Discrimination of amplitude-modulation depth by subjects with normal and impaired hearing

Task-dependent Modulation Masking of 4 Hz Envelope Following Responses

The perception and recognition of natural sounds, like speech, rely on the processing of slow amplitude modulations. Perception can be hindered by interfering modulations at similar rates, a phenomenon known as modulation masking. Cortical envelope following responses (EFRs) are highly sensitive to these slow modulations, but it is unclear how modulation masking impacts these cortical envelope responses. To dissociate stimulus-driven and attention-driven effects, we recorded EEG responses to 4 Hz modulated noise in a two-way factorial design, varying the level of modulation masking and intermodal attention. Auditory stimuli contained one of three random masking bands in the stimulus envelope, at various proximities in modulation frequency to the 4 Hz target, or an unmasked reference condition. During EEG recordings, the same stimuli were presented while participants performed either an auditory or a visual change detection task. Attention to the auditory modality resulted in a general enhancement of sustained EFR responses to the 4 Hz target. In the visual task condition only, EFR 4 Hz power systematically decreased with increasing modulation masking, consistent with psychophysical masking patterns. However, during the auditory task, the 4 Hz EFRs were unaffected by masking and remained strong even with the highest degrees of masking. Rather than indicating a general bottom-up modulation selective process, these results indicate that the masking of cortical envelope responses interacts with attention. We propose that auditory attention allows robust tracking of masked envelopes, possibly through a form of glimpsing of the target, whereas envelope responses to task-irrelevant auditory stimuli reflect stimulus salience.

Investigating the effects of age and hearing loss on speech intelligibility and amplitude modulation frequency selectivity

The perception of amplitude modulation (AM), characterized by a frequency-selective process in the modulation domain, is considered critical for speech intelligibility. Previous literature has provided evidence of an age-related decline in AM frequency selectivity. Additionally, a notable sharpening of AM tuning with hearing loss has been observed, which was proposed to be related to a perceptual advantage resulting from peripheral compression loss. This study explores whether such changes in AM tuning with age and hearing loss contribute to the speech intelligibility challenges older listeners often face in noisy environments. Young (n = 10, aged 22-28 years) and older (n = 9, aged 57-77 years) listeners with normal hearing as well as older listeners with hearing impairment (n = 9, aged 64-77 years) participated in the study. All had previously taken part in studies on AM tuning. Speech-reception thresholds (SRTs) were collected under conditions including stationary, fluctuating, and competing-speech maskers. The results revealed an age-related increase in SRTs, with an additional negative impact of hearing loss. Beyond age and audibility, a measure of AM tuning significantly contributed to explaining the variance in SRTs across listeners for specific maskers. These findings motivate further exploration of the relationship between AM frequency selectivity and speech intelligibility in noise.

Impact of reduced spectral resolution on temporal-coherence-based source segregation

Hearing-impaired listeners struggle to understand speech in noise, even when using cochlear implants (CIs) or hearing aids. Successful listening in noisy environments depends on the brain's ability to organize a mixture of sound sources into distinct perceptual streams (i.e., source segregation). In normal-hearing listeners, temporal coherence of sound fluctuations across frequency channels supports this process by promoting grouping of elements belonging to a single acoustic source. We hypothesized that reduced spectral resolution-a hallmark of both electric/CI (from current spread) and acoustic (from broadened tuning) hearing with sensorineural hearing loss-degrades segregation based on temporal coherence. This is because reduced frequency resolution decreases the likelihood that a single sound source dominates the activity driving any specific channel; concomitantly, it increases the correlation in activity across channels. Consistent with our hypothesis, our physiologically inspired computational model of temporal-coherence-based segregation predicts that CI current spread reduces comodulation masking release (CMR; a correlate of temporal-coherence processing) and speech intelligibility in noise. These predictions are consistent with our online behavioral data with simulated CI listening. Our model also predicts smaller CMR with increasing levels of outer-hair-cell damage. These results suggest that reduced spectral resolution relative to normal hearing impairs temporal-coherence-based segregation and speech-in-noise outcomes.

Hearing Sensitivity to Gliding Rippled Spectra in Hearing-Impaired Listeners

OBJECTIVES

Sensitivity to the gliding of ripples in rippled-spectrum signals was measured in both normal-hearing and hearing-impaired listeners.

