Temporal Fine Structure Processing Research Articles

Individual Differences Elucidate the Perceptual Benefits Associated with Robust Temporal Fine-Structure Processing.

The auditory system is unique among sensory systems in its ability to phase lock to and precisely follow very fast cycle-by-cycle fluctuations in the phase of sound-driven cochlear vibrations. Yet, the perceptual role of this temporal fine structure (TFS) code is debated. This fundamental gap is attributable to our inability to experimentally manipulate TFS cues without altering other perceptually relevant cues. Here, we circumnavigated this limitation by leveraging individual differences across 200 participants to systematically compare variations in TFS sensitivity to performance in a range of speech perception tasks. TFS sensitivity was assessed through detection of interaural time/phase differences, while speech perception was evaluated by word identification under noise interference. Results suggest that greater TFS sensitivity is not associated with greater masking release from fundamental-frequency or spatial cues, but appears to contribute to resilience against the effects of reverberation. We also found that greater TFS sensitivity is associated with faster response times, indicating reduced listening effort. These findings highlight the perceptual significance of TFS coding for everyday hearing.

Relation between temporal fine structure processing and global processing speed

Strelcyk et al. (2019) recently found that interaural phase discrimination in older hearing-impaired listeners was correlated with both visuospatial processing speed and interaural level discrimination. This suggests that temporal fine structure (TFS) processing relies on global processing speed and/or spatial cognition, though it is possible that, generally, complex auditory discrimination engages multiple cognitive domains. Here, 50 Veterans (mean age = 48.1, range = 30–60) with normal or near-normal hearing completed batteries of temporal processing and cognitive tests. Composite cognitive test scores reflecting processing speed/executive function (PS-EX) and working memory (WM) were obtained. Temporal processing tasks included measures of envelope (ENV; gap duration discrimination, forward masking) and TFS (frequency modulation detection, interaural phase modulation detection) processing. Bayesian hierarchical regression was used to fit psychometric functions simultaneously to all ENV and TFS tasks in all subjects. Fixed effects of PS-EX and WM on thresholds and slopes were estimated for the psychometric functions in each temporal task. In general, (i) PS-EX and WM influenced both TFS and ENV thresholds, but not slopes; and (ii) TFS thresholds were best explained by PS-EX scores while ENV thresholds were best explained by WM scores. These findings suggest a specific relation between global processing speed and TFS.

Traditional Martial Arts and Shooting Training Have Different Effects on Auditory Fine Structure Processing Ability - Evidence from Behavioral Tests and fMRI

In this work, we explore the influence of traditional martial arts and shooting training on the ability of auditory Temporal Fine Structure (TFS) processing. Twenty-five college students participated in the experiment, including 8 traditional martial arts practitioners, 8 high-level shooting athletes, and 9 control groups without any regular exercise habits. The BIC (break in interaural correlation) delay threshold and TFS1 test were used to evaluate the transient storage capacity and sensitivity of TFS, respectively. An fMRI test was performed after the behavioral tests. The results found that the traditional martial arts group had stronger TFS sensitivity, while the shooting group had stronger TFS storage capacity and the performance of the behavioral tests of the shooting group is related to the fALFF value of the brain areas of interest. The effects of exercise training on hearing ability may have a specific neural basis and can be used for early intervention in potential hearing impairment.

Effect of Musical Training on Temporal Resolution and Temporal Fine Structure Processing

Background and Aim: Many aspects and features of auditory system can be improved by musical training. This study aimed to investigate the effects of a stringed musical instrument playing course on temporal resolution and temporal fine structure (TFS) processing. Methods: This analytical cross-sectional study was conducted on 44 normal-hearing adults aged 20–40 years divided to two groups. The first group included 22 stringed musical instruments players (13 males) with at least three years of experience, and the second group were 22 non- players (13 males). The random gap detection test (RGDT) was used to measure temporal resolution aspect of hearing. For TFS processing, latest version of temporal fine structure-adaptive frequency (TFS-AF) test was used. Results: The TFS-AF results showed no statistically significant difference between groups in different interaural phase differences (IPDs). The RGDT results showed significant differences between groups at 500, 1000 and 2000 Hz, but not at 4000 Hz. Spearman correlation test results showed no statistically significant correlation between the results of TFS-AF and RGDT. Conclusion: Musical training has no effect on TFS processing but considerably enhances gap detection ability. Their underlying mechanisms for TFS processing and gap detection are different.

