big thanks to my fellow students for being there for me through thick and thin Eric Thompson, Sebastien Santurette, Kostas Aggelakis, Mihalis Kabanis, Paris Kerketsos, Gilles Pigasse, Tobias Piechowiak and Marusha Dekleva. I especially wish to thank Stephan Ewert for always believing in me, for being supportive throughout this project, and for patiently improving my scientific skills.

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7 CONTENTS INTRODUCTION 1 1 THE AUDITORY SYSTEM Anatomy Hearing Impairment TEST SUBJECTS 9 3 PSYCHOPHYSICAL TESTS Apparatus and procedure Frequency discrimination Method Results Gap detection Method Results Binaural masking level difference (BMLD) Method Results Modulation detection Method Results Modulation detection for sinusoidal carrier Frequency selectivity Method Results Discussion SPEECH INTELLIGIBILITY Method Results Discussion

8 5 AUDITORY BRAINSTEM RESPONSES Introduction Method Apparatus Stimulation Procedure Results Discussion SUMMARY AND CONCLUSION 53 APPENDIXES A-G 57

9 INTRODUCTION Elderly people with (bilateral symmetrical) high-frequency hearing loss often have difficulties understanding speech, particularly in the presence of background noise. It has been found, however, that speech intelligibility strongly varies among people with similar audibility as usually characterized by means of pure-tone audiometry. Apparently, the audiogram does not carry sufficient information to characterize hearing loss in a meaningful manner. Therefore, research is needed in order to develop a set of experiments that characterizes hearing loss in a more appropriate way and is suited to relate it to speech intelligibility in noise. In addition to subjective methods like the audiogram and other psychophysical measures, there are objective methods like auditory evoked potentials (ABR) that give information about the state of the hearing system. An advantage of objective methods is that they can also be used in newborns and other subjects incapable to participate in subjective methods. The purpose of the present study is to (i) further investigate the relation between psychoacoustical measures of temporal processing and speech intelligibility in hearing-impaired and normal-hearing listeners and to (ii) try to relate these findings with results obtained from an objective method such as ABR. The current study focuses on temporal processing of the auditory system at low frequencies where the audiograms of the hearing-impaired listeners were close to those observed in normal-hearing listeners. In an auditory system with temporal processing disorders it is expected that the nerve firing insufficiently follows the phase of the stimulating waveform on the basilar membrane for frequencies below about 1-2 khz. Furthermore the transmitted temporal information about the stimulus to some higher neural stages which compare the temporal information from the two ears in binaural listening conditions might be reduced for hearing-impaired listeners with temporal processing problems. The hypothesis is that the analysis of the time pattern occurring within each auditory filter and the comparison of the time patterns across auditory filters can be reduced differently in hearing-impaired listeners that have normal pure-tone thresholds in the low frequency region. These differences might be observable in psychophysical experiments related to temporal processing and the results might be correlated with degradations in speech intelligibility. Synchronization of neural discharges carries important information for auditory per-

10 2 ception (e.g., Stopfer et al., 1997). In the auditory system, neurons can generate action potentials synchronized to stimulus frequencies (Johnson, 1980) and preserve the relative timing of these action potentials passed through several synaptic stages. The synchronous activities in auditory neurons may encode basic auditory perceptions such as pitch and extract complex sound features such as spectral peaks and waveform envelopes for speech recognition (e.g., Young and Sachs, 1979). The synchronous activity of the auditory nerve and auditory brainstem pathways can be recorded from scalp electrodes as the averaged far-field potential, known as the auditory brainstem response (ABR). The acoustic click is commonly used for neurodiagnostic purposes under the assumption that it elicits synchronous discharges from a large proportion of cochlear fibers because of its wide spectral spread (e.g., Kodera et al., 1977). Recent studies (e.g., Dau et al., 2000) have shown that an appropriate chirp stimulus, which is tailored to activate the entire cochlear concurrently, evokes a larger wave-v amplitude than a click for normal-hearing listeners. In chapter 3, psychophysical data related with timing perception tasks such as frequency discrimination at low frequencies, temporal gap detection, binaural masking level differences and temporal modulation detection are presented. In addition to the data of the tasks related to temporal processing, frequency selectivity data at low frequencies will be presented providing an estimate of the bandwidth of the auditory filters. In chapter 4, speech intelligibility in noise is be measured using the DANTALE II Danish sentence test (Wagener et al., 2003). The results of the psychophysical experiments are expressed as a function of speech intelligibility in noise and discussed. In chapter 5, the differences between ABRs obtained with a click and the chirp are investigated. The differences between the hearing-impaired and the normal-hearing listeners are investigated and related to the performance in the psychophysical experiments and speech intelligibility test. The last chapter summarizes the results of the present study and gives an overall conclusion.

11 1. THE AUDITORY SYSTEM In order to understand the responses of the auditory brainstem as well as the psychophysical aspects of the presented study, a brief description of the auditory system is presented in the following chapter. Also presented are the physiological and functional differences between the normal-hearing auditory system and the auditory system with sensorineural hearing loss. 1.1 Anatomy The peripheral auditory system is composed of the outer, middle, and inner ear as shown in figure 1.1. The function of the middle ear is to act as an impedance-matching transformer improving the efficiency of the energy transfer between the surrounding air and the fluid-filled cochlea. It also assists in the prevention of transmission of bone-conducted sound to the cochlea. FIG. 1.1: Left: The ear consists of the of the external pinna, the external auditory canal, the ear drum, the middle ear containing the auditory ossicles, and the inner ear consisting of the labyrinth and cochlear. Right: The cochlear is a spirally round hollow tube containing three tubes running in parallel. The smallest tube is the scala media and contains the organ of Corti. The organ of Corti is situated on the basilar membrane and contains the hair cells [

12 4 The basilar membrane (BM) runs along the length of the cochlea, dividing it into two chambers. Sounds produce waves along the BM, and for each frequency there is a given location where the pattern of vibration reaches a maximum. Low frequencies produce maximum vibration close to the apex, and high frequencies produce maximum vibration close to the base of cochlea. The BM thus acts as a filter bank, splitting complex sounds into frequency components. The sharp tuning of the BM is physiologically vulnerable. The organ of Corti is located on the BM and contains four rows of hair cells consisting of one row of inner hair cells and three rows of outer hair cells. On the tips of the inner hair cells stereocilia are situated. Movement of the BM causes a displacement of these stereocilia that changes the receptor potential of the inner hair cells. Action potentials are generated in the auditory nerve fibers when the electrical potentials of the receptor are above a certain threshold value. The inner hair cells contact with afferent nerve fibers that sent information from the auditory nerve up to the brain. There are also efferent nerve fibers that bring information from the brain to the auditory nerve which are connected with the outer hair cells. The outer hair cells probably actively influence the vibration patterns on the BM, contributing to its sharp tuning and high sensitivity (Moore, 2003). The auditory nerve (vestibulocochlear nerve) carries the signal into the brainstem and synapses onto the cochlear nuclei. From the cochlear nuclei, auditory information is divided into at least two streams. The one related with this study starts with auditory fibers going to the ventral cochlear nuclei synapse on target cells with giant, hand-like terminals. The cells of the ventral cochlear nuclei then project to a collection of nuclei in the medulla called the superior olivary nucleus. The olivary nucleus projects, via a fiber tract called the lateral lemniscus, up to the inferior colliculus. The stream of information proceeds to the sensory thalamus. Medial geniculate body of thalamus Primary auditory cortex in temporal lobe Inferior colliculus Superior olivary nucleus (pons-medulla juncton) Cochlear nuclei Midbrain Lateral lemniscus Medulla Vestibulocochlear nerve Spiral ganglion of cochlear nerve Spiral organ of Corti FIG. 1.2: The auditory nerve (vestibulocochlear nerve) carries the signal to the brainstem. Auditory information transmitted by synapses passes to the cochlear nuclei. The ventral cochlear nuclei project the information to the olivary nucleus, which then project up to inferior colliculus. Finally the geniculate nucleus receives the information and projects to the primary auditory cortex.

13 The auditory system 5 The auditory nucleus of thalamus is the medial geniculate nucleus. The medial geniculate projects to the primary auditory cortex. The latter one is located at the temporal lobes, as shown in Fig. 1.2 [ 1.2 Hearing Impairment The functioning of a normal cochlea is strongly dependent on an active mechanism, which is physiologically vulnerable. This mechanism appears to depend upon the integrity of the outer hair cells (OHCs), and particularly their stereocilia. The mechanism is responsible for the high sensitivity and sharp tuning of the basilar membrane (BM). It is also responsible for a variety of compressive effects that can be observed both in BM responses and in neural responses. These nonlinear effects include: the nonlinear input-output functions of the BM, the reduction in sharpness of tuning with increasing sound level, two-tone suppression, and combination-tone generation. The active mechanism strongly influences responses on the BM at low and medium sounds levels, but its contribution progressively reduces as the sound level increases. In the healthy system, the vibration of the BM is nonlinear meaning that the magnitude of the response does not grow in proportion with the magnitude of the input (Sellick et al., 1982; Ruggero et al., 1992). An example is shown in Fig. 1.3, which shows the inputoutput function of the BM for a place with a characteristic frequency (CF) of 8 khz from Robles et al. (1986). Each curve of figure 1.3 represents the velocity of basilar membrane as a function of the sound level for a particular stimulation frequency, indicated by a number (in khz) close to the curve. FIG. 1.3: Input-output functions for a place on the BM with CF = 8 khz. A number close to each curve indicates the stimulating frequency, in khz. The dashed line indicates the slope that would be obtained if the responses were linear (velocity directly proportional to sound pressure). The slight shift between the two curves for stimulation frequency of 8 khz was probably caused by a deterioration in the condition of the animal (Robles et al., 1986).

14 6 The velocity of vibration is plotted on log scale as a function of the input sound level (in db SPL). If the response of BM was linear, all the curves would be parallel to the dashed line. For a CF tone of 8 khz two functions are shown one for low levels and a second one for higher levels. The difference of the curves with stimulation frequency of 8 khz for input level of 50 db SPL was probably caused by a deterioration in the condition of the animal. The nonlinearity mainly occurs when the stimulation frequency is close to the CF of the monitored point of the BM (8 khz). For worse physiological cochlear conditions with OHC damage, the nonlinearity decreases and after death the response functions becomes linear. The active processes that are responsible for sharp tuning and high sensitivity on the BM are also responsible for the nonlinearity. Khanna and Leonard (1982), Sellick et al. (1982), Leonard and Khanna (1984), Robles et al. (1986) and Ruggero et al. (1992), using different techniques in living animals, have shown that the sharpness of the tuning of the BM critically depends on the physiological condition of the animal. In those studies, the physiological status of the cochlea was monitored by placing an electrode in or near the auditory nerve, and measuring the combined responses of the neurones to tone bursts or clicks. This response was called compound action potential (AP or CAP). The lowest sound level at which an AP can be detected is called the AP threshold. This is illustrated in Fig. 1.4, which shows the input sound pressure level (in db SPL) required to produce a constant velocity of motion (AP threshold) at a particular point on the BM, as a function of stimulus frequency (Sellick et al., 1982). In the healthy system, AP threshold is low (solid circles) and the tuning of the BM is sharp. In the impaired system (open circles), the tuning becomes broader and the AP threshold (sound level required to produce the criterion response) increases markedly around the tip. Results from the post-mortem condition (solid squares), where the active mechanisms are absent, are close to those from the deteriorated system (solid circles). The sharp tuning and high sensitivity depend on biological structures which can actively influence the mechanics of the BM (Pickles, 1986, 1988; Yates, 1986). The most likely structure to play this role are the OHCs. Noise exposure, toxic chemicals, infection and metabolic disturbances easily damage the OHCs. When damaged, the active mechanism is reduced in effectiveness or even destroyed completely. This has several important consequences: (i) Sensitivity is reduced, so that the tips of tuning curves are elevated by up to db (ii) The sharpness of tuning on the BM is greatly reduced. The tips of the tuning curve may be elevated or may disappear completely, leaving only the broad tuning of the passive BM transfer function, in absence of any active mechanisms (post mortem), and (iii) The nonlinearities are reduced or disappear such as the ability to resolve the sinusoidal components in a complex sound.

15 The auditory system 7 FIG. 1.4: Tuning curves measured at a single point on the basilar membrane. Each curve shows the sound pressure level required to produce a constant velocity on the BM (AP threshold), plotted as a function of stimulus frequency. The curve marked by solid circles was obtained at the start of the experiment when the animal (guinea pig) was in a good physiological condition and shows low AP threshold values. The open circles indicate a deteriorated physiological condition of cochlear and the post-mortem situation is indicated by solid squares (Sellick et al., 1982). The inner hair cells (IHCs) are the transducers of the cochlea. They are less susceptible to damage than the OHCs. If they become damaged, sensitivity is reduced. Damage primarily to the IHCs, with intact OHCs, is rare. When it does occur, sharp tuning may be preserved, but the sensitivity is decreased. More commonly, damage occurs both to the OHCs and IHCs. In this case, the whole tuning curve would be elevated, with a greater elevation around the tip, originating from OHCs and IHCs damage in combination, than around the tail, originating from IHCs damage only. Damage to the IHCs can also result in a reduction of phase locking. In this case, the precision with which neural impulses are synchronized to the cochlear-filtered stimulating waveform is reduced. The reason why this occurs is unclear, but it may have important perceptual consequences (Moore, 1995). The experiments of the present study are focused on effects of audibility, temporal processing capabilities and frequency selectivity on auditory perception, which all of them are affected by sensorineural hearing loss.

