Interactive Effects of Low-Pass Filtering and Masking Noise on Word Recognition

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1 J Am Acad Audiol 12 : (21) Interactive Effects of Low-Pass Filtering and Masking Noise on Word Recognition Teri Scott* Walter B. Green* Andrew Stuart' Abstract A word recognition in noise paradigm was employed to examine temporal resolution in individuals with simulated hearing loss. Word recognition scores were obtained for low-pass filtered speech (i.e., cutoff frequencies of 1, 125, and 15 Hz) presented in continuous and interrupted noise at signal-to-noise ratios (s) of-1,, and 1 db. Performance improved with increasing and low-pass frequency filter settings. Generally, word recognition performance was better in the interrupted noise condition than the continuous noise condition. This effect was greatest in the -1 db condition. Since the continuous/interrupted performance difference steadily declined as a function of low-pass filter cutoff frequency, these findings suggest that one factor leading to poorer speech recognition in individuals with high-frequency hearing impairment may be their dependence on low-frequency hearing channels that are inherently poorer than high-frequency channels for temporal resolution. Key Words : Hearing loss, temporal resolution, word recognition Abbreviations : ANOVA = analysis of variance, MS = multiple sclerosis, NU-6 = Northwestern University Auditory Test No. 6, = signal-to-noise ratio t has been established that individuals with hearing loss demonstrate greater difficulty than individuals with normal hearing in understanding speech in background noise (Dubno et al, 1984 ; Pekkarinen et al, 199 ; Payton et al, 1994). Environmental noise, including speech, is characterized by envelope variations over time. Consequently, the ability of the auditory system to separate auditory stimuli over time has been of interest. This is known as temporal resolution (Moore, 1989 ; Rappaport et al, 1994). The importance of examining this phenomenon is evidenced through the deficiencies demonstrated by hearing-impaired listeners compared with normal-hearing listeners on tasks requiring temporal processing. This has been exhibited through poorer speech understanding in modulated (Gustafsson and Arlinger, *School of Human Communication Disorders, Dalhousie University. Halifax, Nova Scotia, tdepartment of Communication Sciences and Disorders, East Carolina University, Greenville, North Carolina Reprint requests : Teri Scott, Audiology Department, Health Science Center, 3 Prince Philip Dr., St. John's, NF A113 3V6 1994) and fluctuating backgrounds (Festen and Plomp, 199 ; Bacon et al, 1994). It has been suggested that the inferior performance of hearing-impaired listeners in conditions requiring temporal resolving abilities is a result of the functional loss of the high-frequency region of the cochlea because the population of high-frequency cochlear output channels has the best temporal resolving abilities (Florentine and Buus, 1984 ; Bacon and Viemeister, 1985 ; Moore et al, 1989 ; Shailer and Moore, 1987 ; Stuart et al, 1995). To explore the above speculation, some resarchers have conducted a series of experiments employing a word recognition in noise paradigm (Phillips et al, 1994 ; Rappaport et al, 1994 ; Stuart et al, 1995 ; Stuart & Phillips, 1996, 1997 ; Stuart and Carpenter, 1999). This paradigm was employed to evaluate temporal processing in different hearing-impaired and normal-hearing populations. The paradigm requires that listeners identify words presented in spectrally identical competing backgrounds of continuous and interrupted noise as the signalto-noise ratio () is varied. The interrupted noise is composed of bursts of noise and silent intervals varying randomly in length from 5 to 437

2 Journal of the American Academy of Audiology/Volume 12, Number 9, October msec (Stuart and Phillips, 1998). Comparisons were made between word recognition performance in both the continuous and interrupted noise conditions. Generally, performance is superior for normal-hearing listeners versus hearingimpaired listeners, in quiet versus both noise conditions, and in the interrupted versus the continuous noise condition. These studies have been consistent with other studies examining the effects of continuous noise: performance is inferior in noise conditions relative to quiet (Givens and Jacobs-Condit, 1981 ; Pekkarinen et al, 199 ; Wilson et al, 199), and individuals with sensorineural hearing loss show greater deficits in noise than individuals with normal hearing (Dubno et al, 1984 ; Pekkarinen et al, 199 ; Needleman and Crandell, 1995). This has been attributed to the attenuation and distortion of the speech signal for those with hearing loss (Dubno et al, 1984 ; Needleman and Crandell, 1995). Superior performance in the interrupted noise condition was attributed to the ability of the listener's auditory system to resolve speech fragments in the silent gaps between