The Effects of 40-Hz Binaural Beat on Working Memory

Aim of the study is to determine whether the auditory processing of temporal fine structure (TFS) is affected with normal aging, even in the presence of normal audiometric hearing and fine cognitive state; and, if it is, to see whether a comparable effect is also observed in the processing of a diotic change in sound envelope. The event-related potentials (ERPs) to binaural beats (BBs), which are the responses of the binaural mechanisms processing TFS of a sound, and the ERPs to diotic amplitude modulation (AM) stimuli, which are the responses of the monaural mechanisms processing the changes in its envelope, were recorded from thirteen young university students and ten senior but active university professors, all with normal hearing in low frequencies. To obtain directly the specific BB responses without confounding monaural frequency change-evoked responses, we used single-cycle BB stimuli with temporary sub-threshold frequency shifts. BBs of a 250-Hz tone and diotic AM of the same tone with similar perceptual salience were presented with 2-second stimulus onset asynchrony. The N1 components of the ERPs to both stimuli displayed notable age-dependent changes in their scalp topography and significant amplitude reduction and latency prolongation in the elderly. These amplitude and latency changes were at similar rates for the two stimulus types, implying that the auditory TFS and envelope processing mechanisms are proportionally affected by physiological aging. These results may serve as control data in future studies investigating the effect of aging-associated cognitive pathologies on auditory TFS processing.

Temporal integration of monaural and dichotic frequency modulation.

Frequency modulation (FM) detection at low modulation frequencies is commonly used as an index of temporal fine-structure processing. The present study evaluated the rate of improvement in monaural and dichotic FM across a range of test parameters. In experiment I, dichotic and monaural FM detection was measured as a function of duration and modulator starting phase. Dichotic FM thresholds were lower than monaural FM thresholds and the modulator starting phase had no effect on detection. Experiment II measured monaural FM detection for signals that differed in modulation rate and duration such that the improvement with duration in seconds (carrier) or cycles (modulator) was compared. Monaural FM detection improved monotonically with the number of modulation cycles, suggesting that the modulator is extracted prior to detection. Experiment III measured dichotic FM detection for shorter signal durations to test the hypothesis that dichotic FM relies primarily on the signal onset. The rate of improvement decreased as duration increased, which is consistent with the use of primarily onset cues for the detection of dichotic FM. These results establish that improvement with duration occurs as a function of the modulation cycles at a rate consistent with the independent-samples model for monaural FM, but later cycles contribute less to detection in dichotic FM.

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.

Portable Automated Rapid Testing (PART) for auditory assessment: Validation in a young adult normal-hearing population.

This study aims to determine the degree to which Portable Automated Rapid Testing (PART), a freely available program running on a tablet computer, is capable of reproducing standard laboratory results. Undergraduate students were assigned to one of three within-subject conditions that examined repeatability of performance on a battery of psychoacoustical tests of temporal fine structure processing, spectro-temporal amplitude modulation, and targets in competition. The repeatability condition examined test/retest with the same system, the headphones condition examined the effects of varying headphones (passive and active noise-attenuating), and the noise condition examined repeatability in the presence of recorded cafeteria noise. In general, performance on the test battery showed high repeatability, even across manipulated conditions, and was similar to that reported in the literature. These data serve as validation that suprathreshold psychoacoustical tests can be made accessible to run on consumer-grade hardware and perform in less controlled settings. This dataset also provides a distribution of thresholds that can be used as a normative baseline against which auditory dysfunction can be identified in future work.

Spatial Hearing by Bilateral Cochlear Implant Users With Temporal Fine-Structure Processing.