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17 2. TEST SUBJECTS Eleven test subjects participated in the current study. Table 2.1 summarizes their audiological information identified by a code name (initials, gender and year of birth). The hearing-impaired group was composed of three males and three females, ranging between 60 and 80 years in age. They were diagnosed with bilateral (both ears were affected) symmetrical (the degree of hearing loss was similar to both ears), high-frequency (frequencies above about 2000 Hz were affected) sensorineural hearing losses (BSH). The hearing-impaired group was divided in three categories. The subjects with a pure-tone threshold of 40 to 60 db HL for one or more tested frequencies were characterized as having a moderate hearing loss, those with 60 to 80 db HL as severe and those with more than 80 db HL as profound. For better illustration, the audiograms for the hearing-impaired listeners are shown in Fig hgf42 hbm45 blm34 lkf33 fhm25 apf Pure Tone threshold (db HL) Pure Tone threshold (db HL) left ear right ear Frequency (khz) Frequency (khz) FIG. 2.1: Pure-tone audiograms of the six hearing-impaired test subjects with bilateral (both ears were affected), symmetrical (similar degree of hearing loss at both ears), high-frequency (affected frequencies above about 2000 Hz) sensorineural hearing loss.

18 10 left ear frequency subjects blm fhm hbm lkf apf hgf htm lnf pkm slvf jxm right ear frequency subjects blm fhm hbm lkf apf hgf htm lnf pkm slvf jxm Hearing Impaired Normal Hearing Hearing Impaired Normal Hearing TABLE 2.1: Audiometric results in db hearing level (HL) of all subjects left ears (top panel) and right ears (bottom panel). The results for the hearing-impaired subjects start on the top of each panel from the subjects with profound high-frequency (frequencies above about 2000 Hz were affected) hearing loss to those with moderate high-frequency loss. For the normal-hearing subjects the results are presented in alphabetical order. Subjects with normal hearing as defined by pure-tone thresholds of 10 db HL (American National Standards Institute (ANSI), 1989) or less for frequencies between Hz, and 20 db HL or less for Hz were used as a control group. The normalhearing group consisted of three males and two females, ranging between 23 and 28 years in age. All the subjects were asked if they had ear infection(s) in the past. Only normalhearing subject lnf80 went through ear infections when she was a child. The pure-tone audiometric testing of the normal-hearing listeners was measured in a soundproof booth using an audiometer AA222 and Sennheiser HDA200 headphones. In order to keep the same procedure for all the test subjects, the automatic version of the audiometer was used. Each ear was tested for the frequencies 0.125, 0.25, 0.5, 1, 2, 3, 4, 7, and 8 khz. During the automatic procedure, a recalculation of the hearing threshold at 1 khz occurred at the end. The hearing thresholds of the hearing-impaired subjects reported here were measured with the same equipment. The tested frequencies were in the same range as for the normal-hearing.

19 Test subjects 11 During the listening tests reported in section 3, 4 and 5 the subjects had three to four two-hour sessions of experiments or more one-hour sessions, depending on how long they could keep their concentration. Subjects were ask to take breaks whenever they felt like. When the standard deviation for the three first runs exceeded 30% of the mean value, the subject repeated the complete experiment (three runs). The data of the first three runs were discarded and the final mean value was calculated from the data of the three runs of the repeated test. Each subject read an information form and signed an approval form before participating in the current study. The ethical committee of Technical University of Denmark (DTU) approved both forms. All subjects received an hourly compensation for their participation in the study. Eleven subjects participated in total, including two internal subjects from the department. The experiments conducted in this study are presented in detail in chapters 3, 4, and 5. Table 2.2 lists the experimental tests that the individual subject participated in. Chapter 3 describes the psychophysical results on frequency discrimination (FRJND), temporal gap detection (GAP), binaural masking level differences (BMLD), modulation detection with bandlimited noise carrier (MODI), modulation detection with sinusoid carrier of 500 Hz (MODII), and equivalent rectangular bandwidth of the auditory filters (ERB). Chapter 4 deals with the speech reception threshold (SRT) in noise. Chapter 5 describes auditory brainstem responses (ABR). Chapter 3 Ch.4 Ch.5 test subject FRJND GAP BMLD MODI MODII ERB SRT ABR blm fhm hbm apf lkf hgf htm lnf pkm slvf jxm Hearing Impaired Normal Hearing TABLE 2.2: List of experiments in which the subject participated (+) or not participated (-).

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21 3. PSYCHOPHYSICAL EXPERIMENTS For low frequencies in the region below about 1000 Hz, the hearing-impaired subjects were able to detect pure tones at comparable levels as the normal-hearing listeners in the control group. Investigation of their hearing abilities in this low frequency region, and a correlation analysis with speech intelligibility and auditory brainstem responses were the purposes of the current study. This chapter deals with timing related perception such as frequency discrimination at low frequencies, temporal gap detection, binaural masking level differences and temporal modulation detection. In addition, the frequency selectivity at low frequencies was measured. Speech perception and auditory evoked potentials are investigated in the subsequent chapters 4 and 5, respectively. 3.1 Apparatus and procedure The subjects performed all psychophysical tasks in a double-walled, sound attenuating booth. All stimuli were generated in MATLAB and converted to analog signals using a 16- bit D/A converter at Hz sampling rate. Sennheiser HD580 headphones were used in combination with a (RME DIGI96/8) soundcard in a PC. All experiments were provided within the MATLAB AFC framework with the names freqjnd, gapdetection, bmld, modulationdetection, modulationdetection500 and notchednoise, respectively. During the calibration of the Sennheiser HD580 headphones a sound level meter B & K Type 2607 and an artificial ear (IEC 318) was used. The maximum output of Sennheiser HD580 headphones, at 1000-Hz was db SPL. An adaptive, three-interval, three-alternative, forced-choice procedure was used in combination with an one-up, two-down tracking rule converging at the 70.7 percent point of the psychometric function (Levitt, 1971). This procedure was used in all psychophysical experiments reported in this chapter. During each trial the subject was presented with three intervals containing sounds that were visually marked by three boxes on a computer screen. One of the three intervals contained the signal, whereas the other two contained the reference. The order of the signal and reference stimuli was randomized. The subject had to choose which interval contained the signal and was given a visual feedback regard-

22 14 ing the correct response. The initial difference between the signal and the reference was sufficiently large for all test subjects. The difference was reduced after two consecutive correct responses and increased after one incorrect response. Three or four step sizes were used. Each run had a total of 12 reversals. In all experiments except frequency selectivity, the threshold value was the average of the last eight reversals. In the frequency selectivity experiment, the last six reversals were averaged to give the threshold value. Each run was repeated three times (see Appendix B). The standard deviation (A.2) and the reported mean threshold (A.1) were calculated from the three threshold estimates obtained per subject. The operator instructed the subjects in the beginning of each experiment. The subjects were informed about the detection task, the involved signals, and that they had to choose one of the intervals even if they had to guess. The subjects were asked if the presentation level was comfortably loud for the frequency discrimination, binaural masking level differences and temporal modulation detection experiment for broadband noise carrier. The instructor adjusted the presentation level according to the subject s preference. A fixed presentation level was used for temporal gap detection, modulation detection for sinusoidal carrier and frequency selectivity experiments for all test subjects. Details for each experiment described in the corresponding sections. 3.2 Frequency discrimination The difference limen for frequency (DLF) is determined by temporal information (phase locking) for frequencies up to at least about 1-2 khz. The temporal theory suggests that there is a tendency of nerve firing to occur at a particular phase of the stimulating waveform on the basilar membrane. For a pure tone, the spikes tend to be phase locked or synchronized to the stimulating waveform. Thus, the time intervals between the spikes are (approximately) integer multiplies of the period of the stimulating waveform. For example, a pure tone at 250-Hz has a period of 4 milliseconds, so it is expected that the intervals between the nerve spikes to be close to 4, 8, 12 ms, etc (Moore, 2003) Method Frequency discrimination was measured at 250 and 1000 Hz. The stimuli had a duration of 500 ms and were separated by 500 ms of silence. The signal was slightly different in frequency from the reference. The subject had to choose which interval produced the higher pitch. The initial frequency difference between the signal and the reference was 25% percent higher. Three step sizes equal to a reduction factor of 2, 1.5 and for the difference were used. A total of six runs (three repeations for each experimental frequency) were obtained for each subject (see Appendix B).

23 Psychophysical experiments Results Figure 3.1 shows frequency discrimination thresholds for 250 Hz (upper panel) and 1000 Hz (lower panel) in the six hearing-impaired subjects (left panels) and in four normalhearing subjects (on the right panels with filled markers), respectively. The subjects with profound high-frequency hearing loss, blm34 and fhm25, are presented on the left side of the left panel. The results for subjects that had musical training during their life are indicated with circle markers. The different performances observed among the hearing-impaired subjects were not related to their sensitivity to pure tones. The test subject apf38, which had a moderate high-frequency hearing loss, required a 90.6 Hz difference from the reference frequency of 250 Hz, while subject blm34 who had profound high-frequency hearing loss only required a 2.2 Hz difference. Subjects fhm25 and apf38 had abnormally high frequency discrimination threshold values for 250 Hz and 1000 Hz. At 1000 Hz, the performance of blm34 is similar or even slightly better than normal-hearing thresholds. The dashed-dotted lines indicate DLFs for normal-hearing subjects measured by Zeng et al. (2005). Their normal-hearing control group required DLFs of 1.5 Hz at 250 Hz and 6 Hz at 1000 Hz, respectively. Frequency Discrimination of pure tone 100 Hearing Impaired Normal Hearing 250 Hz 40 Just Noticeable Difference in frequency (Hz) profound severe moderate Zeng et al. (2005) blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Hearing Impaired Normal Hearing 1000 Hz Just Noticeable Difference in frequency (%) 1 Zeng et al. (2005) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Subjects 0.1 FIG. 3.1: The frequency discrimination limen (DFL) in ten subjects, six with high-frequency sloping hearing loss (left panels) and four with normal hearing (right panels with filled markers). Four of the subjects had musical training marked with circles. DFLs in (Hz) for the reference frequencies 250 Hz (upper panel) and 1000 Hz (lower panel) are plotted for each subject. The left abscissa represents frequency discrimination threshold in Hz and the right abscissa represents just noticeable frequency differences in % of the experimental frequency. The error bars indicate ± one standard deviation of the mean threshold value. The dashed-dotted lines note the normal-hearing DLFs values were measured by Zeng et al. (2005) at 250 Hz and 1000 Hz, respectively.

24 16 In summary, subjects apf38 and fhm25 had severe difficulties to detect changes in frequency at 250 Hz and 1000 Hz. At both frequencies, subject apf38 showed DLFs close to those found by Zeng et al. (2005) for subjects with a disease called auditory neuropathy, where temporal processing capabilities are severely affected while the outerhair cell function is normal or near normal. The hearing-impaired subjects blm34, hbm45, lkf33, and hgf42 showed frequency discrimination threshold values comparable to normalhearing subjects. 3.3 Gap detection One alternative approach of estimating the temporal resolution of the auditory system is to measure the detectability of temporal gaps in noise. The long-term magnitude spectrum of broadband white noise remains the same even if the noise is briefly interrupted. Therefore, no spectral cues are introduced by a temporal gap. When the gap is introduced into a noise band with a large width, the gap creates a dip that is synchronous at the outputs of all the auditory filters that are exited by the noise. Plomp (1964) and Penner (1977) proposed a typical threshold value of 2-3 ms. The threshold increases at very low sound levels, when the level of the noise approaches the absolute threshold, but is relatively invariant with level for moderate to high levels (Moore, 2003) Method A fixed level of 69.3 db SPL was used during the experiment. All of the intervals contained a white noise with a bandwidth of f = 3000 Hz of an upper cut-off frequency of 4000 Hz. One of the intervals contained a gap in the noise. The initial gap duration was 120 ms. Three step sizes equal to a reduction factor of 2, 1.5 and were used. A total of three runs was performed for each subject (see Appendix B) Results Figure 3.2 shows gap detection for each subject at a presentation level of 69.3 db. The results for ten subjects are shown, six with sloping high-frequency hearing loss (left panel) and four with normal-hearing (right panel). The normal-hearing test subjects htm77, lnf82, pkm77 and slvf80 showed a threshold value of about 5 ms, which is slightly higher than the 3 ms found in the Zeng et al. (2005) study. The dashed-dotted line indicates the gap detection threshold for normal-hearing subjects measured by Zeng et al. (2005). Subject hbm45 (severe high-frequency hearing loss) showed a nearly normal threshold value of about 5 ms (compared to the three normal-hearing listeners that showed the lower threshold values), while the other hearing-impaired subjects have higher thresholds. In particular, subjects blm34, lkf33 and apf38 have threshold values around 10 ms, with subject apf38 having the highest threshold of 12.2 ms close to the threshold value of 12 ms measured by Zeng et al. (2005) for subjects with auditory neuropathy at a presentation

