Amplitude Modulation Detection by Listeners with Unilateral Dead Regions DOI: /jaaa

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1 J Am Acad Audiol 20: (2009) Amplitude Modulation Detection by Listeners with Unilateral Dead Regions DOI: /jaaa Brian C.J. Moore* Vinay{ Abstract Background: A dead region is a region in the cochlea where the inner hair cells and/or neurons are functioning very poorly, if at all. We have shown that, for people with sensorineural hearing loss, thresholds for detecting sinusoidal amplitude modulation (AM) of a sinusoidal carrier were lower for ears with high-frequency dead regions, as diagnosed using the threshold-equalizing noise test, calibrated in hearing level, than for ears without dead regions when the carrier frequency was below the edge frequency, fe, of the dead region. Purpose: To measure AM-detection thresholds for subjects with unilateral dead regions, using carrier frequencies both below and above fe. Research Design: Ten subjects with bilateral high-frequency hearing loss, but with unilateral highfrequency dead regions, were tested. The carriers were presented at sensation levels of 5, 10, or 15 db. The values of fe were close to 1000, 1500, or 2000 Hz. Results: For carrier frequencies below fe, AM-detection thresholds were lower for the ears with dead regions than for the ears without dead regions, replicating earlier findings. In contrast, for carrier frequencies above fe, AM-detection thresholds tended to be higher for ears with dead regions than for ears without dead regions. Conclusions: The reason why AM detection was poorer in the ears with dead regions for carrier frequencies above fe is unclear. However, this finding is consistent with the generally poor discrimination of sounds that has been reported previously for sounds with frequency components falling within a dead region. The results have implications for the ability of people with dead regions to use information from frequency components falling inside the dead region. Key Words: Amplitude modulation detection, cortical plasticity, dead region Abbreviations: 2AFC 5 two-alternative forced choice; AM 5 amplitude modulation; ERB N 5 equivalent rectangular bandwidth of the auditory filter averaged across young listeners with normal hearing; fe 5 edge frequency of dead region; IHC 5 inner hair cell; m 5 modulation index; TEN 5 thresholdequalizing noise; TEN(HL) 5 threshold-equalizing noise, calibrated in hearing level Adead region is a region in the cochlea where the inner hair cells (IHCs) and/or neurons are functioning very poorly, if at all (Moore, 2001, 2004). Dead regions are known to occur in humans and maybecausedbyavarietyoffactors,includingexposure to impact sounds and infections (Schuknecht, 1993; Halpin et al, 1994; Borg et al, 1995). Little or no information is transmitted to the brain about basilar membrane vibration in a dead region. However, a tone or other narrowband signal producing peak vibration in that region may be detected by off-place (off-frequency) listening. For example, if there is a dead region at the base of the cochlea, which normally responds to high frequencies, a high-frequency tone may be detected, if it is sufficiently intense, via the spread of basilar membrane vibration to a region tuned to lower frequencies. This is the basis for psychophysical tests for detecting a dead region, such as psychophysical tuning curves (Thornton and Abbas, 1980; Florentine and Houtsma, 1983; Moore and Alcántara, 2001; Kluk and Moore, 2005, 2006b; Sek et al, 2005) and the threshold-equalizing noise (TEN) test (Moore et al, 2000; Moore et al, 2004). The TEN test involves measuring the threshold for detecting a pure tone presented in a background noise *Department of Experimental Psychology, University of Cambridge; {All India Institute of Speech and Hearing, Manasagangothri, Mysore Brian C.J. Moore, Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge, CB2 3EB, England; bcjm@cam.ac.uk; Phone: ; Fax:

2 Journal of the American Academy of Audiology/Volume 20, Number 10, 2009 called threshold-equalizing noise. The noise spectrum is shaped in such a way that, for people with normal hearing, the threshold for detecting a pure tone in the noise is almost constant over a wide range of tone frequencies. The masked threshold is approximately equal to the nominal level of the noise, specified as the level in a 132 Hz wide band centered at 1000 Hz. The value of 132 Hz corresponds to the equivalent rectangular bandwidth of the auditory filter, as determined using young, normally hearing listeners, which is denoted ERB N (Moore, 2003). When the pure-tone signal frequency falls in a dead region, the signal will only be detected when it produces sufficient basilar membrane vibration at a remote region in the cochlea where there are surviving IHCs and neurons. The amount of vibration at