METHODS

The test signal was a 2 oct wide rippled noise centered at 2 kHz, with the ripples gliding downward along the frequency scale. Both the gliding velocity and ripple density were frequency-proportional across the signal band. Ripple density was specified in ripples/oct and velocity was specified in oct/s. The listener's task was to discriminate between the signal with gliding ripples and the non-rippled signal.

RESULTS

In all listener groups, increasing the ripple density decreased the maximal velocity of detectable ripple gliding. The velocity limit of ripple gliding decreased with hearing loss.

CONCLUSIONS

The results can be explained by deteriorated temporal resolution in hearing-impaired listeners.

Relating monaural and binaural measures of modulation sensitivity in listeners with and without hearing loss

Listeners are sensitive to interaural time differences carried in the envelope of high-frequency sounds (ITDENV), but the salience of this cue depends on certain properties of the envelope and, in particular, on the presence/depth of amplitude modulation (AM) in the envelope. This study tested the hypothesis that individuals with sensorineural hearing loss, who show enhanced sensitivity to AM under certain conditions, would also show superior ITDENV sensitivity under those conditions. The second hypothesis was that variations in ITDENV sensitivity across individuals can be related to variations in sensitivity to AM. To enable a direct comparison, a standard adaptive AM detection task was used along with a modified version of it designed to measure ITDENV sensitivity. The stimulus was a 4-kHz tone modulated at rates of 32, 64, or 128 Hz and presented at a 30 dB sensation level. Both tasks were attempted by 16 listeners with normal hearing and 16 listeners with hearing loss. Consistent with the hypotheses, AM and ITDENV thresholds were correlated and tended to be better in listeners with hearing loss. A control experiment emphasized that absolute level may be a consideration when interpreting the group effects.

Characterizing the relationship between modulation sensitivity and pitch resolution in cochlear implant users

Cochlear implants are medical devices that have restored hearing to approximately one million people around the world. Outcomes are impressive and most recipients attain excellent speech comprehension in quiet without relying on lip-reading cues, but pitch resolution is poor compared to normal hearing. Amplitude modulation of electrical stimulation is a primary cue for pitch perception in cochlear implant users. The experiments described in this article focus on the relationship between sensitivity to amplitude modulations and pitch resolution based on changes in the frequency of amplitude modulations. In the first experiment, modulation sensitivity and pitch resolution were measured in adults with no known hearing loss and in cochlear implant users with sounds presented to and processed by their clinical devices. Stimuli were amplitude-modulated sinusoids and amplitude-modulated narrow-band noises. Modulation detection and modulation frequency discrimination were measured for modulation frequencies centered on 110, 220, and 440 Hz. Pitch resolution based on changes in modulation frequency was measured for modulation depths of 25 %, 50 %, 100 %, and for a half-waved rectified modulator. Results revealed a strong linear relationship between modulation sensitivity and pitch resolution for cochlear implant users and peers with no known hearing loss. In the second experiment, cochlear implant users took part in analogous procedures of modulation sensitivity and pitch resolution but bypassing clinical sound processing using single-electrode stimulation. Results indicated that modulation sensitivity and pitch resolution was better conveyed by single-electrode stimulation than by clinical processors. Results at 440 Hz were worse, but also not well conveyed by clinical sound processing, so it remains unclear whether the 300 Hz perceptual limit described in the literature is a technological or biological limitation. These results highlight modulation depth and sensitivity as critical factors for pitch resolution in cochlear implant users and characterize the relationship that should inform the design of modulation enhancement algorithms for cochlear implants.

