Temporal Fine Structure Coding Research Articles

Assessment of Temporal Fine Structure Processing Among Older Adults With Cochlear Implants.

The purpose of this study was to determine if older adults with cochlear implants are able to take advantage of coding schemes that preserve temporal fine structure (TFS) cues. A total of 19 older adults with cochlear implants participated in a prospective, repeated measures, A to B design. Participants entered the study using TFS. The participants used strategy A (high definition continuous interleaved sampling [HDCIS]) for 3 months and strategy B (TFS) for 3 months. Endpoint testing was administered at the end of each 3-month period. Testing included consonant recognition, speech understanding in noise, temporal modulation thresholds, and self-perceived benefit. Older adults were able to use TFS successfully. Speech perception performance was improved using TFS compared with HDCIS for voicing, but not manner or place of articulation. There were no differences between the two strategies for speech understanding in noise, temporal modulation detection, or self-perceived benefit. At the end of the study, 13 out of 19 (68%) of participants chose to continue using TFS processing. Advanced age does not prevent adults with cochlear implants from using TFS coding strategies. Performance outcomes using TFS and HDCIS were similar, with the exception of voicing which was improved when using TFS. The data support the idea of using various sound processing strategies with older adults.

Investigating the role of temporal fine structure in everyday hearing

Human listeners can derive substantial masking release when there are discrepancies in pitch or spatial location between the target and masking sounds. While the temporal fine-structure (TFS) in low-frequency sounds can convey information about both pitch and location, a nuanced debate exists in the literature about the role of these TFS cues in masking release. The long-term goal of the present study is to leverage individual differences to understand the role of TFS in everyday hearing. As a first step, we sought to measure individual TFS sensitivity using monaural frequency modulation (FM) and binaural interaural time difference (ITD) detection tasks. Preliminary data show large individual differences in these measures. Moreover, individual differences in ITD sensitivity were correlated with monaural FM sensitivity suggesting that monaural TFS coding can be a primary bottleneck determining binaural sensitivity. Alternately, both FM and ITD sensitivity variations could be reflecting common non-sensory factors (e.g., attention). To disambiguate between these hypotheses, we designed two passive EEG metrics of TFS coding. Follow-up experiments will compare individual differences in these perceptual and EEG measures to each other, and to speech-in-noise perception in complex environments.

Cortical Correlates of Binaural Temporal Processing Deficits in Older Adults.

This study was designed to evaluate binaural temporal processing in young and older adults using a binaural masking level difference (BMLD) paradigm. Using behavioral and electrophysiological measures within the same listeners, a series of stimulus manipulations was used to evaluate the relative contribution of binaural temporal fine-structure and temporal envelope cues. We evaluated the hypotheses that age-related declines in the BMLD task would be more strongly associated with temporal fine-structure than envelope cues and that age-related declines in behavioral measures would be correlated with cortical auditory evoked potential (CAEP) measures. Thirty adults participated in the study, including 10 young normal-hearing, 10 older normal-hearing, and 10 older hearing-impaired adults with bilaterally symmetric, mild-to-moderate sensorineural hearing loss. Behavioral and CAEP thresholds were measured for diotic (So) and dichotic (Sπ) tonal signals presented in continuous diotic (No) narrowband noise (50-Hz wide) maskers. Temporal envelope cues were manipulated by using two different narrowband maskers; Gaussian noise (GN) with robust envelope fluctuations and low-noise noise (LNN) with minimal envelope fluctuations. The potential to use temporal fine-structure cues was controlled by varying the signal frequency (500 or 4000 Hz), thereby relying on the natural decline in phase-locking with increasing frequency. Behavioral and CAEP thresholds were similar across groups for diotic conditions, while the masking release in dichotic conditions was larger for younger than for older participants. Across all participants, BMLDs were larger for GN than LNN and for 500-Hz than for 4000-Hz conditions, where envelope and fine-structure cues were most salient, respectively. Specific age-related differences were demonstrated for 500-Hz dichotic conditions in GN and LNN, reflecting reduced binaural temporal fine-structure coding. No significant age effects were observed for 4000-Hz dichotic conditions, consistent with similar use of binaural temporal envelope cues across age in these conditions. For all groups, thresholds and derived BMLD values obtained using the behavioral and CAEP methods were strongly correlated, supporting the notion that CAEP measures may be useful as an objective index of age-related changes in binaural temporal processing. These results demonstrate an age-related decline in the processing of binaural temporal fine-structure cues with preserved temporal envelope coding that was similar with and without mild-to-moderate peripheral hearing loss. Such age-related changes can be reliably indexed by both behavioral and CAEP measures in young and older adults.