noise bursts since both noises have the same spectral qualities and differed only in their temporal continuity. Phillips et al (1994) employed the above paradigm to analyze word recognition performance in people with normal hearing and with noise-induced hearing loss. No significant differences existed between the performance of listeners with normal hearing and listeners with noise-induced hearing loss in the continuous noise condition. In the interrupted noise condition, however, the participants with hearing loss displayed significantly lower word recognition performance than normal listeners. It was suggested that those with a high-frequency hearing loss are restricted to low-frequency auditory bandwidths with inherently poor temporal resolution. Furthermore, because the noiseinduced hearing loss group was impaired only in the interrupted noise group, temporal processing problems could not be attributed to decreased signal audibility alone (Stuart and Phillips, 1998). A selective performance deficit in the interrupted relative to the continuous noise condition was reported by Rappaport et al (1994) for patients with multiple sclerosis (MS) who had confirmed demyelinating lesions in the central auditory system (i.e., rostral auditory fiber tracts or the auditory brain stem). That is, patients with MS and normal-hearing listeners exhibited equivalent performance in the continuous noise condition, whereas patients with MS demonstrated reduced performance in the interrupted noise condition compared with normal-hearing subjects. Since the patients with MS displayed normal peripheral auditory function, the authors suggested that deficits in temporal resolution might be the result of damage to the central auditory system, independent of the peripheral system. These findings, coupled with those of Phillips et al (1994), implied that the word recognition in the interrupted and continuous noise paradigms could reveal temporal resolution deficiencies that are both peripheral and central in nature (Stuart and Phillips, 1998). An additional study (Stuart and Phillips, 1996) investigated temporal resolution in listeners with presbyacusis. It was reported that listeners with presbyacusis had reduced performance in interrupted noise compared with older listeners with no hearing loss. Stuart and Phillips attributed this to the dependence on low-frequency listening bandwidths for individuals with presbyacusis. However, there was also a difference between young and old normalhearing subjects, with young normal-hearing participants performing better in the interrupted noise condition compared with their older counterparts. These experimenters concluded that the discrepancy between younger and older normal-hearing listeners demonstrated an agerelated deficit, possibly at a central level, with older participants. With the purpose of confirming the findings of Phillips et al (1994), Stuart et al (1995) examined the performance for people with a simulated high-frequency hearing loss (i.e., lowpass filtered at 2 Hz with a roll-off slope of 48 db/octave). Stuart et al speculated that inasmuch as sensorineural hearing loss is thought to limit the individual to low-frequency hearing channels with inherently poor temporal resolution, a simulated hearing loss should yield parallel results. That is, those with a simulated high-frequency hearing loss should show detriments in performance in the interrupted noise relative to normal listeners. The findings of Stuart et al were in agreement with those of Phillips et al. That is, normal-hearing listeners performed equivalently with and without a simulated high-frequency impairment in the continuous noise conditions. However, when normal-hearing listeners were subjected to the simulated hearing loss, they performed significantly worse in the interrupted noise condition. This suggested that a similar mechanism was responsible for poor temporal resolution per- 438

3 Effect of Low-Pass Filtering on Word Recognition/Scott et al formance in both cases. Despite the corresponding findings, differences between those with normal hearing and noise-induced hearing loss were larger than differences between those with normal hearing and simulated hearing loss. The present study replicated the experiment conducted by Stuart et al (1995) using a series of different low-pass cutoff frequencies for the filtered speech to examine temporal resolving abilities with reduced listening bandwidths in the interrupted noise paradigm. Lowpass filtering (as opposed to noise masking) was employed to simulate hearing impairment for two reasons. First, it has been demonstrated that employing noise-masked normal listeners with elevated thresholds via masking as a model of sensorineural hearing loss does not explain the performance from hearing-impaired listeners on various measures of temporal resolution (Humes et al, 1988). Second, physiologic data suggest that noise masking does not imitate peripheral auditory function in the