this remote region will be less than in the dead region, and so the noise will be very effective in masking it. Thus, the signal threshold is expected to be markedly higher than normal. A masked threshold that is 10 db or more higher than normal is taken as indicating a dead region (Moore et al, 2000; Moore et al, 2004). The extent of a dead region is defined in terms of its edge frequency (or frequencies), fe, which corresponds to the characteristic frequency of the IHCs and/or neurons immediately adjacent to the dead region (Moore et al, 2000). A dead region can be patchy, occurring over one or more restricted regions in the cochlea (Moore and Alcántara, 2001). Here, the term high-frequency dead region is used to refer to a dead region that starts at fe and extends upward from there. Studies using animals have shown that damage to the basal region of the cochlea, effectively producing a highfrequency dead region, can lead to the region of the auditory cortex that previously responded to the damaged region of the cochlea becoming tuned to frequencies corresponding to the adjacent relatively undamaged region (Robertson and Irvine, 1989). The possible functional consequences of this have been explored in several studies; for reviews, see Thai-Van and colleagues (2007) and Moore and Vinay (2009). It has been shown that, for people with high-frequency dead regions, frequency discrimination is enhanced for frequencies that fall just below fe (McDermott et al, 1998; Thai-Van et al, 2003; Kluk and Moore, 2006a; Moore and Vinay, 2009). This has generally been interpreted in terms of cortical plasticity; the expanded cortical representation of frequencies just below fe is assumed to be responsible for the enhanced ability to discriminate those frequencies (McDermott et al, 1998; Thai-Van et al, 2007). Recently, we (Moore and Vinay, 2009) showed that the ability to detect amplitude modulation (AM) was also affected by the presence of a dead region. We tested two groups of subjects, with and without acquired highfrequency dead regions, as assessed using the TEN test calibrated in hearing level, or TEN(HL) (Moore et al, 2004). All subjects in both groups had high-frequency bilateral hearing loss. For the ears with dead regions, the value of fe was close to 1000 or 1500 Hz. The two groups were matched in terms of audiometric thresholds for frequencies below fe and in terms of age. Three subjects with unilateral dead regions (with matched low-frequency audiometric thresholds across ears) were also tested. Thresholds for detecting sinusoidal AM of a sinusoidal carrier were measured. Carrier frequencies of 500 and 800 Hz were used with all subjects, and an additional carrier frequency of 1200 Hz was used for ears with fe close to 1500 Hz and their matched counterparts. The carrier level was 20 db SL. Modulation frequencies were 4, 50, and 100 Hz. AM-detection thresholds were generally lower (better) for the ears with dead regions than for the ears without dead regions, and this effect was statistically significant. For the subjects with unilateral dead regions, thresholds were lower for the ears with dead regions than for the ears without dead regions. The ears with dead regions showed better performance than the ears without dead regions even for a carrier frequency (500 Hz) that was well below fe, and this effect was roughly independent of modulation frequency. One problem in interpreting the improved AMdetection thresholds is that cochlear hearing loss and the associated loudness recruitment can lead to improved AM detection at low SLs; for a review, see Moore (2007). Indeed, this is the basis for the short-increment sensitivity index test (Jerger, 1962; Buus et al, 1982a, 1982b). We have argued that loudness recruitment was unlikely to have influenced the difference between results for the ears with and without dead regions, since the ears were matched in terms of low-frequency audiometric thresholds, and the amount of loudness recruitment caused by cochlear hearing loss is closely related to the amount of hearing loss (Miskolczy-Fodor, 1960; Moore et al, 1999; Moore and Glasberg, 2004). Also, some of the subjects that we tested had normal hearing for frequencies of 1000 Hz and below, but, for those subjects, AM-detection thresholds were lower for the ear with a dead region than for the ear without a dead region. However, it is possible that cortical reorganization might itself lead to a form of loudness recruitment. The cortical overrepresentation of the low-frequency region of the cochlea that occurs following a high-frequency dead region might lead to a more-rapid-than-normal growth of loudness with increasing sound level, and that in itself might lead to improved AM detection. We (Moore and Vinay, 2009) did not assess AM detection for carrier frequencies that fell within a dead region, that is, fell above fe. There is evidence that loudness recruitment is very marked for tones that fall within a dead region (McDermott et al, 1998; Thai-Van et al, 2003). If the enhanced AM detection we found for ears with dead regions was a consequence of greater loudness recruitment in the ears with dead regions, then 598