Disentangling the effects of hearing loss and age on amplitude modulation frequency selectivity

The processing and perception of amplitude modulation (AM) in the auditory system reflect a frequency-selective process, often described as a modulation filterbank. Previous studies on perceptual AM masking reported similar results for older listeners with hearing impairment (HI listeners) and young listeners with normal hearing (NH listeners), suggesting no effects of age or hearing loss on AM frequency selectivity. However, recent evidence has shown that age, independently of hearing loss, adversely affects AM frequency selectivity. Hence, this study aimed to disentangle the effects of hearing loss and age. A simultaneous AM masking paradigm was employed, using a sinusoidal carrier at 2.8 kHz, narrowband noise modulation maskers, and target modulation frequencies of 4, 16, 64, and 128 Hz. The results obtained from young (n = 3, 24-30 years of age) and older (n = 10, 63-77 years of age) HI listeners were compared to previously obtained data from young and older NH listeners. Notably, the HI listeners generally exhibited lower (unmasked) AM detection thresholds and greater AM frequency selectivity than their NH counterparts in both age groups. Overall, the results suggest that age negatively affects AM frequency selectivity for both NH and HI listeners, whereas hearing loss improves AM detection and AM selectivity, likely due to the loss of peripheral compression.

Impact of Reduced Spectral Resolution on Temporal-Coherence-Based Source Segregation

Hearing-impaired listeners struggle to understand speech in noise, even when using cochlear implants (CIs) or hearing aids. Successful listening in noisy environments depends on the brain's ability to organize a mixture of sound sources into distinct perceptual streams (i.e., source segregation). In normal-hearing listeners, temporal coherence of sound fluctuations across frequency channels supports this process by promoting grouping of elements belonging to a single acoustic source. We hypothesized that reduced spectral resolution-a hallmark of both electric/CI (from current spread) and acoustic (from broadened tuning) hearing with sensorineural hearing loss-degrades segregation based on temporal coherence. This is because reduced frequency resolution decreases the likelihood that a single sound source dominates the activity driving any specific channel; concomitantly, it increases the correlation in activity across channels. Consistent with our hypothesis, predictions from a physiologically plausible model of temporal-coherence-based segregation suggest that CI current spread reduces comodulation masking release (CMR; a correlate of temporal-coherence processing) and speech intelligibility in noise. These predictions are consistent with our behavioral data with simulated CI listening. Our model also predicts smaller CMR with increasing levels of outer-hair-cell damage. These results suggest that reduced spectral resolution relative to normal hearing impairs temporal-coherence-based segregation and speech-in-noise outcomes.

Effects of hearing loss, age, noise exposure, and listening skills on envelope regularity discrimination

The envelope regularity discrimination (ERD) test assesses the ability to discriminate irregular from regular amplitude modulation (AM). The measured threshold is called the irregularity index (II). It was hypothesized that the II at threshold should be almost unaffected by the loudness recruitment that is associated with cochlear hearing loss because the effect of recruitment is similar to multiplying the AM depth by a certain factor, and II values depend on the amount of envelope irregularity relative to the baseline modulation depth. To test this hypothesis, the ERD test was administered to 60 older adults with varying degrees of hearing loss, using carrier frequencies of 1 and 4 kHz. The II values for the two carrier frequencies were highly correlated, indicating that the ERD test was measuring a consistent characteristic of each subject. The II values at 1 and 4 kHz were not significantly correlated with the audiometric thresholds at the corresponding frequencies, consistent with the hypothesis. The II values at 4 kHz were significantly positively correlated with age. There was an unexpected negative correlation between II values and a measure of noise exposure. This is argued to reflect the confounding effects of listening skills.

The effect of recreational noise exposure on amplitude-modulation detection, hearing sensitivity at frequencies above 8 kHz, and perception of speech in noise

Psychoacoustic and speech perception measures were compared for a group who were exposed to noise regularly through listening to music via personal music players (PMP) and a control group without such exposure. Lifetime noise exposure, quantified using the NESI questionnaire, averaged ten times higher for the exposed group than for the control group. Audiometric thresholds were similar for the two groups over the conventional frequency range up to 8 kHz, but for higher frequencies, the exposed group had higher thresholds than the control group. Amplitude modulation detection (AMD) thresholds were measured using a 4000-Hz sinusoidal carrier presented in threshold-equalizing noise at 30, 60, and 90 dB sound pressure level (SPL) for modulation frequencies of 8, 16, 32, and 64 Hz. At 90 dB SPL but not at the lower levels, AMD thresholds were significantly higher (worse) for the exposed than for the control group, especially for low modulation frequencies. The exposed group required significantly higher signal-to-noise ratios than the control group to understand sentences in noise. Otoacoustic emissions did not differ for the two groups. It is concluded that listening to music via PMP can have subtle deleterious effects on speech perception, AM detection, and hearing sensitivity over the extended high-frequency range.