Several studies have demonstrated the advantages of the bilateral vs. unilateral cochlear implantation in listeners with bilateral severe to profound hearing loss. However, it remains unclear to what extent bilaterally implanted listeners have access to binaural cues, e.g., accurate processing of interaural timing differences (ITDs) for low-frequency sounds (<1.5 kHz) and interaural level differences (ILDs) for high frequencies (>3 kHz). We tested 25 adult listeners, bilaterally implanted with MED-EL cochlear implant (CI) devices, with and without fine-structure (FS) temporal processing as encoding strategy in the low-frequency channels. In order to assess whether the ability to process binaural cues was affected by fine-structure processing, we performed psychophysical ILD and ITD sensitivity measurements and free-field sound localization experiments. We compared the results of the bilaterally implanted listeners with different numbers of FS channels. All CI listeners demonstrated good sensitivity to ILDs, but relatively poor to ITD cues. Although there was a large variability in performance, some bilateral CI users showed remarkably good localization skills. The FS coding strategy for bilateral CI hearing did not improve fine-structure ITD processing for spatial hearing on a group level. However, some CI listeners were able to exploit weakly informative temporal cues to improve their low-frequency spatial perception.

Effects of aging on event-related potentials to single-cycle binaural beats and diotic amplitude modulation of a tone

Aim of the study is to determine whether the auditory processing of temporal fine structure (TFS) is affected with normal aging, even in the presence of normal audiometric hearing and fine cognitive state; and, if it is, to see whether a comparable effect is also observed in the processing of a diotic change in sound envelope. The event-related potentials (ERPs) to binaural beats (BBs), which are the responses of the binaural mechanisms processing TFS of a sound, and the ERPs to diotic amplitude modulation (AM) stimuli, which are the responses of the monaural mechanisms processing the changes in its envelope, were recorded from thirteen young university students and ten senior but active university professors, all with normal hearing in low frequencies. To obtain directly the specific BB responses without confounding monaural frequency change-evoked responses, we used single-cycle BB stimuli with temporary sub-threshold frequency shifts. BBs of a 250-Hz tone and diotic AM of the same tone with similar perceptual salience were presented with 2-second stimulus onset asynchrony. The N1 components of the ERPs to both stimuli displayed notable age-dependent changes in their scalp topography and significant amplitude reduction and latency prolongation in the elderly. These amplitude and latency changes were at similar rates for the two stimulus types, implying that the auditory TFS and envelope processing mechanisms are proportionally affected by physiological aging. These results may serve as control data in future studies investigating the effect of aging-associated cognitive pathologies on auditory TFS processing.

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.

Bottom-up and top-down neural signatures of disordered multi-talker speech perception in adults with normal hearing.

In social settings, speech waveforms from nearby speakers mix together in our ear canals. Normally, the brain unmixes the attended speech stream from the chorus of background speakers using a combination of fast temporal processing and cognitive active listening mechanisms. Of >100,000 patient records,~10% of adults visited our clinic because of reduced hearing, only to learn that their hearing was clinically normal and should not cause communication difficulties. We found that multi-talker speech intelligibility thresholds varied widely in normal hearing adults, but could be predicted from neural phase-locking to frequency modulation (FM) cues measured with ear canal EEG recordings. Combining neural temporal fine structure processing, pupil-indexed listening effort, and behavioral FM thresholds accounted for 78% of the variability in multi-talker speech intelligibility. The disordered bottom-up and top-down markers of poor multi-talker speech perception identified here could inform the design of next-generation clinical tests for hidden hearing disorders.

Perception of lexical stress cued by low-frequency pitch and insights into speech perception in noise for cochlear implant users and normal hearing adults.