Differential effects of noise trauma and diminished endocochlear potential on neural temporal coding of complex sounds: Implications for speech perception

Communication problems due to cochlear hearing loss are pervasive in today’s society, even with amplification from digital hearing aids. Threshold elevation may arise from noise-induced trauma to hair cells or, as an alternative mechanism, through age-related depletion of the endocochlear potential (EP; the power supply driving hair-cell function). While both noise trauma and diminished EP may frequently underlie hearing loss in older people, it is unclear whether these pathologies have different effects on neural coding of complex sounds, and ultimately, on speech perception. Here, we describe several recent neurophysiological studies of this question based on Wiener-kernel analyses of chinchilla auditory-nerve fiber responses to Gaussian noise. We find that noise trauma causes pronounced distortions in tonotopic coding of temporal fine structure and slower envelope cues, occurring with as little as 20 dB threshold elevation. Similar changes in neural coding occur with diminished EP, but only when thresh...

Distorted Tonotopic Coding of Temporal Envelope and Fine Structure with Noise-Induced Hearing Loss

People with cochlear hearing loss have substantial difficulty understanding speech in real-world listening environments (e.g., restaurants), even with amplification from a modern digital hearing aid. Unfortunately, a disconnect remains between human perceptual studies implicating diminished sensitivity to fast acoustic temporal fine structure (TFS) and animal studies showing minimal changes in neural coding of TFS or slower envelope (ENV) structure. Here, we used general system-identification (Wiener kernel) analyses of chinchilla auditory nerve fiber responses to Gaussian noise to reveal pronounced distortions in tonotopic coding of TFS and ENV following permanent, noise-induced hearing loss. In basal fibers with characteristic frequencies (CFs) >1.5 kHz, hearing loss introduced robust nontonotopic coding (i.e., at the wrong cochlear place) of low-frequency TFS, while ENV responses typically remained at CF. As a consequence, the highest dominant frequency of TFS coding in response to Gaussian noise was 2.4 kHz in noise-overexposed fibers compared with 4.5 kHz in control fibers. Coding of ENV also became nontonotopic in more pronounced cases of cochlear damage. In apical fibers, more classical hearing-loss effects were observed, i.e., broadened tuning without a significant shift in best frequency. Because these distortions and dissociations of TFS/ENV disrupt tonotopicity, a fundamental principle of auditory processing necessary for robust signal coding in background noise, these results have important implications for understanding communication difficulties faced by people with hearing loss. Further, hearing aids may benefit from distinct amplification strategies for apical and basal cochlear regions to address fundamentally different coding deficits. Speech-perception problems associated with noise overexposure are pervasive in today's society, even with modern digital hearing aids. Unfortunately, the underlying physiological deficits in neural coding remain unclear. Here, we used innovative system-identification analyses of auditory nerve fiber responses to Gaussian noise to uncover pronounced distortions in coding of rapidly varying acoustic temporal fine structure and slower envelope cues following noise trauma. Because these distortions degrade and diminish the tonotopic representation of temporal acoustic features, a fundamental principle of auditory processing, the results represent a critical advancement in our understanding of the physiological bases of communication disorders. The detailed knowledge provided by this work will help guide the design of signal-processing strategies aimed at alleviating everyday communication problems for people with hearing loss.

Temporal Fine-Structure Coding and Lateralized Speech Perception in Normal-Hearing and Hearing-Impaired Listeners.

