Frequency Modulation Detection Thresholds Research Articles

The Relative Contribution of Cochlear Synaptopathy and Reduced Inhibition to Age-Related Hearing Impairment for People With Normal Audiograms.

Older people often show auditory temporal processing deficits and speech-in-noise intelligibility difficulties even when their audiogram is clinically normal. The causes of such problems remain unclear. Some studies have suggested that for people with normal audiograms, age-related hearing impairments may be due to a cognitive decline, while others have suggested that they may be caused by cochlear synaptopathy. Here, we explore an alternative hypothesis, namely that age-related hearing deficits are associated with decreased inhibition. For human adults (N = 30) selected to cover a reasonably wide age range (25-59 years), with normal audiograms and normal cognitive function, we measured speech reception thresholds in noise (SRTNs) for disyllabic words, gap detection thresholds (GDTs), and frequency modulation detection thresholds (FMDTs). We also measured the rate of growth (slope) of auditory brainstem response wave-I amplitude with increasing level as an indirect indicator of cochlear synaptopathy, and the interference inhibition score in the Stroop color and word test (SCWT) as a proxy for inhibition. As expected, performance in the auditory tasks worsened (SRTNs, GDTs, and FMDTs increased), and wave-I slope and SCWT inhibition scores decreased with ageing. Importantly, SRTNs, GDTs, and FMDTs were not related to wave-I slope but worsened with decreasing SCWT inhibition. Furthermore, after partialling out the effect of SCWT inhibition, age was no longer related to SRTNs or GDTs and became less strongly related to FMDTs. Altogether, results suggest that for people with normal audiograms, age-related deficits in auditory temporal processing and speech-in-noise intelligibility are mediated by decreased inhibition rather than cochlear synaptopathy.

Binaural Frequency Modulation Detection in School-Age Children, Young Adults, and Older Adults: Effects of Interaural Modulator Phase.

The purpose of this study was to measure low-rate binaural frequency modulation (FM) detection across the lifespan as a gauge of temporal fine structure processing. Children and older adults were expected to perform more poorly than young adults but for different reasons. Detection of 2-Hz FM carried by a 500-Hz pure tone was measured for modulators that were either in-phase or out-of-phase across ears. Thresholds were measured in quiet and in noise. Participants were school-age children (n = 44), young adults (n = 11), and older adults (n = 17) with normal or near-normal hearing. Thresholds were lower for out-of-phase than in-phase modulators among all listening groups. Detection thresholds improved with child age, with larger effects of age for dichotic than diotic FM. Introduction of masking noise tended to elevate thresholds; this effect was larger for the dichotic condition than the diotic condition, and larger for older adults than young adults. In noise, young adults received the greatest dichotic benefit, followed by older adults, then young children. The relative effects of noise on dichotic benefit did not differ for young adults compared to young children and older adults; however, young children saw greater reduction in benefit due to noise than older adults. The difference in dichotic benefit between children and young adults is consistent with maturation of central auditory processing. Differences in the effect of noise on dichotic benefit in young children and older adults support the idea that different factors or combinations of factors limit performance in these two groups. Although dichotic FM detection appears to be more sensitive to the effects of development and aging than diotic FM detection, the positive correlation between diotic and dichotic FM detection thresholds for all listeners suggests contribution of one or more factors common to both conditions.

Age-Related Deficits in Electrophysiological and Behavioral Measures of Binaural Temporal Processing.

Binaural processing, particularly the processing of interaural phase differences, is important for sound localization and speech understanding in background noise. Age has been shown to impact the neural encoding and perception of these binaural temporal cues even in individuals with clinically normal hearing sensitivity. This work used a new electrophysiological response, called the interaural phase modulation-following response (IPM-FR), to examine the effects of age on the neural encoding of interaural phase difference cues. Relationships between neural recordings and performance on several behavioral measures of binaural processing were used to determine whether the IPM-FR is predictive of interaural phase difference sensitivity and functional speech understanding deficits. Behavioral binaural frequency modulation detection thresholds were measured to assess sensitivity to interaural phase differences while spatial release-from-masking thresholds were used to assess speech understanding abilities in spatialized noise. Thirty adults between the ages of 35 to 74 years with normal low-frequency hearing thresholds were used in this study. Data showed that older participants had weaker neural responses to the interaural phase difference cue and were less able to take advantage of binaural cues for speech understanding compared to younger participants. Results also showed that the IPM-FR was predictive of performance on the binaural frequency modulation detection task, but not on the spatial release-from-masking task after accounting the effects of age. These results confirm previous work that showed that the IPM-FR reflects age-related declines in binaural temporal processing and provide further evidence that this response may represent a useful objective tool for assessing binaural function. However, further research is needed to understand how the IPM-FR is related to speech understanding abilities.