impaired cochlea (Phillips and Hall, 1986 ; Phillips, 1987). When Stuart (1995) examined simulated hearing loss with a cutoff of 4 Hz, no significant differences were observed between the filtered and the unfiltered conditions. It was suggested that this occurred because little information contributing to the reception of monosyllabic words is available above 4 Hz (French and Steinberg, 1947 ; Pollack, 1948). Thus, the present investigation used low-pass cutoffs of 15, 125, and 1 Hz, resulting in further reduction in available listening bandwidths. It was hypothesized that as filter cutoff was reduced, performance on word recognition would decrease, word recognition scores would decrease for poorer s, and performance in the interrupted noise condition would be superior to that in the continuous noise condition. Participants METHOD Twenty young adults served as participants (mean = 23.4 years, SD = 3.8; 3 males and 17 females). All participants presented with normal middle ear function (American Speech- Language-Hearing Association, 199) and normal hearing sensitivity defined as having pure-tone thresholds at octave frequencies from 25 to 8 Hz and speech recognition thresholds of <- 25 db HL (American National Standards Institute, 1996). Apparatus The test stimuli consisted of custom twochannel stereo compact disc female talker recordings of 5 monosyllabic words of lists 1 to 4 of the Northwestern University Auditory Test No. 6 (NU-6; Tillman and Carhart, 1966). In an effort to reduce test time, the word lists were edited to remove the carrier phrase and to reduce the interstimulus intervals from 4.2 to 3. sec. The competing masking stimuli consisted of a continuous and an interrupted broadband noise (i.e., varying randomly from 5 to 95 msec). Stuart and Phillips (1996, 1998) provided a complete description of the acoustic characteristics of the interrupted noise. Testing took place in a sound-treated audiometric booth (Industrial Acoustics Company, Inc., Bronx, New York) meeting specifications for permissible ambient noise levels (American National Standards Institute, 1999). The recorded stimuli were routed from a compact disc player to a clinical audiometer (Grason-Stadler Model GSI 61) and delivered to participants through an ER-3A insert earphone. During the simulated hearing loss condition, the stimuli were routed through a passive analog filter (Krohn-Hite Model 3323 filter) before being routed to the audiometer. The word stimuli were low-pass filtered at 15, 12, and 1 Hz with a roll-off slope of 48 db per octave. Procedure Testing took place over several sessions. During each session, the participant listened to NU-6 monosyllabic word lists. The word lists were presented in three conditions : quiet, continuous broadband noise, and interrupted broadband noise, with s of 1,, and -1 db. NU-6 stimuli were presented at 4 db above the participants' speech recognition thresholds. The sequence of test conditions was counterbalanced across participants using a digram-balanced Latin square (Wagenaar, 1969). Moreover, the order of the monosyllabic word lists was randomly assigned. All stimuli were presented monaurally to the right ear. RESULTS ord recognition performance was scored as W the percentage of correct whole words. The mean word recognition scores in quiet were 78.8 percent (SD = 7.9), 66.6 percent (SD = 9.56), and 53.6 percent (SD = 1.61) for filter settings 439

4 Journal of the American Academy of Audiology/Volume 12, Number 9, October 21 Table 1 Mean Percent Correct Word Recognition Scores and SDs as a Function of Noise,, and Low-Pass Cutoff Frequency. Continuous Noise Interrupted Noise (db) (SD) (db) (SD) Low-Pass Filter Cutoff Frequency (Hz) (7.9) 43.9 (9.7) 62.8 (11.9) 33.2 (11.1) 46.9 (12.7) 59.8 (9.4) (9.8) 31.5 (11.3) 52.3 (1.3) 25. (9.2) 38.4 (12.7) 46.9 (12.8) (9.1) 28.8(1.1) 39.8(1.1) 2.4 (9.8) 3.9 (9.9) 42. (11.4) of 15, 125, and 1 Hz, respectively. Mean percent correct word recognition scores and standard deviations as a function of noise type,, and low-pass cutoff frequency are found in Table 1. Two analyses of variance (ANOVAs) were undertaken to examine mean percent correct word recognition score differences. Prior to performing inferential statistics, the proportional word recognition scores were transformed into rationalized arcsine units (Studebaker, 1985). A one-way repeated ANOVA was completed to investigate word recognition in quiet as a function of low-pass cutoff frequency. A significant main effect of cutoff frequency was found (F = 46.4, df = 2, 19, Geisser-Greenhouse p <.1). In general, word recognition performance improved with increasing low-pass filter cutoff frequency. A number of single-df comparisons were undertaken to further assess the main effect of low-pass cutoff frequency. It was found that all pair-wise comparisons were statistically significant (Geisser-Greenhouse p <.5). A three-factor repeated ANOVA was undertaken to examine differences in mean word recognition scores as