25 Psychophysical experiments 17 level of 50 db. Subjects fhm25 and hgf42 showed values between the normal-hearing and the elevated hearing-impaired group. 20 Hearing Impaired Gap detection in 3000 Hz wide noise Normal Hearing 10 Gap threshold (ms) 5 2 profound severe moderate Zeng et al. (2005) blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Subjects FIG. 3.2: Gap detection threshold values are plotted for ten subjects measured for broadband noise at a presentation level of 69.3 db. Data for six subjects with high-frequency sloping hearing loss (left panel) and four with normal hearing (right panel with filled markers) is shown. The error bars indicate ± one standard deviation of the mean threshold value. The dashed-dotted line indicates the normal-hearing gap detection threshold as measured by Zeng et al. (2005) at a presentation level of 50 db SPL. 3.4 Binaural masking level difference (BMLD) Many previous studies have shown that the threshold for detecting a signal masked by noise can be much lower when the noise and the signal are presented in a particular way to both ears (Zurek and Durlach, 1987; van de Par and Kohlrausch, 1999). In those studies, the masked threshold with both masker and signal interaurally in phase was measured. In the second measurement, the interaural phase difference of the signal was changed from 0 to 180 degrees, while the noise was still identical in both ears. In order to describe these types of experiments, a special terminology was introduced. The symbol N was used to symbolize the noise and S for the signal, each followed by a suffix denoting the interaural phase relation. For example, a phase difference of π for the signal was denoted by S π. Thus, N 0 S 0 referred to the condition where both, noise and signal, were in phase at both ears (upper panel of Fig. 3.3), and N 0 S π referred to the condition where the noise was in phase at both ears, but the signal was inverted in phase (lower picture of Fig. 3.3). The difference between the two masked-threshold levels is known as the masking-level difference (MLD) or the binaural masking-level difference (BMLD). The configuration N 0 S π normally results in 15 db lower masked thresholds at low frequencies. The effect decreases to 2-3 db for frequencies above 1500 Hz (Durlach and Colburn, 1978). At 1500 Hz and above, the ability of the auditory system to compare phases at the two ears decreases. Thus, the presence of a noticeable BMLD depends in part on the transmission

26 18 FIG. 3.3: Illustration of an experimental condition where the binaural masking level difference (BMLD) occurs. In the upper picture, where both noise and signal are in phase at both ears N 0S 0, detectability is poor. In the lower picture, where the interaural relation of the signal and masker is different (the noise is in phase at both ears, but the signal is inversed N 0S π), the detectability is better (Moore, 2003). of temporal information about the stimulus to some higher neural center which compares the temporal information from the two ears. Thus, BMLD is related to the phase locking ability at low frequencies which is reduced at high frequencies (Moore, 2003) Method The signals were 480-ms sinusoids with an interaural phase difference of 0 and 180 degrees, respectively. Signal frequencies of 250 and 1000 Hz were used. The masker was 500-ms noise, two octaves wide, and geometrically centred at the signal frequency. The presentation of the masker started 10 ms before the signal and ended 10 ms after the signal. The same masker was presented at both ears. The reference stimulus was the masker alone and one of the intervals contained the signal added to the masker. A total of 12 measurements (two conditions N 0 S 0 and N 0 S π, two frequencies, repeated three times) were performed by each subject. The initial level difference between the sinusoids and the broadband noise was 10 db in order to provide an easily detectable signal for the subjects. Four step sizes of 8 db, 4 db, 2 db and 1 db were used. The two masked thresholds L(N 0 S 0 ) and L(N 0 S π ) for each test subject were the mean value of the three threshold estimates of each run. The BMLD was calculated by subtracting L(N 0 S 0 ) from L(N 0 S π ). The BMLD standard deviation was calculated using equation A Results Figure 3.4 depicts the BMLDs for the hearing-impaired subjects (left panels) and

27 Psychophysical experiments 19 normal-hearing (right panels) for a pure tone of 250 Hz (upper panel) and 1000 Hz (lower panel). At 250 Hz (upper panel) most of the hearing-impaired subjects had near-normal BMLDs of around db. Subject fhm25 showed at 250 Hz slightly decreased performance of 11.7 db. Subject apf38 was hardly able to use the phase difference cues to detect the 250 Hz tone in the noise showing a BMLD of only 3.4 db. As expected, the amount of BMLD was decreased as the frequency was increased. The group of normal-hearing subjects had lower BMLD values of db at 1000 Hz, compared to db at 250 Hz. At 1000 Hz tone frequency, the group of the hearing-impaired showed smaller BMLD values of only around 5-7 db, in contrast to the group of normal-hearing subjects that showed around db. Only subject blm34 showed near-normal performance. Again, apf38 clearly showed the lowest BMLD, as at 250 Hz. Most of the hearing-impaired subjects had near-normal BMLDs at 250 Hz, their performance was decreased at 1000 Hz, indicating that their temporal binaural information can not be evaluated very efficiently at 1000 Hz. Binaural Masking Level Defference Hearing Impaired Normal Hearing 250 Hz BMLD (db) 5 0 profound severe moderate v. d. Par and Kohlrausch (1999) blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 20 Hearing Impaired Normal Hearing 1000 Hz 15 v. d. Par and Kohlrausch (1999) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Subjects FIG. 3.4: The binaural level differences between the masked-threshold level L(N 0S 0) and the maskedthreshold level L(N 0S π) for pure tone frequencies of 250 Hz (upper panel) and 1000 Hz (lower panel). Six subjects with high-frequency sloping hearing loss (left panels) and four with normal hearing (right panels with filled markers). The error bars indicate ± one standard deviation of the mean threshold value. The dashed-dotted lines indicate the BMLD values measured by van de Par and Kohlrausch (1999) for normal-hearing subjects at 250 and 1000 Hz, respectively. Audibility was most likely not the problem. For example, subject blm34 (profound high-frequency hearing loss) showed near-normal BMLD at 1000 Hz and a even better BMLD value compared to the normal-hearing subjects performances at 250 Hz. The

28 20 dashed-dotted lines indicate the BMLDs measured by van de Par and Kohlrausch (1999) for normal-hearing listeners at 250 and 1000 Hz, respectively. The normal-hearing subjects of the present study performed better at 250 Hz and slightly worse at 1000 Hz compared to the BMLD values found in van de Par and Kohlrausch (1999). 3.5 Modulation detection The last experiment evaluating the temporal resolution was modulation detection. It was important to investigate if the subjects were able to distinguish changes in stimuli over time. The detectability of sinusoidal amplitude modulation as a function of the modulation frequency was estimated. For a given signal s(t), a carrier waveform n(t), a modulation depth m and a modulation frequency f m, the sinusoidal amplitude modulation is described by: s(t) = n(t) (1 + m cos(2πf m )) (3.1) Method A bandlimited noise carrier was used with an upper cut-off frequency of 4000 Hz and a bandwidth of 3000 Hz. Two modulation frequencies of 8 and 64 Hz were used. The modulation depth at subject s threshold was obtained for the two modulation frequencies. The reference stimulus was the unmodulated bandlimited noise. One of the intervals contained the sinusoidal amplitude modulated noise. A total of six measurements (two modulation frequencies, three repetitions) was performed by each subject. The initial modulation depth was -6 db, where 0 db corresponds to 100% modulation (m=1). Three step sizes of 4 db, 2 db and 1 db were used. The mean threshold and standard deviation was calculated accross the three runs of each modulation frequency and test subject Results Figure 3.5 shows modulation detection thresholds in broadband noise at 8 Hz (upper panel) and 64 Hz (lower panel) modulation frequency, respectively. The hearing-impaired subjects are presented at the left panels and the normal-hearing subjects at the right panels with filled markers. The results for 8 Hz modulation frequency seem to be correlated with audibility as reflected in the audiograms Fig Subjects with profound high-frequency hearing loss, such as subjects blm34 and fhm25, tend to have a higher modulation detection threshold than those with moderate loss, as subjects apf38 and hgf42. The results for modulation frequency of 64 Hz are uncorrelated with the audibility of the subjects. Subject lkf33 (severe high-frequency hearing loss) showed reduced sensitivity for the sinusoidal modulation of noise, while subject fhm25 (profound high-frequency hearing loss) showed a threshold value of db, comparable to the normal-hearing performances of around -17 to -14 db. Again, subject blm34 showed the highest modulation detection threshold

29 Psychophysical experiments 21 of -10 db in broadband noise. The dashed-dotted lines indicate the modulation detection thresholds for broadband noise carrier found by Viemeister (1979) of -25 and db for modulation frequencies of 8 and 64 Hz, respectively. The modulation detection thresholds for the normal-hearing test subjects of the present study showed higher values (worse performance) for both modulation frequencies when compared to those found by Viemeister (1979). Only subject hgf42 with moderate high-frequency loss showed a threshold value of -20 db for 64 Hz modulation frequency, which is close to the measured value for normal-hearing listener by Viemeister (1979) of db. Modulation Detection broadband noise carrier 10 Hearing Impaired Normal Hearing 8 Hz Modulation depth m (db) Viemeister (1979) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Normal Hearing 64 Hz Hearing Impaired Viemeister (1979) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Subjects FIG. 3.5: Modulation detection threshold for modulation frequencies of 8 Hz (upper panel) and 64 Hz (lower panel) obtained in ten subjects. Six subjects with high-frequency sloping hearing loss (left panels) and four with normal-hearing (right panels with filled markers) are shown. The error bars indicate ± one standard deviation of the mean threshold value. The dashed-dotted lines indicate the modulation detection threshold values measured by Viemeister (1979) for broadband sinusoidally amplitude modulated noise at 8 and 64 Hz, respectively Modulation detection for sinusoidal carrier The bandlimited noise carrier that was used had an upper cut-off frequency of 4000 Hz. For subjects with profound high-frequency hearing loss (e.g. blm34), the effective bandwidth might have been smaller than for other subjects. The reduction of effective bandwidth might have caused the threshold elevation as observed in Fig An additional modulation detection experiment was performed in order to investigate the modulation detection ability of the subjects at 500 Hz were all had similar hearing. The carrier was a sinusoid at 500 Hz and the level was fixed at 63.7 db SPL. Two modulation frequencies

30 22 of 8 and 32 Hz were used. Figure 3.6 shows the modulation detection results for a sinusoidally amplitude modulated pure tone at 500 Hz using modulation frequencies of 8 Hz (upper panel) and 32 Hz (lower panel). The hearing-impaired subjects are presented on the left panels and one normal-hearing control subject is presented on the right panels with filled markers. Subjects lkf33, apf38, hgf42 and fhm25 showed near-normal performance for a modulation frequency of 8 Hz or even better, with thresholds of around -25 db. Subjects blm34 and hbm45 showed slightly higher thresholds of around -22 to -20 db for a modulation frequency of 32 Hz. For the modulation frequency of 32 Hz (lower panel), hearing-impaired subjects showed near-normal performance or even better with the only exception of subject apf38 which showed a slightly elevated threshold (less sensitivity). The dashed-dotted lines indicate the measured values of -27 db for a sinusoidal carrier at 1000 Hz and modulation frequencies of 8 and 32 Hz found by Kohlrausch et al. (2000), respectively. The modulation detection thresholds for sinusoid carrier at 500 Hz and 1000 Hz are expected to be similar for 8 and 32 Hz rate. 15 Hearing Impaired Modulation Detection sinusoidal carrier at 500 Hz Normal Hearing 8 Hz Modulation depth m (db) profound severe moderate Kohlrausch A. et al. (2000) blm34 fhm25 hbm45 lkf33 apf38 hgf42 lnf82 Hearing Impaired Normal Hearing 32 Hz profound severe moderate Kohlrausch A. et al. (2000) blm34 fhm25 hbm45 lkf33 apf38 hgf42 lnf82 Subjects FIG. 3.6: Modulation detection threshold for a sinusoidal carrier at 500 Hz obtained in six subjects with high-frequency sloping hearing loss (left panel) and one with normal hearing (right panel with filled markers). The results for modulation frequencies of 8 and 32 Hz are presented on the upper and the lower panel, respectively. The error bars indicate ± one standard deviation of the mean threshold value. The dashed-dotted lines indicate the measured values of -27 db for sinusoidal carrier at 1000 Hz and modulation frequencies of 8 and 32 Hz found by Kohlrausch et al. (2000), respectively. In summary, despite the small differences, the modulation detection performance of hearing-impaired subjects at 500 Hz with low modulation frequencies of 8 and 32 Hz is

31 Psychophysical experiments 23 near normal. This indicates that the higher threshold values observed for subject blm34 with broadband noise was most likely associated with a reduction of the affective listening bandwidth, and not with a reduced sensitivity to modulation per se. 3.6 Frequency selectivity The ability to resolve spectral components in a complex sound, or frequency selectivity, plays an important role in many aspects of auditory perception. Frequency selectivity is typically demonstrated and measured by studying masking. Masking is the process by which a certain sound, the signal, may be rendered inaudible by the presence of another sound, the masker. It can also be defined as the amount by which the threshold of audibility of a sound is increased by the presence of a masker. Masking is most effective if the frequency components of the masker are close to, or the same as, those of the signal. Our ability to separate the components of a complex sound is mainly determined by the frequency-resolving power of the basilar membrane, as described in section 1.1 (see also Moore (2003)). Each location on the basilar membrane responds to a limited range of frequencies, resulting in a bank of filters with overlapping passbands. Each filter with a different center frequency corresponds to a different point on the basilar membrane (Moore, 1986; Evans et al., 1989). The shape of an auditory filter at a given centre frequency can be estimated psychophysically by using the so-called power spectrum model. Within this model, it is assumed that the signal is detected using the single auditory filter which is centred at the frequency of the signal and that the threshold corresponds to a constant ratio of signal power to masker power at the output of that filter. Figure 3.7 shows a typical auditory filter transfer function, assuming that the gain of the filter at its tip is 0 db and is symmetric on a linear frequency scale (Patterson, 1976). The filter, determined by using Patterson s method, has a rounded top and relatively steep skirts Method The notched-noise method (Patterson, 1976) was chosen for determining auditory filter shapes because of two main advantages. It allows an accurate measurement of the filter shape and it takes into account effects of off-frequency listening. Off-frequency listening refers to the fact that the listener may use filters centered at slightly different frequency than the frequency of the tone. In some masking conditions, an off-frequency auditory filter might help the listener to hear the tone because it passes less noise when compared to the amount of noise energy may passes through the auditory filter centered at the tone frequency. In the notched-noise method, the frequency of the signal is fixed and the bandwidth of the spectral notch (2 f) is varied. The threshold of the signal is determined as a function of notch width. Since the notched noise is placed symmetrically around the signal frequency, the method is not able to measure asymmetries in the auditory filter.