3 Amplitude Modulation Detection/Moore and Vinay one would expect enhanced AM detection for tones falling above fe, as well as for tones falling below fe. Another consideration is that tones with frequencies above fe are detected via off-frequency or off-place listening; they are detected at a place in the cochlea tuned to frequencies just below fe. Since those frequencies are expected to be overrepresented in the cortex, one might again expect enhanced AM detection for carriers that fall above fe. On the other hand, there is evidence that pure tones with frequencies above fe are often heard as sounding noise-like (Huss and Moore, 2005a) and do not have a clear pitch (Huss and Moore, 2005b). Reports of a noise-like sensation for an unmodulated tone might indicate that the tone sounds as if it is fluctuating to some extent. This might make AM detection more difficult (Stone et al, 2008b), so AM detection might be worse than for an ear without a dead region. In the present study, we assessed how the presence of a dead region affects AM detection by comparing results for the two ears of subjects with bilateral hearing loss but unilateral high-frequency dead regions, including carrier frequencies both below and above fe. The results for frequencies above fe are of clinical relevance since the processing of AM is thought to be important for speech intelligibility (Drullman et al, 1994a, 1994b; Shannon et al, 1995). If the ability to detect AM is poorer for frequencies above than below fe, this would imply a poorer ability to process AM for frequencies above fe, which might be associated with a reduced ability to process AM in speech. METHOD Subject Selection and Audiometric Assessment The subjects were selected from a large pool of potential subjects who had been tested using the TEN(HL) test (see below for details). Subjects generally attended the clinic for the purpose of being fitted with a hearing aid. This usually happened after an audiologic evaluation, following clearance by a medically qualified ear specialist. Ten subjects were chosen who had sensorineural loss in both ears, with a dead region in one ear and no dead region in the other, as diagnosed using the TEN(HL) test. The purpose of the study was explained to the subjects, and their consent was obtained for participation in the study. The subjects are denoted S1 to S10. Details of the subjects are given in Table 1. Audiometric air-conduction thresholds were measured using a Madsen OB922 dual-channel clinical audiometer with Telephonics TDH39 headphones, for frequencies from 0.25 to 8 khz. Bone-conduction thresholds were obtained for frequencies from 0.25 to 4 khz using a Radio Ear B71 bone vibrator. In both cases, the modified Hughson-Westlake procedure described by Carhart and Jerger (1959) was used. All subjects had normal middle ear function, as assessed using a Grason- Stadler Tympstar Immittance meter and indicated by the lack of air bone gaps greater than 10 db. Transient evoked otoacoustic emissions were assessed using click stimuli and an Otodynamics ILO 292 analyzer and were found to be absent for all subjects. The ages of the subjects ranged from 37 to 78 years. There were seven males and three females. All subjects had postlingually acquired hearing loss, and all had a hearing loss for five years or more at the time of testing. None of the subjects had nonauditory neural conditions, and none had any history of ear discharge or a feeling of fullness in the ear. Each ear of each subject was considered separately; the right ear is denoted RE, and the left ear is denoted LE. The diagnosis of dead regions was based on the results of the TEN(HL) test (Moore et al, 2004); see below for details. S1RE, S2LE, S3RE, S4LE, S5LE, S6LE, S7RE, S8RE, S9RE, and S10RE were diagnosed to have dead regions. Generally, absolute thresholds were similar for the two ears of each subject for audiometric frequencies below fe, but audiometric thresholds tended to be higher for the ears with dead regions for frequencies at and above fe. Exceptions were S3 and S9, whose audiometric thresholds were very similar for the two ears over the whole range of audiometric frequencies. For each subject, all testing was done in a single session, with breaks as required. The typical duration of a session was two hours, but this varied across subjects. Application of Threshold-Equalizing Noise Test After the initial diagnosis of sensorineural hearing loss, the TEN(HL) test was conducted using the same audiometer. The TEN(HL) test compact disk (Moore et al, 2004) was replayed