Accounting for masking of frequency modulation by amplitude modulation with the modulation filter-bank concept.

Frequency modulation (FM) is assumed to be detected through amplitude modulation (AM) created by cochlear filtering for modulation rates above 10 Hz and carrier frequencies (fc) above 4 kHz. If this is the case, a model of modulation perception based on the concept of AM filters should predict masking effects between AM and FM. To test this, masking effects of sinusoidal AM on sinusoidal FM detection thresholds were assessed on normal-hearing listeners as a function of FM rate, fc, duration, AM rate, AM depth, and phase difference between FM and AM. The data were compared to predictions of a computational model implementing an AM filter-bank. Consistent with model predictions, AM masked FM with some AM-masking-AM features (broad tuning and effect of AM-masker depth). Similar masking was predicted and observed at fc = 0.5 and 5 kHz for a 2 Hz AM masker, inconsistent with the notion that additional (e.g., temporal fine-structure) cues drive slow-rate FM detection at low fc. However, masking was lower than predicted and, unlike model predictions, did not show beating or phase effects. Broadly, the modulation filter-bank concept successfully explained some AM-masking-FM effects, but could not give a complete account of both AM and FM detection.

Sensorineural hearing loss impairs sensitivity but spares temporal integration for detection of frequency modulation.

The effect of the number of modulation cycles (N) on frequency-modulation (FM) detection thresholds (FMDTs) was measured with and without interfering amplitude modulation (AM) for hearing-impaired (HI) listeners, using a 500-Hz sinusoidal carrier and FM rates of 2 and 20 Hz. The data were compared with FMDTs for normal-hearing (NH) listeners and AM detection thresholds (AMDTs) for NH and HI listeners [Wallaert, Moore, and Lorenzi (2016). J. Acoust. Soc. 139, 3088-3096; Wallaert, Moore, Ewert, and Lorenzi (2017). J. Acoust. Soc. 141, 971-980]. FMDTs were higher for HI than for NH listeners, but the effect of increasing N was similar across groups. In contrast, AMDTs were lower and the effect of increasing N was greater for HI listeners than for NH listeners. A model of temporal-envelope processing based on a modulation filter-bank and a template-matching decision strategy accounted better for the FMDTs at 20 Hz than at 2 Hz for young NH listeners and predicted greater temporal integration of FM than observed for all groups. These results suggest that different mechanisms underlie AM and FM detection at low rates and that hearing loss impairs FM-detection mechanisms, but preserves the memory and decision processes responsible for temporal integration of FM.

Assessing the Role of Place and Timing Cues in Coding Frequency and Amplitude Modulation as a Function of Age.

Natural sounds can be characterized by their fluctuations in amplitude and frequency. Ageing may affect sensitivity to some forms of fluctuations more than others. The present study used individual differences across a wide age range (20-79years) to test the hypothesis that slow-rate, low-carrier frequency modulation (FM) is coded by phase-locked auditory-nerve responses to temporal fine structure (TFS), whereas fast-rate FM is coded via rate-place (tonotopic) cues, based on amplitude modulation (AM) of the temporal envelope after cochlear filtering. Using a low (500Hz) carrier frequency, diotic FM and AM detection thresholds were measured at slow (1Hz) and fast (20Hz) rates in 85 listeners. Frequency selectivity and TFS coding were assessed using forward masking patterns and interaural phase disparity tasks (slow dichotic FM), respectively. Comparable interaural level disparity tasks (slow and fast dichotic AM and fast dichotic FM) were measured to control for effects of binaural processing not specifically related to TFS coding. Thresholds in FM and AM tasks were correlated, even across tasks thought to use separate peripheral codes. Age was correlated with slow and fast FM thresholds in both diotic and dichotic conditions. The relationship between age and AM thresholds was generally not significant. Once accounting for AM sensitivity, only diotic slow-rate FM thresholds remained significantly correlated with age. Overall, results indicate stronger effects of age on FM than AM. However, because of similar effects for both slow and fast FM when not accounting for AM sensitivity, the effects cannot be unambiguously ascribed to TFS coding.