32 24 FIG. 3.7: A typical auditory filter shape determined using Patterson s method (Patterson, 1976). The filter is centered at 1 khz. The relative response of the filter (in db) is plotted as a function of frequency (Moore, 2003). The assumption of this method is that the filter is symmetric on a linear frequency scale. This is indicated in Fig In terms of the power-spectrum model, the threshold is proportional to the shaded area under the filter curve. As the width of the spectral notch increased, less and less noise passes through the auditory filter (Moore, 2003). Thus, the threshold for the signal drops. The amount of noise passing through the auditory filter is proportional to the area under the filter in the frequency range covered by the noise. This is indicated as the shaded areas in Fig The power-spectrum model equation can be written as: P s = kn 0 ( f c f 0 W (f)df + ) f u f W (f)df c+ f, (3.2) where P s is the power of the signal at threshold, W (f) indicates the spectral weighting function (W (f c1 < f < f c1 )=1 and 0 otherwise, for a rectangular filter with cutoff frequencies f c1 and f c1 ), f u denotes the upper cut off of the upper band and N 0 reflects the spectral density of the noise. Taking the assumption of symmetry into account, the frequency variable g can be used: g = f f c f c (3.3) The notched-noise method was set up with a signal tone of 250 Hz and 1000 Hz, respectively. The spectral density of the noise was 40 db/hz and was kept constant during the experiment. The detection threshold for the signal tone was measured at six notch widths of g = 0, 0.05, 0.1, 0.2, 0.3, 0.4. The reference stimulus was the notched noise without the signal. The subject s task was to detect the interval that included the pure-

33 Psychophysical experiments 25 FIG. 3.8: Schematic illustration of notched noise method used by Patterson (1976) to determine the shape of the auditory filter. The threshold of the sinusoidal signal is measured as a function of the width of the notched noise masker. The amount of noise passing through the auditory filter centered at the signal frequency is proportional to the shaded areas (Moore, 2003). tone in the notched noise. A total of 36 measurements (six conditions, two frequencies, two repetitions). The initial level of the sinusoidal signal was 69.3 db (see Appendix B). Three step sizes of 8 db, 4 db and 2 db were used. The mean threshold value and the standard deviation were calculated for each relative notched width g per subject. In order to model the auditory filters with an exponential shape, Patterson et al. (1982) suggested adding a rounded tip to the filters, which gives the following expression for the so-called rounded-exponential filter: W (g) = (1 + pg) exp ( pg) (3.4) The combined power-model equation with equation 3.4 is: P s = 2kf c N 0 (2 + pg n ) exp ( pg n) p (3.5) The parameter p determines the width of the passband summation and the slope of the filter skirts. The parameters p and k were estimated by fitting the predicted threshold curve of the rounded-exponential model to the experiment data. A least square fit was used (see Appendix B). The equivalent rectangular bandwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter centered in frequency f c can be calculated from the equation: f ERB = 4f c p (3.6) Glasberg and Moore (1990) derived an equation that gives the value of the ERB of the auditory filter for normal-hearing listeners as a function of centre frequency f c.

34 26 ERB = f c (3.7) Results Figure 3.9 shows the estimated equivalent rectangular bandwidth (ERBs) of the auditory filters centered at 250 Hz (upper panel) and at 1000 Hz (lower panel). The same subjects as in all previous tasks participated in the experiment. Data for the hearingimpaired subjects are plotted in the left panels of Fig. 3.9 and data for normal-hearing in the right panels with filled markers. Among the normal-hearing subjects, lnf82 who had ear inflections as a child showed the broadest auditory filter at 250 Hz, with a bandwidth of 86.2 Hz, while the other normal-hearing were close to the value of 51.7 Hz given by Glasberg and Moore (1990) (indicated with dashed-dotted line on the upper panel). The hearing-impaired subjects apf38 and fhm25 showed a substantially broadened auditory filter at 250 Hz. Subjects blm34, hbm45, lkf33 and hfg42 had auditory filters bandwidths similar to the normal-hearing listeners at 250 Hz. Subject apf38 showed a significantly broader ERB bandwidth of Hz at 1000 Hz, compared with the normal-hearing value of Hz (indicated with dashed-dotted line on the lower panel). Subject blm34 showed an ERB value of Hz at 1000 Hz, while subject fhm25 and lkf33 had ERB values of around 222 Hz. Subjects hbm45 and hgf42 showed near normal-hearing ERB values at 1000 Hz. Figures show the individual threshold values for five different g values and the fitted rounded-exponential filter at centered frequency 250 Hz. Figures show the individual threshold values for different g values and the fitted roundedexponential filter at 1000 Hz. 3.7 Discussion The pure-tone audiometry of the test subjects in Fig. 2.1 provides only a partial picture of their hearing as it does not give much information about their perception of temporally varying sounds. The subject s temporal resolution was directly evaluated using the frequency discrimination experiment at low frequencies (250 Hz and 1000 Hz) (section 3.2), the gap detection experiment (section 3.3), the binaural masking level difference experiment (section 3.4), and the modulation detection experiment (section 3.5). The sharpness of the auditory filters was also measured (section 3.6). The phychophysical experiments were particularly focused on the low-frequency region, where the hearingimpaired listeners, in terms of pure-tone audiometry, were normal or near-normal. All the experiments show no general correlation between the performance and the pure-tone audiometry results of the hearing-impaired subject, except of the modulation detection results for broadband noise carrier. Subjects with better audibility showed worse results in some of the experiments. Specifically subject apf38, with moderate high-frequency hearing loss, showed poor temporal processing in most of the tasks, while subject blm34,

35 Psychophysical experiments Hearing Impaired Equivalent Rectangular Bandwidth (ERB) for Auditory filter Normal Hearing 250 Hz ERB (Hz) Moore and Glasberg (1990) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Hearing Impaired Normal Hearing 1000 Hz ERB (proportion centre frequency) Moore and Glasberg (1990) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 htm77 lnf82 pkm77 slvf80 Subjects 0.02 FIG. 3.9: Equivalent rectangular bandwidths (ERBs) for the auditory filter with characteristic frequencies of 250 Hz (upper panel) and 1000 Hz (lower panel) for ten subjects. The left panels show the ERBs for hearing-impaired subjects and the right panels show the ERBs for normal-hearing subjects with filled markers. The dashed-dotted lines indicate the ERB values suggested by Glasberg and Moore (1990) for auditory filters in normal-hearing subjects centered at 250 Hz and 1000 Hz, respectively. with profound high-frequency hearing loss, showed normal frequency discrimination and BMLD thresholds. The next step in the present study is to investigate the correlation between the audiometric and psychophysical results with the subject s ability to perceive and understand speech in noise.

36 28

37 4. SPEECH INTELLIGIBILITY In everyday life, people with cochlear hearing loss complain about difficulty in understanding speech. Such problems depended partly on the severity of the hearing loss. Usually, people with moderate losses can understand speech reasonably well when they are in a quiet environment and they have one talker in front of them. However, major difficulties occur when several people are talking at the same time in a noisy environment or in rooms with reverberation. Typically, people with severe or profound losses have problems even when listening to a single talker in a quiet room. They have big problems when background noise is presented. In this chapter, speech intelligibility of hearing-impaired subject s was examined. Since speech is the most important signal for our communication, the question whether the ability to understand speech in noise is correlated with the subject s abilities in the psychoacoustic experiments of chapter 3 is of a large interest. In section 4.3, the results of the speech intelligibility experiment are related to the results of the experiments from chapter Method Since all hearing impaired subjects were native Danish speakers, the Danish sentence test DANTALE II (Wagener et al., 2003) was used. The test is based on the Swedish sentence test by Hagerman (1982). The sentences are relatively slowly spoken, and the speech material contains 50 well-known words. Each sentence is syntactically fixed and consists of five words: 1) a name, 2) a verb, 3) a number 4) an adjective and 5) an object. The test sentences are also semantically unpredictable (nonsence). The DANTALE II consists of 16 test lists of 20 sentences each. The intelligibility score is given as a function of the signal-to-noise ratio, in db. SNR = 20log 10 ( L s L n ), (4.1) where L s denotes the speech sound pressure level in db SPL and L n the noise sound pressure level in db SPL. The lowest SNR, at which the speech signal was intelligible enough to lead to an identification ratio of 50% of the words included in each sentence, was defined as the speech reception threshold (SRT). The interfering noise of DANTALE II has

38 30 a comparable long-term spectrum to varies languages long-term average speech spectrum (LTASS) (Byrne et al., 1994). To generate the speech-shaped interfering noise, the speech material was superimposed using random silence durations and starting time. Wagener et al. (2003) demonstrated that the SRT decreased by increasing the number of lists performed for each subject, because of familiarization with the word material. Thus a training with 60 sentences (three lists) before the measurement was used here, as in Wagener et al. (2000) and Brand and Kollmeier (2002). For a male speech, the longterm average sound pressure level for a large number of speakers is 65 db when measured one meter in front of the speaker s mouth in an anechoic room. During normal speech the level varies ± 15 db from the mean value (Poulsen, 2003). It was taken into consideration that when talking to a hearing-impaired person, one normally raises his voice. The speech signal had been recorded in an anechoic room. A noise floor produced from the electrical equipment is unavoidable. It appears that during the measurement of the SRT, it is better to attenuate the noise instead the speech signal since an increase of the speech level would increase the noise level from the speech signal recording. In addition, changes in speech level may change the perception of normal-speech. Thus during the test, the speech material was presented at a fixed level and the noise was reduced when subjects could hear less than 50% of the words in the sentence. The stationary noise was calibrated to have a starting level of 75 db SPL, where a signal to noise ration (SNR) of zero corresponds to L s and L n having a sound pressure levels of 75 db SPL. Figure 4.1 shows the setup that was used during the SRT measurement. Subjects were seated in a soundproof booth. The sentences were presented via Sennheiser HDA200 headphones. After presentation, the subjects were asked to repeat what they understood. A response microphone was connected to headphones so that the operator could hear the subject s responses. The operator was a native Danish speaker and had to enter the number of words that the subject recognized correctly in each sentence into a computer. The computer adaptively adjusted the noise level in order to reach the speech recognition threshold (SRT). The system was running in an MATLAB environment using the DAN- TALE II CD through the computer CD-player (Poulsen, 2003). The operator instructed the subjects in the beginning of the task. The instructor followed the recommended procedure by the American Speech Language Hearing Association (ASHA) (1988). The subjects were informed about the nature of the task, the involved speech material, and that they were encouraged to repeat the words even if they had to guess. During the familiarization session, the operator could ensure that the subject knew the test vocabulary and that the operator could accurately interpret subject s responses. 4.2 Results Figure 4.2 shows the speech recognition threshold (SRT) for each subject. The data for seven subjects are shown, six with sloping high-frequency hearing loss (left panel) and one with normal-hearing (right panel). The SRTs for the impaired-hearing subjects

39 Speech intelligibility 31 Amplifier Amplifier CD player Amplifier FIG. 4.1: Setup of the speech intelligibility experiment. The speech and interfering noise were played from the CD-player of the measurement computer using the DANTALE II. Both speech material and speechshaped noise were fed through a computer controlled amplifier. The amplifiers outputs were connected to a mixer before connected to the final amplifier. The subjects listened to the DANTALE II sentences via Sennheiser HDA200 headphone which were connected to the output of the final amplifier. The subject repeated the words that were understood to a microphone and the operator was able to listen the subject s response via headphones. The level of the interfering noise was changed adaptively in relation to the number of correct words the operator heard and entered in the measurement computer. are uncorrelated with their audibility as defined in chapter 2. The profound hearingimpaired group had higher SRTs of db and 0.25 db, compared to db and -2.5 db of the severe hearing-impaired group. Subject hgf42 with moderate hearing loss even shows a better SRT of db than the normal-hearing control subject lnf82 (-6.35 db). However, subject apf38 clearly showed the highest SRT of 3.65 db, even though only having a moderate hearing loss in terms of pure-tone audiometry. For the last 15 sentences of the experiment, all the test subjects correctly identified two to three words when the signal to noise ration was around their SRT values (see Appendix E). Signal to Noise Ratio for 50% speech intelligibility (SRT) 4 Hearing Impaired Normal Hearing 2 0 SRT (db) profound severe moderate blm34 fhm25 hbm45 lkf33 apf38 hgf42 lnf82 Subjects FIG. 4.2: The speech recognition thresholds (SRT) for the hearing-impaired subjects with profound hearing loss on the left and the results from one native Danish speaker with normal hearing on the right.