This study investigated the relationship between speech perception performance in spatially complex, lateralized listening scenarios and temporal fine-structure (TFS) coding at low frequencies. Young normal-hearing (NH) and two groups of elderly hearing-impaired (HI) listeners with mild or moderate hearing loss above 1.5 kHz participated in the study. Speech reception thresholds (SRTs) were estimated in the presence of either speech-shaped noise, two-, four-, or eight-talker babble played reversed, or a nonreversed two-talker masker. Target audibility was ensured by applying individualized linear gains to the stimuli, which were presented over headphones. The target and masker streams were lateralized to the same or to opposite sides of the head by introducing 0.7-ms interaural time differences between the ears. TFS coding was assessed by measuring frequency discrimination thresholds and interaural phase difference thresholds at 250 Hz. NH listeners had clearly better SRTs than the HI listeners. However, when maskers were spatially separated from the target, the amount of SRT benefit due to binaural unmasking differed only slightly between the groups. Neither the frequency discrimination threshold nor the interaural phase difference threshold tasks showed a correlation with the SRTs or with the amount of masking release due to binaural unmasking, respectively. The results suggest that, although HI listeners with normal hearing thresholds below 1.5 kHz experienced difficulties with speech understanding in spatially complex environments, these limitations were unrelated to TFS coding abilities and were only weakly associated with a reduction in binaural-unmasking benefit for spatially separated competing sources.

Congenital amusia: A cognitive disorder limited to resolved harmonics and with no peripheral basis

Pitch plays a fundamental role in audition, from speech and music perception to auditory scene analysis. Congenital amusia is a neurogenetic disorder that appears to affect primarily pitch and melody perception. Pitch is normally conveyed by the spectro-temporal fine structure of low harmonics, but some pitch information is available in the temporal envelope produced by the interactions of higher harmonics. Using 10 amusic subjects and 10 matched controls, we tested the hypothesis that amusics suffer exclusively from impaired processing of spectro-temporal fine structure. We also tested whether the inability of amusics to process acoustic temporal fine structure extends beyond pitch by measuring sensitivity to interaural time differences, which also rely on temporal fine structure. Further tests were carried out on basic intensity and spectral resolution. As expected, pitch perception based on spectro-temporal fine structure was impaired in amusics; however, no significant deficits were observed in amusics' ability to perceive the pitch conveyed via temporal-envelope cues. Sensitivity to interaural time differences was also not significantly different between the amusic and control groups, ruling out deficits in the peripheral coding of temporal fine structure. Finally, no significant differences in intensity or spectral resolution were found between the amusic and control groups. The results demonstrate a pitch-specific deficit in fine spectro-temporal information processing in amusia that seems unrelated to temporal or spectral coding in the auditory periphery. These results are consistent with the view that there are distinct mechanisms dedicated to processing resolved and unresolved harmonics in the general population, the former being altered in congenital amusia while the latter is spared.

Bigger is Not Better: Effects of Hearing Loss on Central Processing.