Interactions between amplitude modulation and frequency modulation processing: Effects of age and hearing loss.

Frequency modulation (FM) and amplitude modulation (AM) detection thresholds were measured for a 500-Hz carrier frequency and a 5-Hz modulation rate. For AM detection, FM at the same rate as the AM was superimposed with varying FM depth. For FM detection, AM at the same rate was superimposed with varying AM depth. The target stimuli always contained both amplitude and frequency modulations, while the standard stimuli only contained the interfering modulation. Young and older normal-hearing listeners, as well as older listeners with mild-to-moderate sensorineural hearing loss were tested. For all groups, AM and FM detection thresholds were degraded in the presence of the interfering modulation. AM detection with and without interfering FM was hardly affected by either age or hearing loss. While aging had an overall detrimental effect on FM detection with and without interfering AM, there was a trend that hearing loss further impaired FM detection in the presence of AM. Several models using optimal combination of temporal-envelope cues at the outputs of off-frequency filters were tested. The interfering effects could only be predicted for hearing-impaired listeners. This indirectly supports the idea that, in addition to envelope cues resulting from FM-to-AM conversion, normal-hearing listeners use temporal fine-structure cues for FM detection.

The Influence of Cochlear Mechanical Dysfunction, Temporal Processing Deficits, and Age on the Intelligibility of Audible Speech in Noise for Hearing-Impaired Listeners.

The aim of this study was to assess the relative importance of cochlear mechanical dysfunction, temporal processing deficits, and age on the ability of hearing-impaired listeners to understand speech in noisy backgrounds. Sixty-eight listeners took part in the study. They were provided with linear, frequency-specific amplification to compensate for their audiometric losses, and intelligibility was assessed for speech-shaped noise (SSN) and a time-reversed two-talker masker (R2TM). Behavioral estimates of cochlear gain loss and residual compression were available from a previous study and were used as indicators of cochlear mechanical dysfunction. Temporal processing abilities were assessed using frequency modulation detection thresholds. Age, audiometric thresholds, and the difference between audiometric threshold and cochlear gain loss were also included in the analyses. Stepwise multiple linear regression models were used to assess the relative importance of the various factors for intelligibility. Results showed that (a) cochlear gain loss was unrelated to intelligibility, (b) residual cochlear compression was related to intelligibility in SSN but not in a R2TM, (c) temporal processing was strongly related to intelligibility in a R2TM and much less so in SSN, and (d) age per se impaired intelligibility. In summary, all factors affected intelligibility, but their relative importance varied across maskers.

Suprathreshold auditory processing deficits in noise: Effects of hearing loss and age

People with sensorineural hearing loss generally suffer from a reduced ability to understand speech in complex acoustic listening situations, particularly when background noise is present. In addition to the loss of audibility, a mixture of suprathreshold processing deficits is possibly involved, like altered basilar membrane compression and related changes, as well as a reduced ability of temporal coding. A series of 6 monaural psychoacoustic experiments at 0.5, 2, and 6 kHz was conducted with 18 subjects, divided equally into groups of young normal-hearing, older normal-hearing and older hearing-impaired listeners, aiming at disentangling the effects of age and hearing loss on psychoacoustic performance in noise. Random frequency modulation detection thresholds (RFMDTs) with a low-rate modulator in wide-band noise, and discrimination of a phase-jittered Schroeder-phase from a random-phase harmonic tone complex are suggested to characterize the individual ability of temporal processing. The outcome was compared to thresholds of pure tones and narrow-band noise, loudness growth functions, auditory filter bandwidths, and tone-in-noise detection thresholds. At 500 Hz, results suggest a contribution of temporal fine structure (TFS) to pure-tone detection thresholds. Significant correlation with auditory thresholds and filter bandwidths indicated an impact of frequency selectivity on TFS usability in wide-band noise. When controlling for the effect of threshold sensitivity, the listener's age significantly correlated with tone-in-noise detection and RFMDTs in noise at 500 Hz, showing that older listeners were particularly affected by background noise at low carrier frequencies.

Auditory training during development mitigates a hearing loss-induced perceptual deficit.