40 Discussion Problems to understand speech in everyday life situations may partly depended on the severity of the hearing loss as, e.g., seen in the results for subjects fhm25, lkf33, and hgf42 in Fig Subject hgf42, who had a low SRT value of db (good speech intelligibility), could easily understand the operator but she mentioned large difficulties to hear her colleagues in the choir especially when they performed inside a church. Typically, people with severe or profound losses have problems even when listening to single talker in a quiet room and they have significant problems when background noise is present. Subjects blm34 and fhm25, who had profound hearing loss at high frequencies (see Fig. 2.1) relied their speech understanding on lip reading during the instruction session with the operator. Except for a trend, the SRT results show no direct relation between pure tone audiometry and speech intelligibility. Subject hbm45 had severe hearing loss at high frequencies (60 db HL or more) but lower SRT threshold db, while subject lkf33 who had similar pure-tone hearing thresholds had a SRT value of -2.5 db. In contrast, subject apf38 (moderate hearing high-frequency loss) had the poorest SRT value of 3.65 db. DANTALLE II has been designed such that small changes in the effective Signal-to-Noise-Ratio are related to large changes in the intelligibility. This means that a difference of 1.5 db in the SRT is equivalent to a large difference in speech intelligibility. Thus if the speech level for people with moderate or more severe cochlear hearing loss is above the threshold for detection, one may asked which psychophysical factors have the strongest impact influence on speech intelligibility. Due to the limited time of this study, a very limited number of the subjects participated in the different tasks. In addition, no training sessions for the psychophysical experiments of chapter 3 to ensure stable subjects performances were possible to be performed by the subjects. For comparison, Glasberg and Moore (1989) tested six subjects with bilateral losses of cochlear origin. They investigated different psychophysical tests at the frequencies 500, 1000 and 2000 Hz. They found a correlation between the mean absolute threshold and SRTs in speech-shaped noise with level of 75dB SPL. A correlation between different psychophysical aspects (examined in chapter 3) and speech intelligibility results will be presented in the following. Figures show the results of the different psychophysical experiments of the different hearing-impaired subjects and one normal-hearing subject (lnf82) as a function of their SRTs. Results for subjects with low SRT values are shown on the left of the figures, while those with high SRTs are plotted toward the right. The results for the normal-hearing subject lnf82 is indicated with the filled marker.

41 Speech intelligibility 33 Figure 4.3 is a replot of the frequency discrimination results for 250 Hz and 1000 Hz from section 3.2, as a function of SRT values for all hearing-impaired subjects and the normal-hearing control subject. The two subjects with high SRT values (right side) showed the strongest reduction in their ability to detect a change in frequency. There is no systematic dependency for the four subjects with SRT values db. This is consistent with Glasberg and Moore (1989) who found a higher correlation between the frequency difference limen for pulsed sinusoids and SRTs than between the mean absolute threshold and SRTs. Subjects hgf42 hbm45 lkf33 blm34 fhm25 apf38 signal frequency: 250 Hz Just Noticeable Difference in frequency (Hz) lnf82 Zeng et al. (2005) signal frequency: 1000 Hz Just Noticeable Difference in frequency (%) 10 lnf82 1 Zeng et al. (2005) Speech Recognition Threshold SRT (db) FIG. 4.3: Frequency discrimination threshold at 250 Hz (upper panel) and 1000 Hz (lower panel) as a function of SRT values. Data is shown for six subjects with high-frequency sloping hearing loss and one normal-hearing indicated by the filled marker. The subjects with musical training background are indicated with circle markers. The left abscissa represents frequency discrimination threshold in Hz and the right abscissa represents just noticeable frequency differences in % from the experimental frequency. The error bars indicate ± one standard deviation of the mean threshold value. The dash-dotted lines indicate the normal-hearing DLFs values as measured by Zeng et al. (2005) at 250 Hz and 1000 Hz, respectively. Figure 4.4 shows gap detection thresholds (section 3.3), replotted as a function of SRT. There is no clear correlation between the gap detection thresholds and SRTs. In comparison, Glasberg and Moore (1989) found a higher correlation between gap detection thresholds for narrow band noise and SRT than between the mean absolute threshold ABS and SRTs.

42 34 20 Subjects hgf42 hbm45 lkf33 blm34 fhm25 apf38 Gap Threshold (ms) 10 5 lnf82 Zeng et al. (2005) Speech recognition threshold SRT (db) FIG. 4.4: Gap detection threshold at 69.3 db stimulus level as a function of SRT values. Data for six subjects with high-frequency sloping hearing loss and one normal-hearing indicated by the filled marker is shown. The error bars indicate ± one standard deviation of the mean threshold value. The dash-dotted line indicates the normal-hearing gap detection threshold was measured by Zeng et al. (2005) at a presentation level of 50 db SPL. Figure 4.5 shows the results for binaural masking level differences (BMLD, see section 3.4). The lowest BMLD values are observed for subjects with high SRTs (low speech intelligibility). The BMLD is independent of SRT for subjects with SRT Thus, subjects that show problems in understanding speech in noise take little advantage of binaural phase difference information for both experimental frequencies of 250 and 1000 Hz. The dash-dotted lines indicate the BMLD values measured by van de Par and Kohlrausch (1999) for normal-hearing subjects at 250 and 1000 Hz, respectively.

43 Speech intelligibility 35 Subjects hgf42 hbm45 lkf33 blm34 fhm25 apf38 20 signal frequency: 250 Hz 15 lnf82 10 BMLD (db) v. d. Par and Kohlrausch (1999) signal frequency: 1000 Hz 15 v. d. Par and Kohlrausch (1999) 10 lnf Speech Recognition Threshold SRT (db) FIG. 4.5: The binaural level differences between the masked-threshold level L(N 0S 0) and the maskedthreshold level L(N 0S π) for pure tone frequencies of 250 Hz (upper panel) and 1000 Hz (lower panel)as a function of SRT values. Data for six subjects with high-frequency sloping hearing loss and one normalhearing indicated by the filled marker is shown. The error bars indicate ± one standard deviation of the mean threshold value. The dash-dotted lines indicate the BMLD values measured by van de Par and Kohlrausch (1999) for normal-hearing subjects at 250 and 1000 Hz, respectively. Figure 4.6 shows the results for the amplitude modulation threshold experiment for modulation frequencies of 8 and 64 Hz (section 3.5). There is no clear relation between modulation detection threshold and SRT for a modulation frequency of 8 Hz (upper panel). At 64 Hz (lower panel) a trend, giving slightly higher modulation detection thresholds for subjects with high SRT, can be observed. Figure 4.7 shows the modulation detection thresholds for frequencies of 8 (upper panel) and 32 Hz (lower panel) for a sinusoidal carrier at 500 Hz. The modulation detection abilities for all the subjects were near-normal. Figure 4.8 shows the dependency of bandwidths of the auditory filters (section 3.6) and SRTs. The two subjects with the highest SRTs showed higher ERB estimates for 250 Hz. The ERB estimates for 250 Hz (upper panel) are independent of SRT and similar across normal-hearing and hearing-impaired with SRT db. A trend toward higher ERB estimates for subjects with high SRT is observed at 1000 Hz (lower panel of Fig. 4.8).

44 36 10 subjects hgf42 hbm45 lkf33 blm34 fhm25 apf38 f mod : 8 Hz 15 Modulation depth m (db) lnf82 Viemeister (1979) f mod : 64 Hz 15 lnf Viemeister (1979) Speech Recognition Threshold SRT (db) FIG. 4.6: Modulation detection threshold for modulation frequencies of 8 Hz (upper panel) and 64 Hz (lower panel) obtained in seven subjects as a function of their SRTs. Data for six subjects with high-frequency sloping hearing loss and one normal-hearing indicated by the filled marker is shown. The error bars indicate ± one standard deviation of the mean threshold value. The dash-dotted lines indicate the modulation detection threshold values measured by Viemeister (1979) for broadband sinusoidally modulated noise at 8 and 64 Hz, respectively In summary, some of the results of the temporal processing tasks of chapter 3 are related with the speech reception thresholds (SRT). A correlation is found mainly for high SRT values. Subjects with SRTs showed elevated threshold values for some of the psychophysical experiments without indicating any systematical correlation with speech intelligibility in noise. The were subjects categorized in three categories related with their performances in the different tasks of chapter 3 and 4. Table 4.1 presents the range of performance values for each category and test. Table 4.2 shows performances of all hearing-impaired subjects and the normal-hearing control subject lnf82 for all the tests of chapters 3 and 4, using the scaling presented in table 4.1. Subject apf38 (moderate high-frequency hearing loss) showed severely reduced performance for all temporal processing tasks except for the modulation detection tasks. The speech intelligibility in noise was also severely reduced. On the contrary, subject hbm45 (severe high-frequency hearing loss) had near-normal performances for all the temporal resolution tasks except for gap detection, where his performance was reduced and binaural masking level difference at 250 Hz where his performance was severely reduced. In this case, the speech reception threshold was close to the SRT value for the normal-hearing control subject lnf82. The next step in the current study was to investigate the relation

45 Speech intelligibility f mod : 8 Hz Subjects hgf42 hbm45 lkf33 blm34 fhm25 apf38 20 Modulation depth m (db) lnf82 Kohlrausch A. et al. (2000) f mod : 32 Hz 25 lnf82 30 Kohlrausch A. et al. (2000) Speech Recognition Threshold SRT (db) FIG. 4.7: Modulation detection threshold for a sinusoidal carrier at 500 Hz obtained in seven subjects for modulation frequencies of 8 (upper panel) and 32 Hz (lower panel). Data for six subjects with highfrequency sloping hearing loss and one normal-hearing indicated by the filled marker is shown. The error bars indicate ± one standard deviation of the mean threshold value. The dash-dotted lines indicate the measured values of -27 db for a sinusoidal carrier at 1000 Hz and modulation frequencies of 8 and 32 Hz as found by Kohlrausch et al. (2000), respectively. between the speech intelligibility in noise and the psychophysical performances with the objective auditory brainstem responses.

46 hgf42 hbm45 lkf33 blm34 fhm25 apf38 signal frequency: 250 Hz Subjects ERB (Hz) lnf82 Moore and Glasberg (1990) signal frequency: 1000 Hz ERB (proportion centre frequency) lnf82 Moore and Glasberg (1990) Speech Recognition Threshold SRT (db) 0.02 FIG. 4.8: Equivalent rectangular bandwidths (ERBs) for the auditory filters with characteristic frequencies of 250 Hz (upper panel) and 1000 Hz (lower panel) for seven subjects as a function of their SRT values. Data for six subjects with high-frequency sloping hearing loss and one normal-hearing indicated by the filled marker is shown. The dash-dotted lines indicate the ERB values suggested by Glasberg and Moore (1990) for the auditory filters in normal-hearing listerners centered at 250 Hz and 1000 Hz, respectively. tests categories FRJND 250 in (Hz) FRJND 1000 in (Hz) GAP in (ms) BMLD 250 in (db) BMLD 1000 in (db) MODI 8 in (db) MODI 64 in (db) MODII 8 in (db) MODII 32 in (db) ERB 250 in (Hz) ERB 1000 in (Hz) SRT in (db) normal-hearing near-normal hearing ( * ) hearing-impaired reduced performance ( ** ) hearing-impaired severely reduced performance ( *** ) 1 to 4 4 to 7 7 or higher 5 to to or higher 4 to to 9 9 or higher 15 to to or lower 9 to 12 6 to 9 6 or lower -21 to to or lower -19 to to or lower -28 to to or lower -28 to to or lower 55 to to or higher 130 to to or higher -6.6 to to or higher TABLE 4.1: Range of threshold values for each experiment in the three categories: (i) normal or nearnormal performance (one star), (ii) reduced performance (two stars) and (iii) severely reduced performance (three stars).

47 Speech intelligibility 39 tests subjects apf38 fhm25 blm34 lkf33 hbm45 hgf42 lnf82 FRJND 250 Hz *** *** * ** * * * FRJND 1000 Hz *** *** * * * * * GAP *** ** *** *** * ** *** BMLD 250 Hz *** ** * ** ** *** * a BMLD 1000 Hz *** ** * * * * * MODI 8 Hz ** *** *** ** * * * MODI 64 Hz ** ** *** *** * * ** MODII 8 Hz * * * * * * * MODII 32 Hz * * * * * * * b ERB 250 *** *** * ** * ** ** ERB 1000 **** ** *** ** * * * c SRT *** *** ** ** * * * TABLE 4.2: Summary of the performances of the hearing-impaired subjects and the normal-hearing control subject using the categories of table 4.1 for: a) Summarizing temporal processing experiments such as frequency discrimination at 250 Hz and 1000 Hz (FRJND), temporal gap detection (GAP), binaural masking level differences at 250 Hz and 1000 Hz (BMLD) and modulation detection for noise carrier (MODI) and sinusoidal carrier at 500 Hz (MODII). b) Bandwidth (ERB) of auditory filter at 250 Hz and 1000 Hz. c) Speech recognition threshold (SRT). One star indicates normal or near-normal performance. Two stars indicate reduced performance. Three stars indicate severely reduced performance.