Figure: A: For the envelope-dominated lower frequencies, older adults with hearing loss have larger amplitudes than older adults with normal hearing. B: These differences are not seen in the temporal fine structure (TFS)-dominated high frequencies. C: There is a greater ratio of envelope to temporal fine structure in older adults with hearing loss. (Adapted from J Acoust Soc Am 2013;133[5]:3030-3038 http://scitation.aip.org/content/asa/journal/jasa/133/5/10.1121/1.4799804.)Older adults who wear hearing aids often report that speech is too loud, yet they have difficulty with the clarity of the message. With new hearing aid wearers, we often use the analogy of coming out of a theater in the middle of the day and being momentarily blinded by the brightness of the sun. As we know, however, the retinal adjustment to increased light occurs quickly, while adjustment to amplified sound is a longer process. Part of this difficulty is rooted in the well-known phenomenon of recruitment. However, changes in speech representation in the central auditory system also play a role. SWAMPING OUT THE TFS Recruitment effects can be seen in the auditory brainstem response (ABR) to clicks. In an individual with sensorineural hearing loss, brainstem latencies are delayed at low intensities but drop to normal latencies at higher intensities. The effects of hearing loss on auditory processing of a speech syllable were recently evaluated using the cABR—the ABR to complex sounds (J Acoust Soc Am 2013;133[5]:3030-3038 http://scitation.aip.org/content/asa/journal/jasa/133/5/10.1121/1.4799804). Brainstem responses were recorded to a 40-ms speech syllable /da/ in quiet and in noise in two groups of older adults. One group had clinically normal hearing and the other had mild to moderate sensorineural hearing loss and had never worn hearing aids. The group with hearing loss had higher representation of the low frequencies that comprise the speech envelope. In response to the temporal fine structure (TFS), there was no difference in frequency representation between the groups. Overall, in the group with hearing loss, there was a greater ratio of envelope to temporal fine structure than there was in the group with normal hearing, suggesting that the augmented response to the speech envelope swamps out the temporal fine structure, leading to a deficit in fine structure coding. ROLE OF TRAINING AND AMPLIFICATION These results are in line with the work of Sushrut Kale and Michael G. Heinz, who found greater coding of the envelope in auditory nerve fibers of chinchillas who had noise-induced hearing loss (J Assoc Res Otolaryngol 2010;11[4]:657-673 http://link.springer.com/article/10.1007/s10162-010-0223-6 ). When the stimuli were presented in quiet, Kale and Heinz did not find any differences in temporal fine structure coding. However, when the stimuli were presented in noise during a later study, chinchillas with hearing loss had a deficit in temporal coding compared with normal-hearing chinchillas (Nat Neurosci 2012;15[10]:1362-1364 http://http://www.nature.com/neuro/journal/v15/n10/full/nn.3216.html ). Work with cochlear implants has taught us that access to the speech envelope is adequate for hearing in quiet situations, but temporal fine structure cues may be important for hearing in background noise (Science 1995;270[5234]:303-304 http://http://www.sciencemag.org/content/270/5234/303 ). Therefore, a relative deficit in fine structure coding in individuals who wear hearing aids may account for difficulty understanding speech in background noise.Figure: Dr. KrausWork is under way to examine the effects of training and amplification on the balance of envelope and temporal fine structure representation of the speech signal.Figure: Dr. AndersonThis work was supported by the National Institutes of Health (grants T32 DC009399-01A10 and RO1 DC10016) and the Knowles Hearing Center.