Sensory experience during early development can shape the central nervous system and this is thought to influence adult perceptual skills. In the auditory system, early induction of conductive hearing loss (CHL) leads to deficits in central auditory coding properties in adult animals, and this is accompanied by diminished perceptual thresholds. In contrast, a brief regimen of auditory training during development can enhance the perceptual skills of animals when tested in adulthood. Here, we asked whether a brief period of training during development could compensate for the perceptual deficits displayed by adult animals reared with CHL. Juvenile gerbils with CHL, and age-matched controls, were trained on a frequency modulation (FM) detection task for 4 or 10 days. The performance of each group was subsequently assessed in adulthood, and compared to adults with normal hearing (NH) or adults raised with CHL that did not receive juvenile training. We show that as juveniles, both CHL and NH animals display similar FM detection thresholds that are not immediately impacted by the perceptual training. However, as adults, detection thresholds and psychometric function slopes of these animals were significantly improved. Importantly, CHL adults with juvenile training displayed thresholds that approached NH adults. Additionally, we found that hearing impaired animals trained for 10 days displayed adult thresholds closer to untrained adults than those trained for 4 days. Thus, a relatively brief period of auditory training may compensate for the deleterious impact of hearing deprivation on auditory perception on the trained task.

A Sensitive Period for the Impact of Hearing Loss on Auditory Perception

Manipulations of the sensory environment typically induce greater changes to the developing nervous system than they do in adulthood. The relevance of these neural changes can be evaluated by examining the age-dependent effects of sensory experience on quantitative measures of perception. Here, we measured frequency modulation (FM) detection thresholds in adult gerbils and investigated whether diminished auditory experience during development or in adulthood influenced perceptual performance. Bilateral conductive hearing loss (CHL) of ≈30 dB was induced either at postnatal day 10 or after sexual maturation. All animals were then trained as adults to detect a 5 Hz FM embedded in a continuous 4 kHz tone. FM detection thresholds were defined as the minimum deviation from the carrier frequency that the animal could reliably detect. Normal-hearing animals displayed FM thresholds of 25 Hz. Inducing CHL, either in juvenile or adult animals, led to a deficit in FM detection. However, this deficit was greater for juvenile onset hearing loss (89 Hz) relative to adult onset hearing loss (64 Hz). The effects could not be attributed to sensation level, nor were they correlated with proxies for attention. The thresholds displayed by CHL animals were correlated with shallower psychometric function slopes, suggesting that hearing loss was associated with greater variance of the decision variable, consistent with increased internal noise. The results show that decreased auditory experience has a greater impact on perceptual skills when initiated at an early age and raises the possibility that altered development of CNS synapses may play a causative role.

The development of speech-in-noise perception in Hebrew-speaking school-age children

Several studies suggest that auditory perception in general and the perception of speech in noise in particular continue to develop until late childhood or early adolescence. It remains unclear, however, whether this prolonged development results from the maturation of the cognitive factors associated with the performance of auditory tasks or from the slow development of auditory sensory processing. We investigated the perception of monosyllabic words embedded in white noise in Hebrew-speaking school-age children and adults. Although identification thresholds did not become adult-like until 11 years of age, we found no evidence linking this prolonged development to non-sensory factors associated with performance consistency. Therefore, we suggest that similar to the development of amplitude and frequency modulation detection thresholds, this protracted development is related to the maturation of auditory sensory processing.

The effects of unmodulated carrier fringes on the detection of frequency modulation

Detection thresholds for 100 ms of either 5- or 20-Hz frequency modulation (FM) were measured at various temporal positions within a 600-ms, 4-kHz pure-tone carrier. The results indicated that the temporal position of the signal relative to the fringe influences detection thresholds, including an effect that is reminiscent of auditory backward recognition masking. A task involving frequency increments, rather than sinusoidal FM, yielded similar results. Additional manipulation of total carrier duration indicated that FM detection thresholds improve as the duration of the forward fringe increases, while a backward fringe only degrades performance in the absence of any forward fringe. The results suggest that listeners are insensitive to subtle frequency changes that occur at the onset of a longer stimulus and that the interaction between the opposing effects of the forward and backward fringes is not additive.