48 40

49 5. AUDITORY BRAINSTEM RESPONSES 5.1 Introduction By placing electrodes at the surface of the head, one is able to record evoked responses representing the summation of responses from many individual neurons that are excited by the stimulus (Jewett, 1970). Auditory evoked potentials can be recorded from all levels along the auditory pathway. Usually, these potentials are grouped by the time of occurrence after the onset of the stimulus, and this grouping corresponds roughly to the site of generation. The simultaneous discharge of a large number of nerve cells in the auditory brainstem evokes a potential, the auditory brainstem response (ABR). The assumption has been that the ABR is an electrophysiological event evoked by either the onset or the offset of an acoustic stimulus (Hecox et al. (1976); Kodera et al. (1977); Debruyne and Forrez (1982); Gorga and Abbas (1981); vancampen et al. (1997)). The acoustic click has an abrupt onset, which made it considered to be the ideal stimulus for eliciting ABR. Generally, an click-evoked ABR waveform consist of seven peaks within 10ms after the stimulation onset. Three of them, waves I, III and V are evaluated in clinic diagnostics. The current study is focused on the most robust peak, wave-v. The positive voltage of wave-v is related to neural excitation in the olivery complex before it enters the inferior colliculus, while the negative is attributed to the dendritic potentials within the inferior colliculus (Hall, 1992). The click has wide spectral spread, inherent in transient signals. Thus, it excites a large proportion of cochlear fibers. However, when a transient stimulus progresses apically along the basilar membrane, single-unit activity is less synchronous with the preceding activity from basal units because of temporal delays imposed by the cochlear traveling wave (Tsuchitani, 1983). This results in an asynchronous pattern of auditory-nerve-fiber firing along the partitions of the basilar membrane (BM). Additionally, the activity generated from the single units in more synchronous basal regions would be out of phase with activity from some apical units (Fobel, 2003). Thus, the combination of phase cancellation and loss of synchronization bias the click evoked potentials to reflect activity from more basal, high-frequency regions of the cochlea (Neely et al., 1988).

50 42 Dau et al. (2000) developed a chirp, rising in frequency, which is tailored to activate the entire cochlea concurrently at a particular point in time. The chirp theoretically produces simultaneous displacement maxima by canceling traveling-time differences along the cochlear partition. They determined the temporal course of the chirp by an equation based on the traveling wave velocity along cochlear partitions by de Boer (1980) and the functional relationship between stimulus frequency and place of maximum displacement derived by Greenwood (1990). The use of a broadband rising chirp was shown to reflect activity also from low-frequency regions, whereas neural synchrony across the cochlear partition is decreased for the click in accordance with the reduction in traveling wave velocity in the apical region of the cochlea (Dau et al., 2000). At low and moderate stimulation levels, the chirp showed a higher wave-v response amplitude than the click. It can be assumed that all frequencies contribute to the evoked response at the low and moderate stimulation levels. At high stimulation levels, with respect to wave-v amplitude, no advantage was found for the chirp over the click because effects like upward spread of excitation come into play and cause more complex potential patterns, particularly for the chirp (Dau et al., 2000). The present study investigates the ABR in normal-hearing and hearing-impaired subjects, using a click and the flat-spectrum rising frequency chirp (flchp). Auditory brainstem responses elicited by the chirp were recorded for the first time in hearing-impaired subjects in the present study. If hearing-impaired listeners have impaired temporal processing capabilities and reduced speech perception performance, they may also show a reduced wave-v response amplitude. The rising frequency chirp (with flat spectrum), developed by Dau et al. (2000), may therefore be of clinical use in assessing the temporal processing in the peripheral auditory system. This chapter compares the differences in the click-evoked responses and chirp-evoked responses of hearing-impaired and normal-hearing listeners and among the hearing-impaired listeners. First, the results for the normal-hearing subject s responses will be presented. Second, the results for two representative hearing-impaired subjects will be presented. The responses of the hearing-impaired subject apf38 will be compared to the responses of the hearing-impaired subject hbm45. Subject apf38 (moderate high-frequency hearing loss) showed severely reduced temporal processing capabilities, significantly broader auditory filters and large difficulties to understand speech in noise (see table 4.2). Subject hbm45 (severe high-frequency hearing loss) showed near-normal performance at almost all the psychophysical tasks and near-normal speech intelligibility in noise (see table 4.2). This chapter investigates if differences in the temporal processing capabilities affect the auditory evoked potentials and if differences occur between the stimuli. 5.2 Method Apparatus The experiments were carried out with two PC-based computer systems as shown in Fig The first one, the stimulus PC, controlled the presentation of the acoustic stimuli.

51 Auditory brainstem responses 43 The second one, the recording PC, controlled the recording of the evoked potentials. A D/A converter (RME ADI-8 Pro), connected via an optical digital interface (S/PDIF) to a multi-channel audio card (RME DIGI 96), converted the digitally generated stimulus to an analog waveform. The analog output of the D/A converter was connected to an audio amplifier (TDT HB7) which presented the stimulus through an insert earphone (E A R TONE 3A) to the subject. Electroencephalic activity was recorded via silver/silver chloride (Ag/AgCl) electrodes that were attached to the scalp of the subject. The forehead served as the site for the ground electrode, the vertex (positive) as the reference electrode and the ipsilateral mastoid (negative) as the only additional electrode. Interelectrode impedance was maintained below 5 kω. The electrodes were connected to the so-called headbox. The headbox and the insert earphones were placed in a acoustically and electrically shielded booth. The headbox was responsible for the amplification and A/D conversion of the potentials. Outside of the shielded booth, a Synamps 2 system unit was connected with the output of the headbox. The recording was started at the system unit according to the trigger signal, which was provided from an additional channel of the D/A converter. The recording PC controlled the recording procedure and stored the data. Acoustically and Electrically Shielded Booth + Synaps 2 Headbox Synamps 2 System Unit Recording PC Trigger - D/A Converter (RME ADI-8 Pro) Insert Earphone (E A R TONE 3A) Audio Amplifier (TDT HB6) Stimulus PC with multi-channel Audio Card (RME Digi96) FIG. 5.1: Setup for recording of auditory evoked potentials. The stimulation PC is shown in the lower part and the recording PC in the upper part Stimulation Dau et al. (2000) developed an acoustic signal based on the following considerations: (i) The mechanical properties of the cochlear partitions are responsible for a spatial separation of frequency components of an acoustic signal. (ii) Along the cochlear partition, a temporal dispersion of displacement maxima occurs. They concluded that the desired stimulus must have a wideband frequency spectrum to excite a maximal number of nerve fibers (Shore and Nuttall, 1985). In addition, an adjustment of the temporal spacing of frequency

52 44 components of the wideband signal could provide maximum synchrony of discharge across frequency. The waveform and the (acoustic) magnitude spectrum of the click are shown in the upper left and the upper middle panel of Fig. 5.2, respectively. The click had a duration of 113 µs. The lower left panel of Fig. 5.2 shows the waveform of the flat-spectrum chirp rising in frequency, referred to as the chirp throughout the current study. The chirp has a flat amplitude spectrum corresponding to that of the click (upper right panel) Fig The chirp starts with very small amplitudes at low frequencies and increases nonlinearly in amplitude with time. The chirp started and ended with zero amplitude (lower left panel) of Fig The chirp had duration of ms. The instantaneous frequency changes slowly at low frequencies relative to the changes in the high frequencies (lower right panel of Fig. 5.2). No windowing was applied to the stimuli. Temporal waveform click Acoustic spectrum click Acoustic spectrum of chirp and click Amplitude Level (db) 20 0 Level (db) Time (μs) Frequency (khz) Frequency (khz) Temporal waveform of chirp Acoustic spectrum flat spectrum chirp Instantaneous frequency Amplitude Time (ms) Level (db) Frequency (khz) Time (ms) Frequency (khz) FIG. 5.2: Upper left panel: Waveform of the click with duration of 113 µs. Upper middle panel: Acoustic magnitude spectrum of click stimulus. Lower left panel: Waveform of the rising in frequency chirp with a duration of ms. lower middle panel: Acoustic magnitude spectrum of the chirp. Lower right panel: Instantaneous frequency of the chirp Procedure Each test subject lay on a couch in an electrically-shielded, sound-proof booth and electrodes were attached. The subject was instructed to keep movement at a minimum, and to sleep if possible. The first five minutes of the session were used for the subject s relaxation. The lights were turned off at the beginning of each trial. The subject had the opportunity to use a bell to ask for a break. Electrode impedances were checked at the middle of the session to ensure good quality of the recordings. Each trial consisted of 4000 stimulus presentation, 2000 epochs for each of the two stimuli. The total time of the recording for

53 Auditory brainstem responses 45 the experiment was one hour and was completed in one session. For each subject, one ear of stimulation was chosen depending on the site the subject preferred to lay on the couch. The stimulation ear was the same as that in the speech intelligibility experiment and the measurement of the bandwidth of the auditory filters at 250 Hz and 1000 Hz. The acoustic signals were delivered at a mean repetition rate of 22 Hz ( see equation G.1). A temporal jitter with mean value of 5 ms was introduced to minimize response superimposition from preceding stimuli. Thus, the resulting interstimulus interval (ISI) was equally distributed between 17 ms and 27 ms ( see equations G.2 and G.3). All the stimuli are specified as short duration signals (duration of less than 200 ms) according to the International Electrotechnical Commission (IEC) (1994) standards. In the present study, the calibration procedure for click and chirp was carried out following the International Electrotechnical Commission (IEC) (1994) standards. The peak-to-peak equivalent sound pressure level (pespl) was used when defining wave-v amplitude. The pespl is the numerical value of the sound pressure of a long duration sinusoidal signal which, when fed to the same transducer under the same test conditions has the same peak-to-peak sound pressure amplitude as the short-duration signal. The calibration levels were measured in an ear simulator B & K Type In the current study, the stimuli for the normal-hearing and hearing-impaired subjects were presented at five different stimulation levels of 20, 30, 40, 50, and 60 db sensation level (SL). The sensation level is the level of the stimulus in pespl, relative to the absolute threshold level in pespl of the individual listener. The hearing threshold measurement was determined individually using an adaptive three-alternative forced-choice (3AFC) procedure, as in section 3.1. The experiment was provided within the MATLAB AFC framework. During each run, three intervals were presented to the subject, only one of the intervals contained the stimulus. The subject s task was to choose which interval contained the stimulus. The starting level was 80 db pespl, which was sufficiently high for all the subjects. The average threshold level obtained over three repetitions for each stimulus was considered as representing 0 db SL for an individual subject (see Appendix B). At the beginning of each ABR-recording session, the first trial was a 60 db SL presentation of a stimulus. Two independent trials of the same presentation level were saved in a different buffer in order to keep the one with the better quality. The intensity was decreased in steps of 10 db down to 20 db SL. Wave-V (peak-to-peak) amplitude was analyzed in the different stimulus and level conditions. The amplitude was measured from the peak to the largest negativity following it. Throughout this study, the responses of five subjects are discussed explicitly, three normal-hearing subjects and two hearing-impaired. The responses of all the remaining subjects are shown in Appendix F. 5.3 Results Figure 5.3 shows the auditory brainstem responses (ABR) for the three normal-hearing subjects pkm77 (left column), jxm80 (middle column) and lnf82 (right column). The ABRs

54 46 elicited by the click are presented in the upper panels and the responses obtained with the chirp are shown in the lower panels. Responses at different stimulus levels are shown on separate axes displaced along the ordinate and labeled with the sensation level (db SL). The hearing thresholds for the individual subject for each stimulus are noted above each panel. For the click stimulus, the abscissa represents recording time relative to onset. In the case of the chirp stimulus, a dual abscissa is used representing recording time relative to stimulus onset and offset. Small vertical bars for the two stimuli mark the wave-v peaks. In addition, the variation in the responses is indicated by the shaded area between the averaged ABR response plus and minus one standard deviation. The upper left panel of Fig. 5.3 shows the click-evoked responses of the subject pkm77. First, it can be seen that the data show a clear wave-v peak in response to the click, at the two highest stimulation level of 50 and 60 db SL. For the levels 30 and 40 db SL wave-v peak is clearly weaker, but still visible. For the levels 20 db SL, no wave-v peak in response to the click can be seen. The wave-v response amplitude relative to the click is 0.25 µv at 60 db SL stimulation level. Wave-V latency is 6.02 ms for the highest stimulation level. The corresponding peak-equivalent sound pressure level hearing threshold of subject pkm77 s left ear for the click stimulus was 30 db pespl. The 60 db SL sensation level of subject pkm77 for click stimulus corresponds to 90 db SPL. The lower left panel of Fig. 5.3 shows the chirp-evoked responses for the same subject. First, it can be seen that the data show a higher wave-v response amplitude in comparison to the click responses. The results show a clear wave-v for all the stimulation levels. The chirp-evoked wave-v response amplitude is 0.54 and 0.40 µv for the highest and lower stimulation level, respectively. Wave-V latency relative to chirp offset is about 4.34 ms, for the highest stimulation level. The hearing threshold of subject pkm77 s left ear for the chirp stimulus was 35 db pespl. The 60 db SL sensation level of subject pkm77 for chirp stimulus corresponds to 95 db SPL. At the two highest stimulation levels, the early low-frequency energy in the chirp stimulates basal regions of the basilar membrane, probably due to cochlear upward spread of excitation and produces a response. The observations related to this phenomenon are: (i) Earlier response relative to the chirp becomes visible, with a first peak wave-i at about 1 ms after chirp offset. (ii) The wave- V response amplitude for the chirp stimulus was around 0.66 µv higher at moderate stimulation level (40 db SL), where it can be assumed that all the frequencies contribute to the evoked response. The results for subjects jxm80 and lnf82 show the same behavior as for pkm77. Figure 5.4 shows the ABR for the two hearing-impaired subjects, apf38 (moderate high-frequency hearing loss) with severely reduced speech perception in noise (left column), and hbm45 (severe high-frequency hearing loss) with near-normal speech perception (middle column). In addition, the normal-hearing subject lnf82 (right column) is shown for direct comparison. The ABRs elicited by the click are presented in the upper panels and those by the chirp are shown in the lower panels. The hearing thresholds of the individual subject for each stimulus are noted above each panel. The upper left panel of Fig. 5.4 shows the click-evoked responses of the hearing-