Bigger is Not Better

Figure: A: For the envelope-dominated lower frequencies, older adults with hearing loss have larger amplitudes than older adults with normal hearing. B: These differences are not seen in the temporal fine structure (TFS)-dominated high frequencies. C: There is a greater ratio of envelope to temporal fine structure in older adults with hearing loss. (Adapted from J Acoust Soc Am 2013;133[5]:3030-3038 http://scitation.aip.org/content/asa/journal/jasa/133/5/10.1121/1.4799804.)Older adults who wear hearing aids frequently report that speech is too loud, yet they have difficulty with the clarity of the message.Figure: Nina Kraus, PhDWith new hearing aid wearers, we often use the analogy of coming out of a theater in the middle of the day and being momentarily blinded by the brightness of the sun. As we know, however, the retinal adjustment to increased light occurs quickly, while adjustment to amplified sound is a longer process.Figure: Samira Anderson, AuD, PhDPart of this difficulty is rooted in the well-known phenomenon of recruitment. However, changes in speech representation in the central auditory system also play a role. SWAMPING OUT THE TFS Recruitment effects can be seen in the auditory brainstem response (ABR) to clicks. In an individual with sensorineural hearing loss, brainstem latencies are delayed at low intensities but drop to normal latencies at higher intensities. The effects of hearing loss on auditory processing of a speech syllable were recently evaluated using the cABR—the ABR to complex sounds (J Acoust Soc Am 2013;133[5]:3030-3038http://scitation.aip.org/content/asa/journal/jasa/133/5/10.1121/1.4799804). Brainstem responses were recorded to a 40-ms speech syllable /da/ in quiet and in noise in two groups of older adults. One group had clinically normal hearing and the other had mild to moderate sensorineural hearing loss and had never worn hearing aids. The group with hearing loss had higher representation of the low frequencies that comprise the speech envelope. In response to the temporal fine structure (TFS), there was no difference in frequency representation between the groups. Overall, in the group with hearing loss, there was a greater ratio of envelope to temporal fine structure than there was in the group with normal hearing, suggesting that the augmented response to the speech envelope swamps out the temporal fine structure, leading to a deficit in fine structure coding. ROLE OF TRAINING AND AMPLIFICATION These results are in line with the work of Sushrut Kale and Michael G. Heinz, who found greater coding of the envelope in auditory nerve fibers of chinchillas who had noise-induced hearing loss (J Assoc Res Otolaryngol 2010;11[4]:657-673http://link.springer.com/article/10.1007/s10162-010-0223-6). When the stimuli were presented in quiet, Kale and Heinz did not find any differences in temporal fine structure coding. However, when the stimuli were presented in noise during a later study, chinchillas with hearing loss had a deficit in temporal coding compared with normal-hearing chinchillas (Nat Neurosci 2012;15[10]:1362-1364http://www.nature.com/neuro/journal/v15/n10/full/nn.3216.html). Work with cochlear implants has taught us that access to the speech envelope is adequate for hearing in quiet situations, but temporal fine structure cues may be important for hearing in background noise (Science 1995;270[5234]:303-304http://www.sciencemag.org/content/270/5234/303). Therefore, a relative deficit in fine structure coding in individuals who wear hearing aids may account for difficulty understanding speech in background noise. Work is under way to examine the effects of training and amplification on the balance of envelope and temporal fine structure representation of the speech signal. This work was supported by the National Institutes of Health (grants T32 DC009399-01A10 and RO1 DC10016) and the Knowles Hearing Center. HJ Return to www.thehearingjournal.com

Modeling disrupted tonotopicity of temporal coding following sensorineural hearing loss

Perceptual studies suggest that sensorineural hearing loss (SNHL) affects neural coding of temporal fine structure (TFS) more than envelope (ENV). Although the “quantity” of TFS coding is degraded only in background noise, Wiener-kernel analyses suggest SNHL disrupts tonotopicity (i.e., the “quality”) of TFS coding for complex sounds more than ENV coding. Specifically, auditory-nerve (AN) fibers in noise-exposed chinchillas can have their dominant TFS component located within their tuning-curve tail (i.e., the wrong place) while their ENV response remains centered at CF. Here, the ability of a AN model [Zilany and Bruce (2007)] to replicate this dissociation between TFS and ENV tonotopicity was evaluated. By varying the degree of outer- and inner-hair-cell damage, hypothesized factors such as hypersensitive tails and tip-to-tail ratio were evaluated. The model predicted the main trends in our physiological data: (1) no loss of tonotopicity for lower CFs without a clear tip/tail distinction, (2) more easily disrupted TFS tonotopicity than ENV (without requiring hypersensitive tails), and (3) disruption of both TFS and ENV tonotopicity for severely degraded tips. This computational approach allows exploration of the interaction between tip-tail ratio and phase-locking roll-off, and whether amplification strategies can restore cochlear tonotopicity. [Work supported by NIH grants R01-DC009838 and F32-DC012236.]

Complementary correlation may offer a new approach to better understand temporal fine structure coding

Historically, sound has been separated into a slow-varying envelope and a component that rapidly varies, called the temporal fine structure (TFS). Perceptual studies suggest that users of cochlear implants can benefit from better TFS coding strategies, as they lead to improved speech understanding in noise. Yet given the standard signal processing methodology used in the auditory neuroscience community, namely the Hilbert transform, findings in such prior studies have been limited by their use of the heavily distorted estimates of the TFS information (through the Hilbert phase of the related analytic signal). Complementary correlation is a new mathematical tool with potential to advance our understanding of temporal coding in neuroscience. This new mathematical formulation can be defined simply by dropping the usual complex conjugation operation from the canonical definitions of correlation, coherence, variance, magnitude-squared estimators of power, and other similar common second-order statistical quantities and their estimators. We will show that a complementary correlation approach, which provides a more complete characterization of TFS information, will provide measurable perceptual benefits by reducing distortion in the original speech signal.