C034 The FOX community: A working group for assessing new cochlear implant fitting approaches and practices

People with sensorineural hearing loss generally suffer from a reduced ability to understand speech in complex acoustic listening situations, particularly when background noise is present. In addition to the loss of audibility, a mixture of suprathreshold processing deficits is possibly involved, like altered basilar membrane compression and related changes, as well as a reduced ability of temporal coding. A series of 6 monaural psychoacoustic experiments at 0.5, 2, and 6 kHz was conducted with 18 subjects, divided equally into groups of young normal-hearing, older normal-hearing and older hearing-impaired listeners, aiming at disentangling the effects of age and hearing loss on psychoacoustic performance in noise. Random frequency modulation detection thresholds (RFMDTs) with a low-rate modulator in wide-band noise, and discrimination of a phase-jittered Schroeder-phase from a random-phase harmonic tone complex are suggested to characterize the individual ability of temporal processing. The outcome was compared to thresholds of pure tones and narrow-band noise, loudness growth functions, auditory filter bandwidths, and tone-in-noise detection thresholds. At 500 Hz, results suggest a contribution of temporal fine structure (TFS) to pure-tone detection thresholds. Significant correlation with auditory thresholds and filter bandwidths indicated an impact of frequency selectivity on TFS usability in wide-band noise. When controlling for the effect of threshold sensitivity, the listener's age significantly correlated with tone-in-noise detection and RFMDTs in noise at 500 Hz, showing that older listeners were particularly affected by background noise at low carrier frequencies.

Relations between frequency selectivity, temporal fine-structure processing, and speech reception in impaired hearing

Frequency selectivity, temporal fine-structure (TFS) processing, and speech reception were assessed for six normal-hearing (NH) listeners, ten sensorineurally hearing-impaired (HI) listeners with similar high-frequency losses, and two listeners with an obscure dysfunction (OD). TFS processing was investigated at low frequencies in regions of normal hearing, through measurements of binaural masked detection, tone lateralization, and monaural frequency modulation (FM) detection. Lateralization and FM detection thresholds were measured in quiet and in background noise. Speech reception thresholds were obtained for full-spectrum and lowpass-filtered sentences with different interferers. Both the HI listeners and the OD listeners showed poorer performance than the NH listeners in terms of frequency selectivity, TFS processing, and speech reception. While a correlation was observed between the monaural and binaural TFS-processing deficits in the HI listeners, no relation was found between TFS processing and frequency selectivity. The effect of noise on TFS processing was not larger for the HI listeners than for the NH listeners. Finally, TFS-processing performance was correlated with speech reception in a two-talker background and lateralized noise, but not in amplitude-modulated noise. The results provide constraints for future models of impaired auditory signal processing.

Frequency modulation detection, frequency selectivity, and speech recognition in noise by hearing‐impaired listeners.

Reduced ability to make use of temporal fine structure (TFS) information may contribute to deficits among hearing impaired (HI) listeners for speech recognition in competing background sounds. The broad tuning typically observed in HI listeners will produce more complex patterns of TFS as input to the auditory nerve, and these patterns may be more difficult to encode and/or interpret by higher auditory centers. In the current study, we tested HI listeners on a frequency‐modulation (FM) detection task thought to require the use of TFS cues, on tone detection in notched‐noise to assess frequency selectivity, and on speech recognition in continuous and modulated background sounds. Performance on the psychoacoustic tasks was tested at four frequencies (500, 1000, 2000, and 4000 Hz) at levels between 60 and 85 dB SPL (approximating the range of third‐octave band levels in the speech stimuli). HI listeners with similar audiometric thresholds showed clear differences in frequency tuning, FM detection thresholds, and speech performance. Relationships between the psychoacoustic measures in the four frequency regions and between these measures and speech performance will be discussed.

Frequency modulation detection with simultaneous amplitude modulation by cochlear implant users

To better represent fine structure cues in cochlear implants (CIs), recent research has proposed varying the stimulation rate based on slowly varying frequency modulation (FM) information. The present study investigated the abilities of CI users to detect FM with simultaneous amplitude modulation (AM). FM detection thresholds (FMDTs) for 10-Hz sinusoidal FM and upward frequency sweeps were measured as a function of standard frequency (75-1000 Hz). Three AM conditions were tested, including (1) No AM, (2) 20-Hz Sinusoidal AM (SAM) with modulation depths of 10%, 20%, or 30%, and (3) Noise AM (NAM), in which the amplitude was randomly and uniformly varied over a range of 1, 2, or 3 dB, relative to the reference amplitude. Results showed that FMDTs worsened with increasing standard frequencies, and were lower for sinusoidal FM than for upward frequency sweeps. Simultaneous AM significantly interfered with FM detection; FMDTs were significantly poorer with simultaneous NAM than with SAM. Besides, sinusoidal FMDTs significantly worsened when the starting phase of simultaneous SAM was randomized. These results suggest that FM and AM in CI partly share a common loudness-based coding mechanism and the feasibility of "FM+AM" strategies for CI speech processing may be limited.