a function of competing noise,, and low-pass cutoff frequency. The results of this analysis are presented in Table 2. As evident in Table 2, all three main effects were statistically significant. In general, word recognition performance was better in the interrupted noise and improved with increasing and increasing low-pass cutoff frequency. In addition, a statistically significant interaction occurred between the noise and the. This interaction is displayed in Figure 1. Post hoc single-df comparisons revealed that all pair-wise comparisons of mean differences in low-pass cutoff frequency were statistically significant (Geisser-Greenhouse p <.5). A number of single-df comparisons were undertaken to examine the noise by interaction. It was found that word recognition performance was significantly better in the interrupted noise than in the continuous noise condition at both -1 and db (Geisser-Greenhouse p <.5), whereas Table 2 Summary for the Three-Factor Repeated ANOVA Examining Differences in Mean Word Recognition Scores Rationalized Arcsine Units as a Function of Noise,, and Low-Pass Cutoff Frequency Source of F p 17 2 Noise < Low-pass cutoff frequency < < Noise X low-pass cutoff frequency Noise X < Low-pass cutoff frequency X Noise X low-pass cutoff frequency X p <.5 ; repeated measures factor p values following a Geisser-Greenhouse correction ; effect size indexed by Q2 ; power indexed by at a of.5. 44

5 Effect of Low-Pass Filtering on Word Recognition/Scott et al U U U ~ V -1~T 1 15 Hz 1, 125 Hz ~ 2 2 n n, Hz o, -1 1 Figure 1 Mean percent correct word recognition scores as a function of competing noise and. Error bars represent ± 1 SD of the means. no difference existed at 1 db (Geisser- Greenhouse p >.5). DISCUSSION T emporal resolution was investigated with a word recognition in noise paradigm, which compared word recognition in quiet, interrupted, and continuous broadband noise. To model the temporal resolution abilities of individuals with high-frequency hearing impairment, a low-pass filtered speech was presented to normal-hearing listeners. Inasmuch as the investigators were interested in whether temporal resolution performance would be degraded with a reduction in available listening bandwidths, three low-pass filter cutoff frequencies were employed (i.e., 15, 125, and 1 Hz). Four findings emerged : as expected, word recognition scores were higher in quiet than in either noise condition, performance increased with increasing, scores were higher as low-pass filter cutoff frequency increased, and interrupted noise resulted in better performance than continuous noise. The results of this investigation are consistent with the findings of Stuart et al (1995). They employed a continuous and interrupted noise paradigm to investigate temporal resolution in simulated hearing loss. These authors found that individuals with normal hearing had better performance than individuals with simulated hearing impairment in the interrupted noise but not in the continuous noise condition. According to Stuart et al, the simulated hearing loss group demonstrated a similar deficit to those with real high-frequency hearing impairment. As such, these authors concluded that a similar mechanism was responsible for the reduced ability to temporally resolve auditory information in both cases. To this end, the simulated hearing loss group continued to have normal cochlear functioning; however, listening was restricted to the low frequencies. Therefore, it was suggested that poor temporal resolving abilities in hearing impairment was caused by listening being limited to lowfrequency channels with inherently inferior temporal resolution. Stuart and Phillips and their colleagues suggested that the differences observed between word recognition in performance in continuous and interrupted noise (viz., superior performance in interrupted noise) reflect the temporal resolving abilities of the auditory system. This difference has been observed in individuals with normal hearing (Phillips et al, 1994 ; Stuart et al, 1995 ; Stuart and Phillips, 1996, 1998). This difference is seen in hearing-impaired listeners as well, although the magnitude of the difference between the continuous and interrupted noise conditions is less than for the size of the difference seen in normal hearers. That is, individuals with noise-induced hearing loss (Phillips et al, 1994), presbyacusis (Stuart and Phillips, 1996), and simulated high-frequency hearing loss (Stuart et al, 1995) do not perform as well as those with normal hearing in the interrupted noise condition. In addition, differences have been noted between individuals with MS who have normal peripheral hearing sensitivity and normal-hearing listeners (Rappaport et al, 1994), as well as between older and young normal hearers (Stuart and Phillips, 1996). Patients with MS and older normal-hearing listeners had reduced performance in the interrupted noise condition compared with their counterparts. In these last two cases, it has been demonstrated that the interrupted noise paradigm may be sensitive to temporal resolving deficits of a central nature. 441