via a Philips 729 K CD player connected to the audiometer. Test frequencies were 0.5, 0.75, 1, 1.5, 2, 3, and 4 khz. The level of the signal and the TEN(HL) were controlled using the attenuators in the audiometer. The TEN(HL) level was 70 db HL/ERB N. The signal level was varied in 2 db steps to determine the masked thresholds, as recommended by Moore and colleagues (2004). A no response (NR) was recorded when the subject did not indicate hearing the signal at the maximum output level of the audiometer, which was 100 db HL for the signals derived from the TEN(HL) CD. NR implies that the threshold was above 100 db HL. The diagnosis of the presence or absence of a dead region at a specific frequency was based on the criteria suggested by Moore and colleagues (2004). If the masked threshold in the TEN(HL) was 10 db or more above the TEN(HL) level/erb N (i.e., was 80 db HL or more), and the TEN(HL) elevated the absolute threshold by 10 db or more, then a dead region was assumed to be present at the signal frequency. If the masked threshold in the TEN(HL) was less than 10 db above the TEN(HL) level/erb N, and the TEN(HL) 599

4 Journal of the American Academy of Audiology/Volume 20, Number 10, 2009 Table 1. Audiometric Thresholds (Upper Entries in Each Row) and Thresholds in the Threshold-Equalizing Noise, Calibrated in Hearing Level, Test (Lower Entries in Each Row) for Each Subject Frequency (Hz) Subject/Ear Age Gender S1RE 76 M * NR NR S1LE S2RE 70 M S2LE * NR NR S3RE 66 M * NR NR NR NR S3LE S4RE 78 M S4LE * NR NR S5RE 77 M S5LE * NR NR S6RE 37 F S6LE * NR NR S7RE 76 M * NR NR S7LE S8RE 57 F * 70* NR S8LE S9RE 73 F * NR NR S9LE S10RE 64 M * NR NR S10LE Note: Bold numbers indicate frequencies falling in a dead region; * indicates the edge frequency, fe; NR means no response. elevated the absolute threshold by 10 db or more, then a dead region was assumed to be absent. It should be noted that the true value of fe lies between the lowest frequency at which the TEN(HL) test criteria were met and the next lowest test frequency. Here, for simplicity, the value of fe was taken as the lowest frequency at which the TEN(HL) test criteria for the presence of a dead region were met. Measurement of Absolute Thresholds In addition to the measurement of audiometric thresholds as described above, absolute thresholds for pure-tone signals were measured with higher accuracy in db SPL using an adaptive two-alternative forcedchoice (2AFC) method. These absolute thresholds were used to set the SLs of the stimuli used in the measurement of AM detection. Stimuli were digitally generated using a personal computer equipped with a SoundMAX sound card. The sampling frequency was 25 khz. The output of the sound card was fed to Sennheiser HD265 earphones. These are closed -type earphones that give good isolation between the two ears. They are diffuse-field equalized. The signal level was started above the threshold level as estimated from the audiogram. The signal could occur either in the first 600

5 Amplitude Modulation Detection/Moore and Vinay interval or in the second interval, selected at random. The signal lasted 200 msec, including 20 msec raisedcosine rise/fall times, and the intervals were separated by 500 msec. The intervals were indicated by virtual buttons on the computer screen (labeled 1 and 2), each of which was lit up in blue during the appropriate interval. The subject responded by clicking on the appropriate button with a computer mouse or by pressing button 1 or 2 on the computer numeric keypad. Feedback was provided by flashing the button in green for a correct answer and red for an incorrect answer. A two-down, one-up procedure was used (Levitt, 1971). Six turnpoints were obtained. The step size was initially 6 db. It was changed to 4 db after one turnpoint and to 2 db after the second turnpoint. The threshold was taken as the mean signal level at the last four turnpoints. Measurement of AM-Detection Thresholds Thresholds for detecting AM were determined using an adaptive 2AFC method, similar to that described for the measurement of absolute thresholds, and using the same equipment. One interval contained an unmodulated carrier, and the other interval contained a carrier that was amplitude modulated with modulation index m.the order of the two was random. The task of the subject was to indicate the interval in which the carrier was modulated. The equation describing the modulated waveform as a function of time, W(t), is W(t) 5 {1 + msin(2pf mod t + Q)}sin(2pf carr t), where f mod is the modulation frequency and f carr is the carrier frequency. A value of m 5 1means 100 percent modulation, while a value of m 5 0 means no modulation. The starting phase of the modulation, Q, was random, and the AM was present throughout the whole stimulus. The total power was equated across the two intervals. The duration