Age-related reduction of amplitude modulation frequency selectivity

The perception of amplitude modulations (AMs) has been characterized by a frequency-selective process in the temporal envelope domain and simulated in computational auditory processing and perception models using a modulation filterbank. Such AM frequency-selective processing has been argued to be critical for the perception of complex sounds, including speech. This study aimed at investigating the effects of age on behavioral AM frequency selectivity in young (n = 11, 22-29 years) versus older (n = 10, 57-77 years) listeners with normal hearing, using a simultaneous AM masking paradigm with a sinusoidal carrier (2.8 kHz), target modulation frequencies of 4, 16, 64, and 128 Hz, and narrowband-noise modulation maskers. A reduction of AM frequency selectivity by a factor of up to 2 was found in the older listeners. While the observed AM selectivity co-varied with the unmasked AM detection sensitivity, the age-related broadening of the masked threshold patterns remained stable even when AM sensitivity was similar across groups for an extended stimulus duration. The results from the present study might provide a valuable basis for further investigations exploring the effects of age and reduced AM frequency selectivity on complex sound perception as well as the interaction of age and hearing impairment on AM processing and perception.

Age-related deficits in dip-listening evident for isolated sentences but not for spoken stories

Fluctuating background sounds facilitate speech intelligibility by providing speech ‘glimpses’ (masking release). Older adults benefit less from glimpses, but masking release is typically investigated using isolated sentences. Recent work indicates that using engaging, continuous speech materials (e.g., spoken stories) may qualitatively alter speech-in-noise listening. Moreover, neural sensitivity to different amplitude envelope profiles (ramped, damped) changes with age, but whether this affects speech listening is unknown. In three online experiments, we investigate how masking release in younger and older adults differs for masked sentences and stories, and how speech intelligibility varies with masker amplitude profile. Intelligibility was generally greater for damped than ramped maskers. Masking release was reduced in older relative to younger adults for disconnected sentences, and stories with a randomized sentence order. Critically, when listening to stories with an engaging and coherent narrative, older adults demonstrated equal or greater masking release compared to younger adults. Older adults thus appear to benefit from ‘glimpses’ as much as, or more than, younger adults when the speech they are listening to follows a coherent topical thread. Our results highlight the importance of cognitive and motivational factors for speech understanding, and suggest that previous work may have underestimated speech-listening abilities in older adults.

Low-sound-level auditory processing in noise-exposed adults

Early signs of noise-induced hearing damage are difficult to identify, as they are often confounded by factors such as age, audiometric thresholds, or even music experience. Much previous research has focused on deficits observed at high intensity levels. In contrast, the present study was designed to test the hypothesis that noise exposure causes a degradation in low-sound-level auditory processing in humans, as a consequence of dysfunction of the inner hair cell pathway. Frequency difference limens (FDLs) and amplitude modulation depth discrimination (MDD) were measured for five center frequencies (0.75, 1, 3, 4 and 6 kHz) at 15 and 25 dB sensation level (SL), as a function of noise exposure, age, audiometric hearing loss, and music experience. Forty participants, aged 33-75 years, with normal hearing up to 1 kHz and mild-to-moderate hearing loss above 2 kHz, were tested. Participants had varying degrees of self-reported noise exposure, and varied in music experience. FDL worsened as a function of age. Participants with music experience outperformed the non-experienced in both the FDL and MDD tasks. MDD thresholds were significantly better for high-noise-exposed, than for low-noise-exposed, participants at 25 dB SL, particularly at 6 kHz. No effects of age or hearing loss were observed in the MDD. It is possible that the association between MDD thresholds and noise exposure was not causal, but instead was mediated by other factors that were not measured in the study. The association is consistent, qualitatively, with a hypothesized loss of compression due to outer hair cell dysfunction.