6 Journal of the American Academy of Audiology/Volume 12, Number 9, October 21 Two plausible explanations have been presented for the inferior performance of listeners with hearing loss on temporal resolution tasks (Stuart and Phillips, 1996, 1998). Both concepts consider the bandwidths available to the listener. The first explanation involves the spectral domain, or the issue of audibility. In other words, "if the signal, or part thereof, is inaudible, then some task performances should suffer for this reason alone" (Stuart and Phillips, 1998, p. 338). Several studies argue this point. For instance, Florentine and Buus (1984) examined minimum detectable gaps in hearing-impaired and simulated hearing-impaired participants. This task was completed at different sound pressure levels. These investigators noted that in most of their participants, poor temporal resolution was the result of decreased audibility. Likewise, in their study of temporal resolution, Turner et al (1995) concluded that if audibility is compensated for, individuals with hearing impairment might not show deficits on tasks requiring temporal resolution relative to people with normal hearing. Stated differently, when the presentation level is loud enough to overcome the sensitivity loss, deficits in temporal resolution are typically not observed in people with hearing impairment. Likewise, Stuart and Phillips (1997) noted that audibility plays a role in the interrupted noise paradigm when the sensation level is manipulated. Specifically, a change in performance in normal listeners is observed in the interrupted noise when the sensation level is increased. The results of the present study offer some support for the audibility argument. First, performance in the quiet condition decreased with decreasing filter cutoffs. This demonstrated that the word recognition scores were poorer as the signal became less audible through the removal of the higher-frequency content of the stimuli. These findings are similar to those of Bornstein et al (1994), who examined word recognition of NU-6 words in low-pass and high-pass filter conditions in quiet. For the low-pass condition, these authors noted that performance deteriorated as filter settings were decreased from 17 to 8 Hz. Second, support for the audibility argument is evidenced with the performance in competing noises as that decreased. That is, as the became less favorable, less of the stimuli was audible ; consequently, performance was reduced. This effect was less pronounced in interrupted noise due to its temporal continuity. That is, when the temporal continuity of the masker is interrupted, the effectiveness of the masker is reduced. In other words, listeners experienced a perceptual advantage (i.e., a release from masking). At the most favorable, there was no perceptual advantage in the interrupted noise, relative to the continuous noise, presumably because the audibility of the stimuli was minimally affected by either competing noise. This is consistent with the results of Turner et al (1995), who stated that once the issue of audibility is resolved, problems on temporal resolution tasks are no longer observed. The second method of approaching deficits in temporal resolution in hearing impairment is through the temporal domain (Stuart and Phillips, 1996, 1998). This argument is based on the description of the peripheral auditory system being composed of a series of bandpass filters with continuously overlapping center frequencies (Glasberg et al, 1984 ; Moore, 1989). In such a system, the bandwidths of auditory filters decrease with decreasing frequency. The theory that temporal resolution is poorer in the low frequencies "is based on the assumption that temporal resolution might be limited by the response time of the auditory filters" (Moore, 1989, p. 141). That is, auditory filters "ring" after the offset of signal input, and the duration of the ringing is inversely proportional to the bandwidth of the filter, suggesting that the response times of high-frequency auditory filters have to be the more rapid. Since narrow bandwidths have longer response times, low frequencies with narrow bandwidths should exhibit poorer temporal resolution than the high frequencies with broader bandwidths. In contrast, Stuart and Phillips (1998) argued that the loss of audibility could not explain all circumstances of inferior temporal resolution in hearing impairment. This may be exemplified through their findings with simulated hearing loss (Stuart et al, 1995). Although performance was degraded for simulated hearing loss compared with normal hearers in the interrupted noise condition that required temporal resolving abilities, no significant word recognition performance differences existed between normal hearers and simulated hearing loss listeners in the continuous noise condition. One would think that if the inability to hear the speech were the cause of the degraded performance, deficits would be seen in both the continuous and the interrupted noise conditions. Further, when the data from Stuart et al (1995) are compared with the present study, an interesting pattern is evident. Figure 2 shows the mean percent correct word recognition scores as 442