55 Auditory brainstem responses 47 click 60 pkm77 30 db pespl 60 jxm db pespl 60 lnf db pespl 1 μv Sensation level (db SL) time (ms) time (ms) time (ms) chirp 60 pkm77 35 db pespl 60 jxm db pespl 60 lnf db pespl 1 μv Sensation level (db SL) relative offset time (ms) relative onset time (ms) 20 relative offset time (ms) relative onset time (ms) 20 relative offset time (ms) relative onset time (ms) FIG. 5.3: Auditory brainstem responses (ABR) for three normal-hearing subjects. The responses for the subject pkm77, jxm80 and lnf82 presented in the left, middle and right panels, respectively. Responses evoked by the click presented in the upper panels and responses evoked by the chirp presented in the lower panels. The stimulation level varied from 20 to 60 db SL, as indicated. ABR waveforms are the average of 4000 responses. The stimulus presentation rate was 22/s for both stimuli. In the lower panels a dual abscissa is used representing recording time relative to stimulus onset and offset for chirp. A gray area indicates the variation in the response. The left ear of subject pkm77, the right ear of jxm80 and the left ear of lnf82 were the stimulation ears. Vertical small bars mark the wave-v peaks for both stimuli, if found. impaired subject apf38 who showed severely reduced performances in almost all the experiments of chapters 3 and 4 (see table 4.2). First, it can be seen that the data show a clear wave-v peak in response to the click at the highest stimulation level of 60 db SL. For the levels 40 and 50 db SL, wave-v peak is clearly weaker but still visible. For the levels 20 and 30 db SL, no wave-v peak in response to the click can be observed. The wave-v response amplitude relative to the click is 0.35 µv at 60 db SL stimulation level. Wave-V latency is 5.6 ms for the highest stimulation level. The corresponding peak-equivalent sound-pressure level hearing threshold of subject apf38 s left ear for the click stimulus

56 48 was 56.5 db pespl. Thus the 60 db SL sensation level of subject apf38 for click stimulus corresponds to db SPL. The lower left panel of Fig. 5.4 shows the chirp-evoked responses for the same subject. First, it can be seen that the data show a wave-v peak in response to the chirp which is comparable to that obtained for the click except for the highest level. The results show a wave-v peak at the two highest stimulation levels of 50 and 60 db SL. For the stimulation levels of 20, 30, and 40 db SL, no wave-v peak is visible. The shirp-evoked wave-v response amplitude is almost constant for these stimulation levels around 0.13 µv. Wave-V latency relative to chirp offset is about 4.8 ms, for the highest stimulation level. The hearing threshold of subject apf38 s left ear for the chirp stimulus was 64.4 db pespl. The 60 db SL sensation level of subject apf38 for chirp stimulus corresponds to db SPL. The upper middle panel of Fig. 5.4 shows the click-evoked responses for the hearingimpaired subject hbm45 who showed near-normal performances almost all the experiments of chapters 3 and 4 (see table 4.2). It can be seen that the data show a clear wave-v peak in response to the click at the highest stimulation level of 60 db SL. For levels 50 db SL wave-v peak is clearly weaker, but still visible. For levels of 20, 30, and 40 db SL, no wave-v peak in response to the click can be seen. The wave-v response amplitude relative to the click is about 0.22 µv for 60 db SL stimulation level. Wave-V latency relative to click is about 6.33 ms for the highest stimulus level. The corresponding hearing threshold of subject hbm45 s left ear for the click stimulus was 47.2 db pespl. The 60 db SL sensation level of subject hbm45 for click stimulus corresponds to db SPL. The lower middle panel of Fig. 5.4 shows the chirp-evoked responses for the same subject. It can be seen that the data show a higher wave-v response amplitude evoked by chirp than by the click stimulus. The data show a clear wave-v peak in response to the chirp, at all stimulation levels. The chirp-evoked wave-v response amplitude is 0.21 and 0.25 µv for the lowest and highest stimulation level, respectively. Wave-V latency relative to chirp offset is about 4.14 ms and 7 ms for the highest and the lowest stimulation level, respectively. The hearing threshold of subject hbm45 s left ear for the chirp stimulus was 55.5 db pespl. The 60 db SL sensation level of subject hbm45 for chirp stimulus corresponds to db SPL. At the highest stimulation level, an earlier response with a first wave-i peak at about 0.5 ms after chirp offset can be seen, possibly due to upward spread of excitation of the cochlear at stimulus onset. Interestingly, the wave-v response amplitude for the chirp stimulus was around 0.30 µv high at a moderate stimulation level (40 db SL). The right panels of Fig. 5.4 show the click-evoked and chirp-evoked responses of the normal-hearing subject lnf82 for direct comparison. In summary, the normal-hearing subject showed the clearest ABR responses, as expected. Among the normal-hearing subjects, the ABR response wave-v amplitude was larger when elicited by the chirp than by the click. All normal-hearing subjects showed a clear chirp-evoked wave-v at all stimulation levels tested. The hearing-impaired subject apf38 had moderate high-frequency loss, while subject hbm45 had severe high-frequency

57 Auditory brainstem responses 49 click 60 apf db pespl 60 hbm db pespl 60 lnf db pespl 1 μv Sensation level (db SL) time (ms) time (ms) time (ms) flchp 60 apf db pespl 60 hbm db pespl 60 lnf db pespl 1 μv Sensation level (db SL) relative offset time (ms) relative onset time (ms) 20 relative offset time (ms) relative onset time (ms) 20 relative offset time (ms) relative onset time (ms) FIG. 5.4: Auditory brainstem responses (ABR) for two hearing-impaired subjects and one normal-hearing. The responses for the hearing-impaired subject apf38 who had severely reduced performance in many psychophysical aspects and speech reception in noise are presented in the left column. The responses for the hearing-impaired subject hbm45 who had near-normal results for the most psychophysical tests and speech perception in noise are presented in the middle column. The responses of the normal-hearing subject lnf82 are presented in the right column. Responses evoked by the click are presented in the upper panels and responses evoked by the chirp presented in the lower panels. The stimulation level varied from 20 to 60 db SL, as indicated. ABR waveforms are the average of 4000 responses. The stimulus presentation rate was 22/s for both stimuli. In the lower panels a dual abscissa is used representing recording time relative to stimulus onset and offset for chirp. A shaded area indicates the variance of the response. The left ears of subject apf38 and hbm45 were the stimulation ears. Vertical small bars mark the wave-v peaks for both stimuli, if found. loss. The subject hbm45 s left ear had slightly better hearing threshold at 2000 Hz, the same at 3000 Hz, worse at 4000 and 7000 Hz, and similar at 8000 Hz than subject apf38 s left ear (see Fig. 2.1). For both hearing-impaired subjects, the wave-v evoked by the click was clear only at the highest stimulation level. The wave-v response amplitude evoked by the click was larger for the subject apf38 than for the subject hbm45 at the highest stimulation level. The subject apf38, with severely reduced timing processing and speech

58 50 intelligibility in noise, showed clearly smaller wave-v response amplitude for the chirp than subject hbm45. Subject hbm45, with near-normal performance for the most of the timing tasks and speech reception threshold in noise, showed clear wave-v peaks at all the stimulation levels. This observation suggests that there might be a correlation between the ABR response amplitude and the performance in temporal processing tasks and speech intelligibility. 5.4 Discussion The results for the normal-hearing subjects demonstrated a large difference between ABR response wave-v amplitudes for the click and the chirp. The chirp was more effective than the click and produced clear wave-v peaks at all the stimulation levels. The hearingimpaired subject hbm45, with worse hearing at high-frequencies than hearing-impaired subject apf38, but with significantly better temporal processing performance, showed clear wave-v peaks elicited by the chirp at all stimulation levels. The ABR responses evoked by the click, however, were similar for both hearing-impaired subjects. The upper panels of Fig. 5.5 show the wave-v response amplitude evoked by the click (left) and evoked by the chirp (right) at a stimulation level of 40 db SL as a function of the speech reception threshold (SRT) of the hearing-impaired subjects. The lower panels of Fig.5.5 show the wave-v response amplitude evoked by the click (left) and evoked by the chirp (right) as related with the audibility results of the hearing-impaired subjects. The results for the normal-hearing subject lnf82 are indicated with the filled marker. The results for the click-evoked wave-v response amplitude at 40 db SL were not related with the audibility and the SRT values of the hearing-impaired subjects. In contrast the results of the chirpevoked wave-v response amplitude show a trend to be reduced when the audibility is reduced and when the SRT value is increased. In order to test how the ABR obtained with normal-hearing listeners would look like when no information from the frequency region above 1000 Hz would be available, a control experiment was performed by the normal-hearing subject pkm77. A sixth-order Butterworth filter with cut-off frequency of 1000 Hz was used to remove the high frequencies of the chirp. The ABR responses elicited by the filtered chirp were recorded at 60 db SL stimulation level. Figure 5.6 shows the waveform, the (acoustic) magnitude spectrum and the ABR response of the chirp (three left most panels) and the same for the 1000-Hz low-pass filtered chirp (three right most panels). The both superimposed chirps in the upper third panel of Fig. 5.6 illustrates that the phase information was not effected by the filtering. Thus, the filtered chirp should still produce simultaneous displacement maxima along the cochlear partition in the frequency range from 100 to 1000 Hz. Even though the filtered chirp-evoked response of pkm77 was not evoked from activity beyond 1000 Hz, the wave-v response amplitude is clearly visible (lower right panel of Fig. 5.6). In contrast to the simulated hearing of subject pkm77 using the filtered chirp, even the hearing-impaired listeners with profound high-frequency hearing loss typically still had some remaining hearing at high frequencies.

59 Auditory brainstem responses 51 click evoked wave V amplitude (μv) hgf hbm lkf blm fhm apf SRT (db) chirp evoked wave V amplitude (μv) hgf hbm lkf blm fhm apf SRT (db) click evoked wave V amplitude (μv) moderate severe profound hgf apf lkf hbm fhm blm audibility chirp evoked wave V amplitude (μv) moderate severe profound hgf apf lkf hbm fhm blm audibility FIG. 5.5: Upper left panel: Click-evoked wave-v amplitude as a function of SRT values for the six hearingimpaired subjects. Upper right panel: Chirp-evoked wave-v amplitude as a function of SRT values for the six hearing-impaired subjects. Lower left panel: Click-evoked wave-v amplitude for the different subjects audibility. Lower right panel: Chirp-evoked wave-v amplitude for the different subjects audibility. The results for the normal-hearing subject lnf82 are indicated by the filled markers. Thus, if the recorded potential patterns evoked by the chirp for hearing-impaired listeners do not show a visible wave-v response, this indicates that their temporal synchronization, mainly in their available low-frequency region, might be abnormal. For subject apf38, a significantly reduced response amplitude is seen, even though the loss of audibility at high-frequencies is only moderate. This suggest that the timing in the low-frequency region is severely hampered and the subject does not gain from some supposedly still available information in the high-frequency region. In comparison, the second hearingimpaired subject, hbm45, showed a clear wave-v response even though audibility for high frequencies is severely reduced. This indicates that the synchronization of neural activity is normal or near-normal in the low-frequency region, comparable to the filtered chirp condition in the normal-hearing subject pkm77. Interestingly, hbm45 also showed near-normal performance in the temporal processing tasks and good speech intelligibility in noise. Thus, the ABR responses elicited by the chirp might reflect impairments in the temporal processing. Therefore, the chirp might be used to indicate if the temporal processing, in terms of neural synchrony, is normal or impaired.