Frequency modulation detection as a measure of temporal processing: Age-related monaural and binaural effects

The detection of low-rate frequency modulation (FM) carried by a low-frequency tone has been employed as a means of assessing the fidelity of temporal fine structure coding. Detection of low-rate FM can be made more acute, relative to the monaural case, by the addition of a pure tone to the contralateral ear. This study examined whether FM detection in the 500-Hz region could be further improved by using a binaural stimulation mode where the modulator was antiphasic across the two ears. The study also sought to determine whether these dichotic FM conditions were beneficial in identifying the emergence of a temporal fine structure processing deficiency relatively early in the aging process. Young, mid-aged, and older listeners (n = 12 per group) were tested. The results demonstrated better FM acuity in the dichotic task irrespective of listener age. Dichotic FM detection also differentiated between age groups more definitively than diotic detection, especially in terms of distinguishing mid-aged from older listeners. In the group of older listeners, dichotic FM detection was weakly associated with absolute sensitivity to the carrier. In addition, this group failed to show a dichotic benefit in the presence of a marked asymmetry in sensation level across ears. The overall pattern of results suggests that dichotic FM measurements have advantages over monaural measurements for the purposes of assessing age-related temporal processing effects, although a marked asymmetry in absolute thresholds across ears could undermine these advantages.

Modeling the effects of sensorineural hearing loss on temporal coding in the auditory nerve

Recent psychoacoustical studies have suggested a temporal-fine-structure (TFS) deficit in listeners with sensorineural hearing loss (SNHL). Physiological studies generally have not found a reduction in the strength of TFS coding in auditory-nerve responses; however, a number of other effects on temporal coding have been found that may be related to these perceptual temporal processing deficits [e.g., Kale and Heinz, JARO (2010); Scheidt et al., Hear. Res. (2010)]. These physiological effects of SNHL include enhanced envelope (ENV) coding, changes in relative TFS/ENV coding that depend on stimulus bandwidth, broadened tuning, loss of tonotopicity for complex sounds, reduced latencies and traveling-wave delays, and altered temporal dynamics such as enhanced onset responses, faster adaptation, and slower recovery from stimulation. The current study evaluated which of these physiological effects were accounted for by an existing computational model of auditory-nerve responses [Zilany et al., JASA (2009)], and the extent to which these properties are predicted to depend on outer- vs. inner-hair-cell dysfunction. The degree to which existing models account for these effects of SNHL on temporal coding will help to elucidate the responsible mechanisms, and is important for future diagnostic and rehabilitative applications. [Work supported by NIH Grant No. R01DC009838.]

Electrophysiological Measurement of Binaural Beats

The purpose of this study was to determine the reliability of the electrophysiological binaural beat steady state response as a gauge of temporal fine structure coding, particularly as it relates to the aging auditory system. The hypothesis was that the response would be more robust in a lower, than in a higher, frequency region and in younger, than in older, adults. Two experiments were undertaken. The first measured the 40 Hz binaural beat steady state response elicited by tone pairs in two frequency regions: lower (390 and 430 Hz tone pair) and higher (810 and 850 Hz tone pair). Frequency following responses (FFRs) evoked by the tones were also recorded. Ten young adults with normal hearing participated. The second experiment measured the binaural beat and FFRs in older adults but only in the lower frequency region. Fourteen older adults with relatively normal hearing participated. Response metrics in both experiments included response component signal-to-noise ratio (F statistic) and magnitude-squared coherence. Experiment 1 showed that FFRs were elicited in both frequency regions but were more robust in the lower frequency region. Binaural beat responses elicited by the lower frequency pair of tones showed greater amplitude fluctuation within a participant than the respective FFRs. Experiment 2 showed that older adults exhibited similar FFRs to younger adults, but proportionally fewer older participants showed binaural beat responses. Age differences in onset responses were also observed. The lower prevalence of the binaural beat response in older adults, despite the presence of FFRs, provides tentative support for the sensitivity of this measure to age-related deficits in temporal processing. However, the lability of the binaural beat response advocates caution in its use as an objective measure of fine structure coding.