7 Effect of Low-Pass Filtering on Word Recognition/Scott et al -.- Continuous Noise -o-- Interrupted Noise Figure 2 Mean percent correct word recognition scores as a function of competing noise,, and low-pass cutoff frequency The data in the unfiltered and 2-Hz lowpass cutoff frequency come from Stuart et al (1995). Open circles and filled squares represent performance in interrupted and continuous noise, respectively. a function of competing noise,, and low-pass cutoff frequency from the present study and the data in the unfiltered and 2-Hz low-pass filter from Stuart et al. There are two important observations to be made regarding this figure. First, overall word recognition performance decreases in both competing noises with decreasing low-pass cutoff frequency. This can be attributed to loss of signal audibility with restricted listening bandwidth. Second, and more important, is the reduction in the overall advantage in the interrupted noise condition (i.e., release from masking) relative to the continuous noise condition with decreasing low-pass cutoff frequency. Keeping in mind that the continuous and interrupted noises were equated in overall sound power, it is apparent that listeners experienced a reduction in temporal resolution ability with decreasing low-pass cutoff frequency. The most parsimonious explanation for this reduced release from masking is the listeners' restricted listening bandwidth and their dependence on low-frequency hearing channels. That is, as listeners are constrained by more restrictive lowpass filtering, they are compelled to use exclusively their low-frequency cochlear channels that have inherently poorer temporal resolution. Consequently, their performance in the interrupted noise approaches that in the continuous noise as temporal resolving power diminishes and a concomitant release from masking is lost. The present study offers strong support for hearing-impaired listeners having difficulty in the temporal domain in addition to lost audibility of signal. This is exhibited through the differences between the interrupted and the contin- uous noise conditions. This discrepancy cannot be rationalized by a reduction in audibility alone. Both types of noise contained the same spectral qualities, with the only difference existing in the temporal characteristics of the interrupted noise. Consequently, if audibility were a factor, then both noise conditions should have yielded similar results. Since this was not the case, one must conclude that differences between these two conditions were the result of the listeners being limited to the low frequencies when hearing loss was simulated. Again, the findings are in accordance with others who have used the interrupted noise paradigm (Phillips et al, 1994 ; Rappaport et al, 1994 ; Stuart et al, 1995 ; Stuart and Phillips, 1996, 1997, 1998). To conclude, this study demonstrated that differences exist between continuous and interrupted noise when high-frequency hearing loss is simulated through low-pass filtered speech. These differences are evident at several filter settings and s. Because these discrepancies are noted, one may assume that audibility is not the only factor contributing to poor performance on temporal resolution tasks by hearing-impaired individuals. Specifically, these listeners are limited to low-frequency channels that have poorer temporal resolution compared with high-frequency channels. REFERENCES American National Standards Institute. (1996). Specifications for Audiometers. (ANSI S ). New York : ANSI. American National Standards Institute. (1999). Criteria for Permissible Ambient Noise During Audiometeric Testing. (ANSI S ). New York : ANSI. American Speech-Language-Hearing Association. (199). Guidelines for screening for hearing impairments and middle ear disorders. ASHA 32(Suppl 2): Bacon SP, Opie JM, Montoya DY (1994). Speech recognition thresholds in temporally complex backgrounds : effects of hearing loss and noise masking. J Acoust Soc Am 95 :2993. Bacon SP, Viemeister NF. (1985). Temporal modulation transfer functions in normal-hearing and hearingimpaired listeners. Audiology 24 : Bornstein SP, Wilson RH, Cambron NK. (1994). Low- and high-pass filtered Northwestern University Auditory Test No. 6 for monaural and binaural evaluation. JAm Acad Audiol 5: Dubno JR, Dirks DD, Morgan DE. (1984). Effects of age and mild hearing loss on speech recognition in noise. J Acoust Soc Am 76 : Festen JM, Plomp R. (199). Effects of fluctuating noise and interfering speech on the speech-reception thresh- 443

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