60 52 1 chirp ms 20 acoustic spectrum chirp 1 filtered chirp 14 ms filter (but,6,1000) acoustic spectrum filtered chirp filter (but,6,1000) 20 Amplitude Level in db Amplitude Level in db Time (sec) Frequency (khz) Time (sec) Frequency (khz) pkm77 chirp 35 db pespl pkm77 filtered chirp 2.05 db pespl Sensation level (db SL) 60 relative offset time (ms) relative onset time (ms) 60 1 μv relative offset time (ms) relative onset time (ms) FIG. 5.6: Upper left panel: Waveform of the chirp stimulus. Second upper panel: Acoustic magnitude spectrum of the chirp. Third upper panel: Superposed waveforms of the chirp and the filtered chirp. Upper right panel: Acoustic magnitude spectrum of the filtered chirp. Lower left panel: Auditory brainstem responses (ABR) for normal-hearing subject pkm77 evoked by the chirp. Lower right panel: Auditory brainstem responses (ABR) for normal-hearing subject pkm77 evoked by the filtered chirp. The stimulation level was 60 db SL, as indicated. ABR waveforms are the average of 4000 responses. The small vertical lines indicate wave-v peaks. The stimulus presentation rate was 22/s for both stimuli. In the lower panels a dual abscissa is used representing recording time relative to stimulus onset and offset for the chirp and the filtered chirp. The shaded area indicates the average ABR ± one standard deviation of the measurement. The left ear of subject pkm77 was the stimulation ear. The small vertical bars mark the wave-v responses for both stimuli. The hearing threshold value for the subject pkm77 for the chirp stimulus was 35 db pespl and for the filtered chirp db pespl.

61 SUMMARY AND CONCLUSION The current study investigated the processing capabilities of the auditory system in the frequency region below about 1000 Hz in hearing-impaired and normal-hearing subjects using subjective and objective methods. The main idea was to establish a relationship between speech intelligibility and the results of basic psychoacoustical experiments (subjective) as well as a relation between the subjective results and objective auditory brainstem response (ABR) recordings. In the first part (chapters 3 and 4), the timing related perception in the low frequency region was measured with psychophysical experiments addressing frequency discrimination, temporal gap detection, binaural masking level difference, and temporal modulation detection. In addition, the frequency resolution was estimated with the notched-noise method. Speech intelligibility was characterized using the speech reception threshold (SRT) measure performed with the DANTALE II Danish sentence test in noise. In the second part (chapter 5), ABR recordings were presented using a click and a rising frequency chirp for normal-hearing subjects and hearing-impaired subjects with high-frequency hearing loss. In the following, a summary of the results and the main findings of the present study is given and conclusions are drawn. Chapter 3 presented psychophysical experiments performed by six hearing-impaired subjects with bilateral symmetrical high-frequency hearing loss and four normal-hearing subjects as a control group. The first experiment dealt with frequency discrimination of pure tones at 250 Hz and 1000 Hz. Threshold values for both experimental frequencies were not related to audibility for the hearing-impaired subjects. At both frequencies, subjects apf38 (moderate high-frequency hearing loss) and fhm25 (profound high-frequency hearing loss) had the highest frequency difference limen values. The subject apf38 showed results similar to those found by Zeng et al. (2005) for subjects with a disease called auditory neuropathy (normal outer hair-cells function but disrupted auditory nerve function). In contrast subject hbm45 (severe high-frequency loss) showed results similar to those found by Zeng et al. (2005) for subjects for normal-hearing listeners. The second experiment investigated the detection of a temporal gap contained in a broadband white noise. Gap detection thresholds were not correlated with audibility for the hearing-impaired subjects: Subjects blm34 (profound high-frequency hearing loss), lkf33 (severe high-frequency loss) and apf38 (moderate high-frequency loss) showed ele-

62 54 vated thresholds. Again, subject apf38 showed gap detection threshold similar to those found for subjects clinically diagnosed with auditory neuropathy (Zeng et al., 2005). Subject hbm45 showed threshold values close to those of the normal-hearing control group of the current study. Binaural temporal processing was addressed using the binaural masking level difference (BMLD) paradigm between the condition N 0 S 0 (no interaural phase difference of sinusoidal signal) and N 0 S π (interaural phase difference of sinusoidal signal of π). The BMLDs were measured for signal frequencies of 250 Hz and 1000 Hz. Most of the hearing-impaired subjects had near-normal BMLDs for 250 Hz. At 1000 Hz, the hearing-impaired group showed smaller BMLD values in comparison with the normal-hearing control group. The BMLDs of the hearing-impaired for pure tone at 250 Hz and 1000 Hz were not related with the subject s audibility results. The subject apf38 (moderate high-frequency loss) was hardly able to use the phase difference cues to detect the tone in noise for both experimental frequencies, while hearing-impaired subject hbm45 (severe high-frequency hearing loss) had near-normal performance at 250 Hz and reduced performance at 1000 Hz. The fourth measure was modulation detection at 8 Hz and 64 Hz using a broadband noise and 8 Hz and 32 Hz using a sinusoidal carrier (500Hz). When using the noise carrier, the results for modulation frequency of 8 Hz were correlated with the audibility of the hearing-impaired subjects. The subject blm34 had a reduced modulation-detection threshold for broadband-noise carrier due to the profound hearing loss at high frequencies. The nearly normal threshold values of all the hearing-impaired subjects for the sinusoidal carrier indicated near normal temporal processing. Overall, the timing related perception tasks at low frequencies below about 1000 Hz were not related with the audibility of the hearing-impaired subjects. The last psychophysical measure was the equivalent rectangular bandwidth (ERB) of the auditory filters at 250 Hz and 1000 Hz. Again, the results were not correlated with the audibility for the hearing-impaired subjects and with the absolute threshold in db HL at the tested frequency. The subjects apf38 (moderate high-frequency hearing loss) and fhm25 (profound high-frequency hearing loss) showed a substantially broadened auditory filter at 250 Hz. The subject hbm45 (severe high-frequency hearing loss) showed similar ERBs to those suggested by Glasberg and Moore (1990) for normal-hearing listeners. In chapter 4, the speech reception thresholds (SRT) of the same hearing-impaired subjects as in the previous experiments and one of the previous normal-hearing listeners were presented. The speech intelligibility in noise was performed with the DANTALE II test, where the speech signal level was fixed at 75 db SPL. As expected from the literature, SRTs were not related with the subject s audibility. The subject apf38 (moderate highfrequency hearing loss) showed clearly the highest SRT (low speech intelligibility) of 3.65 db SPL, while subject hbm45 (severe high-frequency hearing loss) showed db SPL resulting in a spread of about 8 db among their SRTs. All the results of the different experiments examined in chapter 3 were presented as a function of SRT. The subjects apf38

63 Summary 55 (moderate high-frequency hearing loss) and fhm25 (profound high-frequency hearing loss) showed the highest SRTs (reduced speech intelligibility in noise) and the strongest reduction in their ability to discriminate differences in frequency. A correlation for frequency discrimination and SRTs for both experimental frequencies was found for the hearingimpaired subjects with the higher SRTs, while no systematic dependence found for the four hearing-impaired subjects with SRTs db SPL. Similarly, the binaural masking level differences (BMLD) were lower (reduced sensitivity) for the subjects with higher SRTs (reduced speech perception in noise), especially for the BMLD at 250 Hz. The BMLDs for subjects with SRT db SPL were independent of SRTs for both frequencies 250 Hz and 1000 Hz. In contrast, gap detection thresholds and modulation detection at low modulation frequencies with broadband noise and sinusoidal carrier did not seem to be related to the SRTs. Broadening of the estimated equivalent rectangular bandwidth (ERB) of the auditory filter centered at 250 Hz was correlated with the higher SRTs. The largest ERBs values indicating reduced frequency resolution were shown for subject apf38 and fhm25 with the highest SRTs (reduced speech perception in noise). The ERBs for 1000 Hz showed a trend toward higher ERB (reduced frequency resolution) estimates among all the hearing-impaired subjects. Overall, frequency discrimination and binaural masking level differences at low frequencies below about 1000 Hz showed a correlation with the SRT, resulting in highest frequency discrimination threshold values and in lowest BMLDs for the subjects with the highest SRTs. Chapter 5, showed the auditory brainstem responses (ABR) evoked by a click and a rising-frequency chirp for five different stimulation levels. Results from the same six hearing-impaired subjects of the previous experiments and three normal-hearing subjects were obtained. Among the normal-hearing subjects, the ABR wave-v response amplitude was larger for the chirp stimulus at all stimulation levels. There were no previous investigations of the ABRs evoked by the chirp in hearing-impaired subjects with high-frequency hearing loss. The results showed a more prominent wave-v peak for lower stimulation levels for the chirp than for the click. The amplitude of ABR wave-v for chirp, compared to the click stimulus, was smaller or similar for the two highest stimulation levels. The ABR wave-v response amplitude elicited by chirp was higher for all the hearing-impaired subjects for a moderate stimulation level of 40 db SL and lower. The measured wave-v response amplitudes at moderate stimulation level (40 db SL) for click were not related with the SRTs and the subject s audibility. There was a trend for the chirp stimulus showing higher wave-v amplitudes for subjects with low SRT. In conclusion, the results of the current study demonstrate the importance of temporal processing and neural synchronization in speech intelligibility. The speech reception threshold is not related with the degree of high-frequency hearing loss. The results showed that frequency discrimination limen and binaural masking level differences at low frequency region were related with the SRTs. Subjects with severe temporal processing impairments had results similar to those found by Zeng et al. (2005) for patients with auditory neuropathy (near-normal outer hair cells function but disrupted auditory nerve function). The use

64 56 of the chirp as a stimulus to elicit ABR in a hearing-impaired listener enables the investigation of listener s auditory system ability to transmit synchronous neural activity in the brainstem. A decrease in ABR wave-v evoked by the chirp was found for listeners with severely reduced sensitivity in timing related psychophysical tasks. In contrast, responses evoked by the click were similar for subjects with different temporal processing abilities. As a trend, both audibility at high-frequencies and speech intelligibility in noise seemed to related with the wave-v response amplitude evoked by the chirp. More data from subjects with the similar hearing-loss at high-frequencies and different temporal processing abilities are required to further investigate the relation between ABR responses evoked by the chirp and temporal processing disorders. The chirp stimulus might be useful as an objective indicator of temporal processing impairments particularly for subjects with available low-frequency hearing and high-frequency hearing loss.

65 APPENDIX A: Statistical equations The subjects mean threshold values were calculated from the equation: x j = 1 n The standard deviation was calculated from the equation: n i=1 x ij (A.1) ( ) 1 1 n std = (x i x) 2 2 n 1 i=1 (A.2) The standard deviation of the binaural masking level difference was calculated from the equation: std bmld = std (L N0 S 0 ) 2 + std (L N0 S π ) 2 (A.3)

66 APPENDIX B: Enclosed DVD The DVD contains the results and the scripts used in the present study.

67 APPENDIX C: Auditory filters at 250 Hz Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 blm34r threshold (db) 40 attenuation (db) relative bandwidth g relative bandwidth g FIG. 10.7: The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject blm34 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above the right panel.

68 60 58 Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) fhm25l threshold (db) 50 attenuation (db) relative bandwidth g relative bandwidth g FIG. 10.8: The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject fhm25 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 hbm45l threshold (db) 45 attenuation (db) relative bandwidth g relative bandwidth g FIG. 10.9: The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject hbm45 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

69 Appendix C Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 56 lkf33r threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject lkf33 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) apf38l threshold (db) 50 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject apf38 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

70 62 56 Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 54 hgf42r threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject hgf42 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 htm77r threshold (db) 45 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject htm77 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

71 Appendix C 63 Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 54 lnf82r threshold (db) 46 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject lnf82 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 50 pkm77l threshold (db) 40 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject pkm77 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

72 64 Measured and fitted Auditory filter for 250Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 slvf80l threshold (db) 45 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 250 Hz and errorbars with ± one standard deviation for the subject slvf80 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 250 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

73 APPENDIX D: Auditory filter at 1000 Hz 56 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) blm34r threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject blm34 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

74 66 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 60 fhm25l threshold (db) 45 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject fhm25 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) hbm45l threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject hbm45 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

75 Appendix D 67 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 60 lkf33r threshold (db) 45 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject lkf33 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 61 apf38l threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject apf38 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

76 68 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 hgf42r threshold (db) 35 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject hgf42 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 htm77r threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject htm77 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

77 Appendix D Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) lnf82r threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject lnf82 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel. 55 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) pkm77l threshold (db) 35 attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject pkm77 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

78 70 Measured and fitted Auditory filter for 1000Hz 0 Fitted Roexfilter (p= ERB: Hz) 55 slvf80l threshold (db) attenuation (db) relative bandwidth g relative bandwidth g FIG : The measured threshold values for the five different notch widths, g relative to the centered frequency at 1000 Hz and errorbars with ± one standard deviation for the subject slvf80 are indicated in the left panel. The solid lines indicate the fitted rounded-exponential filter centered at frequency f c = 1000 Hz. The calculated value of the equivalent rectangular bandiwidth of a rectangular filter f ERB that passes the same power as the rounded-exponential fitted filter is noted above right panel.

79 APPENDIX E: Speech reception threshold FIG : Speech reception threshold of subject blm34. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response.

80 72 FIG : Speech reception threshold of subject fhm25. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response. FIG : Speech reception threshold of subject hbm45. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response.

81 Appendix E 73 FIG : Speech reception threshold of subject lkf33. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response. FIG : Speech reception threshold of subject apf38. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response.

82 74 FIG : Speech reception threshold of subject hgf42. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response. FIG : Speech reception threshold of subject lnf82. The system was running in a MATLAB environment using the DANTALE II Danish sentence test. The speech signal was calibrated at 75 db SPL, not at 65 db SPL as shown in the dialog. The test started with a signal to noise ration (SNR) of zero db SPL (i.e. noise level 75 db SPL) and the noise level was adjusted adaptively based on the subject s response.