of each carrier was 1000 msec, including 20 msec raised-cosine rise/fall times. The value of m was adjusted using a two-down, one-up adaptive procedure. The initial value was chosen to make the modulation clearly audible. The value of m was adjusted by a factor of until two turnpoints had occurred. Then m was adjusted by a factor of until two more turnpoints had occurred. Finally, m was adjusted by a factor of 1.25 until eight further turnpoints had occurred. The threshold was taken as the geometric mean of the values of m at the last eight turnpoints. Four carrier frequencies were used for each subject. They were chosen so that, for the ear with a dead region, two of the carrier frequencies fell below fe and two fell above fe. The carrier frequencies were 500, 750, 2000, and 4000 Hz for ears with fe Hz; 500, 1000, 2000, and 4000 Hz for ears with fe Hz; and 500, 1000, 3000, and 4000 Hz for ears with fe Hz. For each subject, the same carrier frequencies were used for the ears with and without dead regions. The modulation frequencies were 4, 16, and 50 Hz. These were chosen to span the range of modulation frequencies that are thought to be most important for speech perception (Shannon et al, 1995; Stone et al, 2008a) while being low enough to ensure that spectral sidebands produced by the modulation were not resolved (Kohlrausch et al, 2000; Moore and Glasberg, 2001). All subjects except one were tested using carrier levels of 5, 10, and 15 db SL. The exception was S5, who found the stimuli to be too loud in both ears for the 2000 Hz carrier frequency at 15 db SL and who was tested using levels of 5 and 10 db SL only for that carrier frequency. The order of testing the ear, SL, carrier, and modulation frequencies was randomized for each subject. At least two threshold estimates were obtained for each condition; where time permitted, an additional estimate was obtained. The final threshold for a given condition was taken as the mean across all estimates. In what follows, thresholds are expressed as 20log 10 (m), that is, in decibel-like units. For the maximum value of m (5 1), the value of 20log 10 (m) is 0. The smaller the value of m, the more negative 20log 10 (m) is. Thus, more negative numbers indicate better performance. RESULTS The pattern of results was similar across subjects. To simplify presentation, AM-detection thresholds were averaged across subjects with the same value of fe. Figure 1 shows mean AM-detection thresholds for S5 and S8, who both had fe Hz in one ear. Figure 2 shows mean thresholds for S2, S3, and S9, who all had fe Hz in one ear. Figure 3 shows mean thresholds for S1, S4, S6, S7, and S10, who all had fe Hz in one ear. In each figure, data for the ears with no dead region are plotted on the left, and data for the ears with dead regions are plotted on the right. For the ears with no dead region, mean thresholds did not differ systematically for carrier frequencies below the value of fe for the other ear (filled symbols) and above that value (open symbols). In contrast, for ears with dead regions, the mean thresholds were consistently lower for carriers below fe than for carriers above fe. Mean thresholds tended to improve (decrease) with increasing SL, although this effect was most apparent for carrier frequencies below fe presented to the ears with dead regions. For carriers below fe,mean thresholds were consistently lower for the ears with dead regions than for the ears without dead regions. This replicates our 2009 results. For carriers above fe, mean thresholds tended to be higher for the ears with dead regions than for the ears without dead regions. The use of different carrier frequencies across subjects made statistical analysis slightly awkward. However, results were generally similar for the two lower carrier frequencies (below fe) and the two higher carrier frequencies (above fe) for each subject. Therefore, the data were analyzed in the following way: 601

6 Journal of the American Academy of Audiology/Volume 20, Number 10, 2009 Figure 1. Mean amplitude modulation detection thresholds, expressed as 20log 10 (m), for two subjects with a dead region in one ear with fe Hz. Data for ears with no dead regions are shown on the left, and data for ears with dead regions are shown on the right. The carrier sensation level was 5 db (top), 10 db (middle), or 15 db (bottom). Each symbol represents a different carrier frequency, as indicated in the key. Filled symbols indicate carrier frequencies below fe, and open symbols indicate carrier frequencies above fe. Error bars show 6 one standard deviation across subjects. Error bars are omitted when they would be smaller than the symbol used to represent the mean. 1. For each ear of each subject, and for each SL and each modulation frequency, the AM-detection thresholds, expressed as 20log 10 (m), were averaged for the two lower