Hearing loss and brain plasticity: the hyperactivity phenomenon

Many aging adults experience some form of hearing problems that may arise from auditory peripheral damage. However, it has been increasingly acknowledged that hearing loss is not only a dysfunction of the auditory periphery but also results from changes within the entire auditory system, from periphery to cortex. Damage to the auditory periphery is associated with an increase in neural activity at various stages throughout the auditory pathway. Here, we review neurophysiological evidence of hyperactivity, auditory perceptual difficulties that may result from hyperactivity, and outline open conceptual and methodological questions related to the study of hyperactivity. We suggest that hyperactivity alters all aspects of hearing-including spectral, temporal, spatial hearing-and, in turn, impairs speech comprehension when background sound is present. By focusing on the perceptual consequences of hyperactivity and the potential challenges of investigating hyperactivity in humans, we hope to bring animal and human electrophysiologists closer together to better understand hearing problems in older adulthood.

Cortical Responses to the Amplitude Envelopes of Sounds Change with Age

Many older listeners have difficulty understanding speech in noise, when cues to speech-sound identity are less redundant. The amplitude envelope of speech fluctuates dramatically over time, and features such as the rate of amplitude change at onsets (attack) and offsets (decay), signal critical information about the identity of speech sounds. Aging is also thought to be accompanied by increases in cortical excitability, which may differentially alter sensitivity to envelope dynamics. Here, we recorded electroencephalography in younger and older human adults (of both sexes) to investigate how aging affects neural synchronization to 4 Hz amplitude-modulated noises with different envelope shapes (ramped: slow attack and sharp decay; damped: sharp attack and slow decay). We observed that subcortical responses did not differ between age groups, whereas older compared with younger adults exhibited larger cortical responses to sound onsets, consistent with an increase in auditory cortical excitability. Neural activity in older adults synchronized more strongly to rapid-onset, slow-offset (damped) envelopes, was less sinusoidal, and was more peaked. Younger adults demonstrated the opposite pattern, showing stronger synchronization to slow-onset, rapid-offset (ramped) envelopes, as well as a more sinusoidal neural response shape. The current results suggest that age-related changes in the excitability of auditory cortex alter responses to envelope dynamics. This may be part of the reason why older adults experience difficulty understanding speech in noise.SIGNIFICANCE STATEMENT Many middle-aged and older adults report difficulty understanding speech when there is background noise, which can trigger social withdrawal and negative psychosocial health outcomes. The difficulty may be related to age-related changes in how the brain processes temporal sound features. We tested younger and older people on their sensitivity to different envelope shapes, using EEG. Our results demonstrate that aging is associated with heightened sensitivity to sounds with a sharp attack and gradual decay, and sharper neural responses that deviate from the sinusoidal features of the stimulus, perhaps reflecting increased excitability in the aged auditory cortex. Altered responses to temporal sound features may be part of the reason why older adults often experience difficulty understanding speech in social situations.

Language-Independent Hearing Screening Based on Masked Recognition of Ecological Sounds

A language-independent automated self-test on tablet based on masked recognition of ecological sounds, the Sound Ear Check (SEC), was developed. In this test, 24 trials of eight different sounds are randomly presented in a noise that was spectrally shaped according to the average frequency spectra of the stimulus sounds, using a 1-up 2-down adaptive procedure. The test was evaluated in adults with normal hearing and hearing loss, and its feasibility was investigated in young children, who are the target population of this test. Following equalization of perceptual difficulty across sounds by applying level adjustments to the individual tokens, a reference curve with a steep slope of 18%/dB was obtained, resulting in a test with a high test–retest reliability of 1 dB. The SEC sound reception threshold was significantly associated with the averaged pure tone threshold (r = .70), as well as with the speech reception threshold for the Digit Triplet Test (r = .79), indicating that the SEC is susceptible to both audibility and signal-to-noise ratio loss. Sensitivity and specificity values on the order of magnitude of ∼70% and ∼80% to detect individuals with mild and moderate hearing loss, respectively, and ∼80% to detect individuals with slight speech-in-noise recognition difficulties were obtained. Homogeneity among sounds was verified in children. Psychometric functions fitted to the data indicated a steep slope of 16%/dB, and test–retest reliability of sound reception threshold estimates was 1.3 dB. A reference value of −9 dB signal-to-noise ratio was obtained. Test duration was around 6 minutes, including training and acclimatization.