Psychophysiological Analyses Demonstrate the Importance of Neural Envelope Coding for Speech Perception in Noise

Understanding speech in noisy environments is often taken for granted; however, this task is particularly challenging for people with cochlear hearing loss, even with hearing aids or cochlear implants. A significant limitation to improving auditory prostheses is our lack of understanding of the neural basis for robust speech perception in noise. Perceptual studies suggest the slowly varying component of the acoustic waveform (envelope, ENV) is sufficient for understanding speech in quiet, but the rapidly varying temporal fine structure (TFS) is important in noise. These perceptual findings have important implications for cochlear implants, which currently only provide ENV; however, neural correlates have been difficult to evaluate due to cochlear transformations between acoustic TFS and recovered neural ENV. Here, we demonstrate the relative contributions of neural ENV and TFS by quantitatively linking neural coding, predicted from a computational auditory nerve model, with perception of vocoded speech in noise measured from normal hearing human listeners. Regression models with ENV and TFS coding as independent variables predicted speech identification and phonetic feature reception at both positive and negative signal-to-noise ratios. We found that: (1) neural ENV coding was a primary contributor to speech perception, even in noise; and (2) neural TFS contributed in noise mainly in the presence of neural ENV, but rarely as the primary cue itself. These results suggest that neural TFS has less perceptual salience than previously thought due to cochlear signal processing transformations between TFS and ENV. Because these transformations differ between normal and impaired ears, these findings have important translational implications for auditory prostheses.

Relations between perceptual measures of temporal processing, auditory-evoked brainstem responses and speech intelligibility in noise

This study investigates behavioural and objective measures of temporal auditory processing and their relation to the ability to understand speech in noise. The experiments were carried out on a homogeneous group of seven hearing-impaired listeners with normal sensitivity at low frequencies (up to 1 kHz) and steeply sloping hearing losses above 1 kHz. For comparison, data were also collected for five normal-hearing listeners. Temporal processing was addressed at low frequencies by means of psychoacoustical frequency discrimination, binaural masked detection and amplitude modulation (AM) detection. In addition, auditory brainstem responses (ABRs) to clicks and broadband rising chirps were recorded. Furthermore, speech reception thresholds (SRTs) were determined for Danish sentences in speech-shaped noise. The main findings were: (1) SRTs were neither correlated with hearing sensitivity as reflected in the audiogram nor with the AM detection thresholds which represent an envelope-based measure of temporal resolution; (2) SRTs were correlated with frequency discrimination and binaural masked detection which are associated with temporal fine-structure coding; (3) The wave-V thresholds for the chirp-evoked ABRs indicated a relation to SRTs and the ability to process temporal fine structure. Overall, the results demonstrate the importance of low-frequency temporal processing for speech reception which can be affected even if pure-tone sensitivity is close to normal.

Temporal fine structure in cochlear implants: Preliminary speech perception results in Cantonese-speaking implant users

Conclusion: Acute comparisons between continuous interleaved sampling (CIS) and a temporal fine structure (TFS) coding strategy in Cantonese-speaking cochlear implant (CI) users did not reveal any significant differences in speech perception. Performance with the unfamiliar TFS coding strategy was on a par with CIS. Benefits of extended fine structure use observed in other studies should be investigated for tonal languages. Objectives: CIS-based stimulation strategies lack an explicit representation of fine structure, which is crucial for tonal language speech perception. The aim of this study was to assess speech recognition with a TFS coding strategy in Cantonese-speaking CI users with no prior fine structure experience. Methods: The fine structure coding strategy encodes TFS on a few apical channels, while the remaining more basal channels carry CIS stimuli. Twelve MED-EL implantees and long-term CIS users participated in a study comparing recognition for Cantonese lexical tones and CHINT sentences between CIS and fine structure stimulation. Results: Mean tone identification scores in 12 subjects were 59.2% with CIS and 59.2% with fine structure stimulation using 4 TFS channels, mean scores of CHINT sentences in 8 subjects were 54.2% with CIS and 55.9% with TFS stimulation. Differences between the two strategies were not significant for any speech test. Two additional versions of TFS strategy and pulse rates were tested in six subjects. No significant differences between strategies were found.