carrier frequencies and the two upper carrier frequencies. The resulting values are denoted Av(low) and Av(high). Since S5 was not tested using the carrier frequency of 2000 Hz at the SL of 15 db, the value of Av(high) at 15 db SL was based on the AM-detection threshold for the 4000 Hz carrier only. 2. A repeated-measures analysis of variance was conducted on the values of Av(low) and Av(high), with the following factors: ear (with dead region or without dead region), carrier frequency region (below or above fe), modulation frequency (4, 16, or 50 Hz), and SL (5, 10, or 15 db). The Greenhouse-Geisser correction was used where appropriate. The effect of ear was significant: F(1, 9) , p The mean threshold was slightly lower for ears 602

7 Amplitude Modulation Detection/Moore and Vinay Figure 2. Mean amplitude modulation detection thresholds, expressed as 20log 10 (m), for three subjects with a dead region in one ear with fe Hz. Data for ears with no dead regions are shown on the left, and data for ears with dead regions are shown on the right. The carrier sensation level was 5 db (top), 10 db (middle), or 15 db (bottom). Each symbol represents a different carrier frequency, as indicated in the key. Filled symbols indicate carrier frequencies below fe, and open symbols indicate carrier frequencies above fe. Error bars show 6 one standard deviation across subjects. Error bars are omitted when they would be smaller than the symbol used to represent the mean. with a dead region (216.3 db) than for ears without a dead region (215.1 db). The effect of carrier frequency region was significant: F(1, 9) , p,.001. The mean threshold was lower for carriers below fe (217.1 db) than for carriers above fe (214.3 db). The effect of modulation frequency was not significant. The effect of SL was significant: F(2, 18) , p,.001. The mean thresholds were 214.1, 216.2, and db for SLs of 5, 10, and 15 db, respectively. There was a significant interaction between ear and carrier frequency: F(1, 9) , p,.001. Mean AMdetection thresholds were lower for the ears with dead regions (219.1 db) than for the ears without dead regions (215.1 db) for the carrier frequency region below fe. A post hoc test (Tukey HSD) showed that this difference was significant (p,.001). In contrast, for the higher carrier frequency region, the mean threshold was higher for the ears with dead regions (213.2 db) than for the ears without dead regions (215.1 db). A post hoc test showed that this difference was significant (p,.01). Other interactions either were not significant or accounted for only a small proportion of the variance in the data. In summary, the results showed that, for carrier frequencies below fe, AM-detection thresholds were lower for ears with dead regions than for ears without 603

8 Journal of the American Academy of Audiology/Volume 20, Number 10, 2009 Figure 3. Mean amplitude modulation detection thresholds, expressed as 20log 10 (m), for five subjects with a dead region in one ear with fe Hz. Data for ears with no dead regions are shown on the left, and data for ears with dead regions are shown on the right. The carrier sensation level was 5 db (top), 10 db (middle), or 15 db (bottom). Each symbol represents a different carrier frequency, as indicated in the key. Filled symbols indicate carrier frequencies below fe, and open symbols indicate carrier frequencies above fe. Error bars show 6 one standard deviation across subjects. Error bars are omitted when they would be smaller than the symbol used to represent the mean. dead regions. In contrast, for carrier frequencies above fe, AM-detection thresholds tended to be higher for ears with dead regions than for ears without dead regions. DISCUSSION The results indicate that AM-detection thresholds obtained using sinusoidal carriers did not vary markedly with AM frequency for frequencies up to 50 Hz. This is broadly consistent with previous results for normally hearing subjects (Kohlrausch et al, 2000) and hearing-impaired subjects (Moore and Glasberg, 2001), showing that AM-detection thresholds are roughly independent of AM frequency for frequencies up to about 100 Hz, provided that the spectral sidebands produced by the AM are not resolved in the auditory system. However, it has sometimes been reported that AMdetection thresholds are lower for modulation frequencies 604