Binaural summation of amplitude modulation involves weak interaural suppression

The brain combines sounds from the two ears, but what is the algorithm used to achieve this summation of signals? Here we combine psychophysical amplitude modulation discrimination and steady-state electroencephalography (EEG) data to investigate the architecture of binaural combination for amplitude-modulated tones. Discrimination thresholds followed a ‘dipper’ shaped function of pedestal modulation depth, and were consistently lower for binaural than monaural presentation of modulated tones. The EEG responses were greater for binaural than monaural presentation of modulated tones, and when a masker was presented to one ear, it produced only weak suppression of the response to a signal presented to the other ear. Both data sets were well-fit by a computational model originally derived for visual signal combination, but with suppression between the two channels (ears) being much weaker than in binocular vision. We suggest that the distinct ecological constraints on vision and hearing can explain this difference, if it is assumed that the brain avoids over-representing sensory signals originating from a single object. These findings position our understanding of binaural summation in a broader context of work on sensory signal combination in the brain, and delineate the similarities and differences between vision and hearing.

Impaired frequency selectivity and sensitivity to temporal fine structure, but not envelope cues, in children with mild-to-moderate sensorineural hearing loss

Psychophysical thresholds were measured for 8-16 year-old children with mild-to-moderate sensorineural hearing loss (MMHL; N = 46) on a battery of auditory processing tasks that included measures designed to be dependent upon frequency selectivity and sensitivity to temporal fine structure (TFS) or envelope cues. Children with MMHL who wore hearing aids were tested in both unaided and aided conditions, and all were compared to a group of normally hearing (NH) age-matched controls. Children with MMHL performed more poorly than NH controls on tasks considered to be dependent upon frequency selectivity, sensitivity to TFS, and speech discrimination (/bɑ/-/dɑ/), but not on tasks measuring sensitivity to envelope cues. Auditory processing deficits remained regardless of age, were observed in both unaided and aided conditions, and could not be attributed to differences in nonverbal IQ or attention between groups. However, better auditory processing in children with MMHL was predicted by better audiometric thresholds and, for aided tasks only, higher levels of maternal education. These results suggest that, as for adults with MMHL, children with MMHL may show deficits in frequency selectivity and sensitivity to TFS, but sensitivity to the envelope may remain intact.

Discrimination of the phase of amplitude modulation applied to different carriers: Effects of modulation rate and modulation depth for young and older subjects

The discrimination of amplitude modulation (AM) from frequency modulation (FM) of a 1000-Hz carrier, with equally detectable AM and FM, is better for a 2-Hz than for a 10-Hz modulation rate. This might reflect greater sensitivity to temporal fine structure for low than for high rates. Alternatively, AM-FM discrimination may depend on comparing fluctuations in excitation level on the two sides of the excitation pattern, which are in phase for AM and out of phase for FM. Discrimination of the relative phase of fluctuations might worsen with increasing rate, which could account for the effect of rate on AM-FM discrimination. To test this, discrimination of the phase of AM applied to two sinusoidal carriers was assessed, with a band of noise between the two carriers to prevent use of within-channel cues. Young and older subjects with normal hearing were tested. Performance was almost constant for AM rates from 2 to 10 Hz, but worsened at 20 Hz. Performance was near chance for AM depths near the detection threshold. The results suggest that the superior AM-FM discrimination at 2 Hz cannot be explained in terms of comparison of the phase of fluctuations on the two sides of the excitation pattern.

Asymmetric temporal envelope encoding: Implications for within- and across-ear envelope comparison

Separating sound sources in acoustic environments relies on making ongoing, highly accurate spectro-temporal comparisons. However, listeners with hearing impairment may have varying quality of temporal encoding within or across ears, which may limit the listeners' ability to make spectro-temporal comparisons between places-of-stimulation. In this study in normal hearing listeners, depth of amplitude modulation (AM) for sinusoidally amplitude modulated (SAM) tones was manipulated in an effort to reduce the coding of periodicity in the auditory nerve. The ability to judge differences in AM rates was studied for stimuli presented to different cochlear places-of-stimulation, within- or across-ears. It was hypothesized that if temporal encoding was poorer for one tone in a pair, then sensitivity to differences in AM rate of the pair would decrease. Results indicated that when the depth of AM was reduced from 50% to 20% for one SAM tone in a pair, sensitivity to differences in AM rate decreased. Sensitivity was greatest for AM rates near 90 Hz and depended upon the places-of-stimulation being compared. These results suggest that degraded temporal representations in the auditory nerve for one place-of-stimulation could lead to deficits comparing that temporal information with other places-of-stimulation.