Neural coding of envelope and fine structure in noise degraded speech

Numerous perceptual studies have revealed that envelope is sufficient for speech perception in quiet, but that temporal fine structure (TFS) is required for speech perception in noise. However, the neural correlates of these perceptual observations remain unknown. The primary focus of the present work was to develop and evaluate neural cross correlation coefficient (CCC) metrics to quantify envelope and TFS coding in auditory‐nerve responses to noise degraded speech. Shuffled auto‐ and cross‐correlogram analyses were used to compute separate CCCs to quantify stimulus‐related envelope and fine structure based on neural spike train data from a computational auditory‐nerve model. The neural CCCs have a wide dynamic range as revealed by near‐zero values for uncorrelated conditions and near‐one values for correlated conditions based on broadband noise responses. Spectrally matched noise was systematically added to a speech sentence at different signal‐to‐noise ratios (SNRs). Initial analyses reveal that CCC_ENV > CCC_TFS for positive SNRs, whereas CCC_TFS > CCC_ENV for negative SNRs. Predicted effects of hearing loss on envelope and TFS coding will also be discussed. These neural metrics can be used to evaluate temporal coding of speech with implications for cochlear‐implant and hearing‐aid strategies. Supported by NIH‐NIDCD.

Transcriptional Regulation of Cortical Interneuron Development: Figure 1.

People with cochlear hearing loss have substantial difficulty understanding speech in real-world listening environments (e.g., restaurants), even with amplification from a modern digital hearing aid. Unfortunately, a disconnect remains between human perceptual studies implicating diminished sensitivity to fast acoustic temporal fine structure (TFS) and animal studies showing minimal changes in neural coding of TFS or slower envelope (ENV) structure. Here, we used general system-identification (Wiener kernel) analyses of chinchilla auditory nerve fiber responses to Gaussian noise to reveal pronounced distortions in tonotopic coding of TFS and ENV following permanent, noise-induced hearing loss. In basal fibers with characteristic frequencies (CFs) &gt;1.5 kHz, hearing loss introduced robust nontonotopic coding (i.e., at the wrong cochlear place) of low-frequency TFS, while ENV responses typically remained at CF. As a consequence, the highest dominant frequency of TFS coding in response to Gaussian noise was 2.4 kHz in noise-overexposed fibers compared with 4.5 kHz in control fibers. Coding of ENV also became nontonotopic in more pronounced cases of cochlear damage. In apical fibers, more classical hearing-loss effects were observed, i.e., broadened tuning without a significant shift in best frequency. Because these distortions and dissociations of TFS/ENV disrupt tonotopicity, a fundamental principle of auditory processing necessary for robust signal coding in background noise, these results have important implications for understanding communication difficulties faced by people with hearing loss. Further, hearing aids may benefit from distinct amplification strategies for apical and basal cochlear regions to address fundamentally different coding deficits. <b>SIGNIFICANCE STATEMENT</b> Speech-perception problems associated with noise overexposure are pervasive in today9s society, even with modern digital hearing aids. Unfortunately, the underlying physiological deficits in neural coding remain unclear. Here, we used innovative system-identification analyses of auditory nerve fiber responses to Gaussian noise to uncover pronounced distortions in coding of rapidly varying acoustic temporal fine structure and slower envelope cues following noise trauma. Because these distortions degrade and diminish the tonotopic representation of temporal acoustic features, a fundamental principle of auditory processing, the results represent a critical advancement in our understanding of the physiological bases of communication disorders. The detailed knowledge provided by this work will help guide the design of signal-processing strategies aimed at alleviating everyday communication problems for people with hearing loss.