9 Amplitude Modulation Detection/Moore and Vinay around 10 Hz than for frequencies around 80 Hz (Füllgrabe et al, 2003; Füllgrabe et al, 2005). The differences across studies may be related to the carrier duration, which was greater in the studies of Füllgrabe and colleagues than in our study or the other studies cited above. For normally hearing subjects, AM-detection thresholds, expressed as 20log 10 (m), are typically about 215 db for carriers at low SLs (Kohlrausch et al, 2000; Moore and Glasberg, 2001; Füllgrabe et al, 2005), although few data are available for SLs as low as those used here. The typical threshold of 215 db is comparable to the AM-detection thresholds obtained here for ears without dead regions. For the ears with dead regions, AMdetection thresholds appear to be somewhat better than normal for carrier frequencies below fe but are comparable to or slightly worse than normal for carrier frequencies above fe. The finding that AM detection for carrier frequencies below fe was better for ears with dead regions than for ears without dead regions is consistent with the results of our 2009 study. The present results show that this occurred even when the carrier frequency was as much as two octaves below fe (a carrier of 500 Hz with fe Hz). The present results also show that, for carrier frequencies above fe, AM detection tended to be worse for ears with than without dead regions. There may be several reasons for the worse performance of ears with dead regions for carrier frequencies above fe. It could be connected with the fact that, for frequencies above fe, absolute thresholds were usually higher for the ears with than without dead regions (see Table 1). However, the absolute thresholds of S3 and S9 were similar across ears, even for frequencies above fe, and the pattern of their results is very similar to that for the other subjects. Therefore, differences in absolute threshold across ears do not seem to be critical in accounting for the observed results. Another possible factor accounting for the worse AM detection of ears with dead regions for frequencies above fe is the noise-like character of the percept that is usually evoked by tones with such frequencies in ears with dead regions (Huss and Moore, 2005a, 2005b). This noise-like percept may indicate that even an unmodulated carrier appears to fluctuate, which would make it harder to detect AM imposed on a carrier. The noise-like percept may result partly from a mismatch between place information and the temporal information carried in the patterns of phase locking in the auditory nerve. Such a mismatch has been proposed previously to account for the relatively poor frequency discrimination of tones falling in a dead region (Huss and Moore, 2005a, 2005b; Moore and Carlyon, 2005). The noise-like percept may also occur because a modulated or unmodulated carrier falling in a dead region is detected via the IHCs and/or neurons immediately adjacent to the dead region, and these may not be functioning in a normal way; rather, there may be a poorly functioning region just below the dead region (Huss and Moore, 2003). Consistent with this view, for the ears with dead regions, the absolute threshold for the audiometric frequency just below fe was mostly in the range of 60 to 75 db, indicating hearing losses that were probably too large to be explained by deficits in outer hair cell function alone (Robles and Ruggero, 2001; Moore, 2007). Whatever the reasons, the present results show that, for an ear with a dead region, AM detection for sounds with frequencies above fe is relatively poor. This may be important because, as noted earlier, it is thought that the perception of speech depends strongly on the processing of patterns of AM in different frequency bands (Drullman et al, 1994a, 1994b; Shannon et al, 1995). Indeed, since people with cochlear hearing loss generally have reduced frequency selectivity (Glasberg and Moore, 1986; Moore, 2007) and reduced sensitivity to temporal fine structure in sounds (Hopkins and Moore, 2007; Hopkins et al, 2008; Moore, 2008), they may be especially dependent on the processing of temporal envelope cues in order to understand speech. Our results suggest that the processing of such cues is likely to be less efficient for frequency components above fe than for components below fe. This may partly explain the limited benefit for speech intelligibility of amplifying frequency components that fall well above fe (Vickers et al, 2001; Baer et al, 2002; Preminger et al, 2005). Acknowledgments. We thank the director of the All India Institute of Speech and Hearing for providing the facilities to carry out this work. The work of author BCJM was supported by the Medical Research Council (U.K.). We thank Aleksander Sek for writing the software for conducting the amplitude modulation detection task, Brian Glasberg for help with statistical analyses, and Christian Füllgrabe for helpful comments on an earlier version of this article. We also thank three anonymous reviewers for helpful comments on an earlier version of this article. REFERENCES Baer T, Moore BCJ, Kluk K. (2002) Effects of lowpass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies. J Acoust Soc Am 112: Borg E, Canlon B, Engström B. (1995) Noise-induced hearing loss Literature review and experiments in rabbits. 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