Effect of age on envelope regularity discrimination

The ability to discriminate irregular from regular amplitude modulation was compared for young and older adults with audiometric thresholds within the normal range for frequencies from 250 to 8000 Hz, using the "envelope regularity discrimination" (ERD) test. The amount of irregularity was parametrically varied and quantified by an "irregularity index." The carrier frequency was 2000 Hz, the modulation rate was 8 Hz, and the baseline modulation index was 0.3. Stimuli were presented both at 80 dB sound pressure level (SPL) and at 20 dB sensation level (SL) in the presence of a threshold-equalizing noise. There was a significant effect of level, performance being better at 80 dB SPL than at 20 dB SL. There was also a significant effect of age, performance being worse for the older subjects. There was no significant interaction of level and age. The thresholds for the ERD test were not significantly correlated with absolute thresholds at the test carrier frequency of 2000 Hz, for either group, or for the two groups combined. The worse envelope regularity discrimination for the older group may be related to the age-related synaptopathy that has been established from recent studies of human temporal bones.

Envelope regularity discrimination

The ability to discriminate irregular from regular amplitude modulation was assessed using the "envelope regularity discrimination" test. The amount of irregularity was parametrically varied and quantified by an "irregularity index." Normative data were gathered for young subjects with normal audiometric thresholds. Parameters varied were the carrier and modulation frequencies, f_c and f_m, and the baseline modulation index, m. All tests were performed using a background threshold-equalizing noise. The main findings were (1) using f_c = 4000 Hz, f_m = 8 Hz, and m = 0.3, performance improved over the first two threshold runs and then remained roughly stable, and there was a high correlation between thresholds obtained at 80 dB sound pressure level (SPL) and at 20 dB sensation level; (2) using f_m = 8 Hz and m = 0.3 with a level of 80 dB SPL, thresholds did not vary significantly across f_c = 1000, 2000, and 4000 Hz; (3) using f_m = 8 Hz and f_c = 4000 Hz with a level of 80 dB SPL, thresholds did not vary significantly for m from 0.2 to 0.5; and (4) using m = 0.3 and f_c = 4000 Hz with a level of 80 dB SPL, thresholds improved with increasing f_m from 2 to 16 Hz. For all conditions, there was substantial individual variability, probably resulting from differences in "processing efficiency."

Effects of Age and Hearing Loss on the Discrimination of Amplitude and Frequency Modulation for 2- and 10-Hz Rates

Detection of frequency modulation (FM) with rate = 10 Hz may depend on conversion of FM to amplitude modulation (AM) in the cochlea, while detection of 2-Hz FM may depend on the use of temporal fine structure (TFS) information. TFS processing may worsen with greater age and hearing loss while AM processing probably does not. A two-stage experiment was conducted to test these ideas while controlling for the effects of detection efficiency. Stage 1 measured psychometric functions for the detection of AM alone and FM alone imposed on a 1-kHz carrier, using 2- and 10-Hz rates. Stage 2 assessed the discrimination of AM from FM at the same modulation rate when the detectability of the AM alone and FM alone was equated. Discrimination was better for the 2-Hz than for the 10-Hz rate for all young normal-hearing subjects and for some older subjects with normal hearing at 1 kHz. Other older subjects with normal hearing showed no clear difference in AM-FM discrimination for the 2- and 10-Hz rates, as was the case for most older hearing-impaired subjects. The results suggest that the ability to use TFS cues is reduced for some older people and most hearing-impaired people.

Neural envelope encoding predicts speech perception performance for normal-hearing and hearing-impaired adults

Peripheral hearing impairment cannot fully account for speech perception difficulties that emerge with advancing age. As the fluctuating speech envelope bears crucial information for speech perception, changes in temporal envelope processing are thought to contribute to degraded speech perception. Previous research has demonstrated changes in neural encoding of envelope modulations throughout the adult lifespan, either due to age or due to hearing impairment. To date, however, it remains unclear whether such age- and hearing-related neural changes are associated with impaired speech perception. In the present study, we investigated the potential relationship between perception of speech in different types of masking sounds and neural envelope encoding for a normal-hearing and hearing-impaired adult population including young (20-30 years), middle-aged (50-60 years), and older (70-80 years) people. Our analyses show that enhanced neural envelope encoding in the cortex and in the brainstem, respectively, is related to worse speech perception for normal-hearing and for hearing-impaired adults. This neural-behavioral correlation is found for the three age groups and appears to be independent of the type of masking noise, i.e., background noise or competing speech. These findings provide promising directions for future research aiming to develop advanced rehabilitation strategies for speech perception difficulties that emerge throughout adult life.