Detection of amplitude modulation and frequency modulation in tactual gratings: a critical bandwidth for active touch.

Since most natural surfaces are complex and vary in amplitude and spatial frequency, it might be interesting to consider gratings not in the spatial domain, but in the spatial-frequency domain. Detection thresholds for amplitude modulation (AM) and frequency modulation (FM) in sinusoidal gratings were measured for seven participants. Participants moved their fingers actively across the gratings. Although the two types of modulation are quite different in the spatial domain, they have many features in common in the frequency domain. In previous research (Nefs et al 2001 Perception 30 1263-1274) we measured the discrimination thresholds for amplitude and frequency for sinusoidal gratings. We hypothesised then that these thresholds could be used to predict the discriminability of other types of gratings. In the present study, we did indeed find that the FM and AM detection thresholds can be understood quite well by these discrimination thresholds. The results indicate that the tactual system contains parallel psychophysical channels that filter and integrate the power of stimuli within critical bands. With these results, we are also able to calculate the critical bandwidth for active dynamic touch. We estimated the critical bandwidth surrounding the spatial frequency of 2 cycles cm(-1) to be about 125% of that spatial frequency. This value for the critical band for spatial frequency is incompatible with previous findings for temporal frequencies in vibrotactile research. This indicates that dynamic spatial-frequency discrimination is not likely to be done by temporal frequency.

Modulation rate-discrimination and frequency modulation detection thresholds in the amplitude modulation domain

The present study tested the existence of modulation filters by transposing the concept of excitation pattern to the amplitude modulation (AM) domain. In the first experiment, AM rate-discrimination thresholds (DLFs) for white-noise carriers with AM reference rates of 4, 16, and 64 Hz were measured in five normal-hearing listeners. AM depth was either (i) fixed at 0.2, 0.6, or 1.0, or (ii) varied randomly between 0.2 and 1.0. Randomization of the modulation depth occurred either between trials or between the two intervals of the same trial. In a second experiment, detection thresholds of slow (1 or 4 Hz) frequency modulation (FMDLs) applied to the AM carrier (of identical rates as in the previous experiment) were measured, with and without the presence of a slow sinusoidal modulation applied to the depth of the AM carrier. Overall, the data show that DLFs are less affected than FMDLs by variations in the depth of AM. The results are discussed in light of current models of AM perception.

Detection of frequency modulation by hearing-impaired listeners: Effects of carrier frequency, modulation rate, and added amplitude modulation

It has been proposed that the detection of frequency modulation (FM) of sinusoidal carriers can be mediated by two mechanisms: a place mechanism based on FM-induced amplitude modulation (AM) in the excitation pattern, and a temporal mechanism based on phase-locking in the auditory nerve. The temporal mechanism appears to be "sluggish" and does not play a role for FM rates above about 10 Hz. It also does not play a role for high carrier frequencies (above about 5 kHz). This experiment examined FM detection in three young subjects with normal hearing and four elderly subjects with cochlear hearing loss. Carrier frequencies were 0.25, 0.5, 1, 2, 4, and 6 kHz and modulation rates were 2, 5, 10, and 20 Hz. FM detection thresholds were measured both in the absence of AM, and with AM of a fixed depth (m = 0.33) added in both intervals of a forced-choice trial. The added AM was intended to disrupt cues based on FM-induced AM in the excitation pattern. Generally, the hearing-impaired subjects performed markedly more poorly than the normal-hearing subjects. For the normal-hearing subjects, the disruptive effect of the AM tended to increase with increasing modulation rate, for carrier frequencies below 6 kHz, as found previously by Moore and Sek [J. Acoust. Soc. Am. 100, 2320-2331 (1996)]. For the hearing-impaired subjects, the disruptive effective of the AM was generally larger than for the normal-hearing subjects, and the magnitude of the disruption did not consistently increase with increasing modulation rate. The results suggest that cochlear hearing impairment adversely affects both temporal and excitation pattern mechanisms of FM detection.