Binaural Pitch Fusion: Effects of Amplitude Modulation

Hearing-impaired adults, including both cochlear implant and bilateral hearing aid (HA) users, often exhibit broad binaural pitch fusion, meaning that they fuse dichotically presented tones with large pitch differences between ears. The current study was designed to investigate how binaural pitch fusion can be influenced by amplitude modulation (AM) of the stimuli and whether effects differ with hearing loss. Fusion ranges, the frequency ranges over which binaural pitch fusion occurs, were measured in both normal-hearing (NH) listeners and HA users with various coherent AM rates (2, 4, and 8 Hz); AM depths (20%, 40%, 60%, 80%, and 100%); and interaural AM phase and AM rate differences. The averaged results show that coherent AM increased binaural pitch fusion ranges to about 2 to 4 times wider than those in the unmodulated condition in both NH and bilateral HA subjects. Even shallow temporal envelope fluctuations (20% AM depth) significantly increased fusion ranges in all three coherent AM rate conditions. Incoherent AM introduced through interaural differences in AM phase or AM rate led to smaller increases in binaural pitch fusion range compared with those observed with coherent AM. Significant differences between groups were observed only in the coherent AM conditions. The influence of AM cues on binaural pitch fusion shows that binaural fusion is mediated in part by central processes involved in auditory grouping.

Effects of Expanding Envelope Fluctuations on Consonant Perception in Hearing-Impaired Listeners

This study examined the perceptual consequences of three speech enhancement schemes based on multiband nonlinear expansion of temporal envelope fluctuations between 10 and 20 Hz: (a) "idealized" envelope expansion of the speech before the addition of stationary background noise, (b) envelope expansion of the noisy speech, and (c) envelope expansion of only those time-frequency segments of the noisy speech that exhibited signal-to-noise ratios (SNRs) above -10 dB. Linear processing was considered as a reference condition. The performance was evaluated by measuring consonant recognition and consonant confusions in normal-hearing and hearing-impaired listeners using consonant-vowel nonsense syllables presented in background noise. Envelope expansion of the noisy speech showed no significant effect on the overall consonant recognition performance relative to linear processing. In contrast, SNR-based envelope expansion of the noisy speech improved the overall consonant recognition performance equivalent to a 1- to 2-dB improvement in SNR, mainly by improving the recognition of some of the stop consonants. The effect of the SNR-based envelope expansion was similar to the effect of envelope-expanding the clean speech before the addition of noise.

Sensorineural hearing loss enhances auditory sensitivity and temporal integration for amplitude modulation

Amplitude-modulation detection thresholds (AMDTs) were measured at 40 dB sensation level for listeners with mild-to-moderate sensorineural hearing loss (age: 50-64 yr) for a carrier frequency of 500 Hz and rates of 2 and 20 Hz. The number of modulation cycles, N, varied between two and nine. The data were compared with AMDTs measured for young and older normal-hearing listeners [Wallaert, Moore, and Lorenzi (2016). J. Acoust. Soc. Am. 139, 3088-3096]. As for normal-hearing listeners, AMDTs were lower for the 2-Hz than for the 20-Hz rate, and AMDTs decreased with increasing N. AMDTs were lower for hearing-impaired listeners than for normal-hearing listeners, and the effect of increasing N was greater for hearing-impaired listeners. A computational model based on the modulation-filterbank concept and a template-matching decision strategy was developed to account for the data. The psychophysical and simulation data suggest that the loss of amplitude compression in the impaired cochlea is mainly responsible for the enhanced sensitivity and temporal integration of temporal envelope cues found for hearing-impaired listeners. The data also suggest that, for AM detection, cochlear damage is associated with increased internal noise, but preserved short-term memory and decision mechanisms.