Cochlear implant (CI) users show great difficulty for understanding speech in noise and this fact may partly stem from their poor low-frequency (LF) pitch perception and temporal fine structure (TFS) processing. Clinical assessment of pitch perception is usually based on non-speech tasks. However, linguistically relevant contexts such as lexical stress may better reflect the role of pitch in speech perception, especially for everyday speech where background noise is inevitable. Hence, the study aimed to assess perception of lexical stress cued by LF pitch and TFS cues for CI and normal hearing (NH) listeners, and to investigate relationships with speech perception in noise. The low-pass-filtered Word Stress Pattern (WSP-LPF) test was used to evaluate perception of lexical stress cued by LF pitch. Speech perception was assessed with the sentence test with adaptive randomized roving level (STARR) test which presented everyday sentences at low, medium, and high levels in a fluctuating noise to estimate a Speech Reception Threshold. This new test intended to give a realistic estimate for real-world listening. Median WSP-LPF scores in NH (N = 18) and CI listeners (N = 18) were 12.0Hz and 67.0Hz, respectively. The corresponding STARR scores were - 9.1dB and 17.3dB. Group differences were statistically significant (p < 0.001). Analysis showed significant positive correlations for NH (rs = 0.50) and CI listeners (rs = 0.60). Present findings reveal stronger correlations than previous studies using non-speech materials, supporting that CI listeners' poor speech perception in noise might be strongly associated with their inability for LF pitch perception and TFS processing.

Uncertainty in location, level and fundamental frequency results in informational masking in a vowel discrimination task for young and elderly subjects

Informational masking (IM) is defined as the compromised ability to perceive and analyze signals from a single sound source in a cacophony of sounds from other sources even if the excitation patterns produced by these signals in the auditory periphery are well separated from those produced by the sounds from the other sources. IM that causes an elevation of discrimination thresholds is affected by the similarity between target and masker and by stimulus uncertainty. Here, six young and six elderly subjects were asked to discriminate between sequentially presented reference and target vowels of the vowel pairs /I/-/i/, /æ/-/ε/, and /α/-/Λ/. Psychometric functions were collected characterizing the discrimination of target vowels from reference vowels. Target vowels differed from the reference by one of seven steps shifting the three formants of a reference vowel towards the formants of the corresponding target vowel. Stimulus statistics were varied, generating uncertainty by non-informative but potentially distracting location, level, and fundamental frequency changes or all three combined. Young subjects tested with distracting changes applied to the target vowels only, the reference vowels only, or the target and reference vowels showed similar amounts of IM for all three conditions. Elderly subjects were tested with distracting changes applied to target vowels only. Applying uncertainty only to the target vowels led to worse vowel discrimination thresholds for young and elderly subjects and thresholds increased most for the three distracting changes combined. Elderly subjects showed higher vowel discrimination thresholds than young subjects, but the increase in vowel discrimination thresholds due to IM did not differ between young and elderly subjects. The temporal fine structure processing of elderly subjects was degraded in comparison to young subjects, but it was only correlated with the discrimination threshold for vowel pair /I/-/i/.

Effects of Musical Training and Hearing Loss on Fundamental Frequency Discrimination and Temporal Fine Structure Processing: Psychophysics and Modeling

Several studies have shown that musical training leads to improved fundamental frequency (F0) discrimination for young listeners with normal hearing (NH). It is unclear whether a comparable effect of musical training occurs for listeners whose sensory encoding of F0 is degraded. To address this question, the effect of musical training was investigated for three groups of listeners (young NH, older NH, and older listeners with hearing impairment, HI). In a first experiment, F0 discrimination was investigated using complex tones that differed in harmonic content and phase configuration (sine, positive, or negative Schroeder). Musical training was associated with significantly better F0 discrimination of complex tones containing low-numbered harmonics for all groups of listeners. Part of this effect was caused by the fact that musicians were more robust than non-musicians to harmonic roving. Despite the benefit relative to their non-musicians counterparts, the older musicians, with or without HI, performed worse than the young musicians. In a second experiment, binaural sensitivity to temporal fine structure (TFS) cues was assessed for the same listeners by estimating the highest frequency at which an interaural phase difference was perceived. Performance was better for musicians for all groups of listeners and the use of TFS cues was degraded for the two older groups of listeners. These findings suggest that musical training is associated with an enhancement of both TFS cues encoding and F0 discrimination in young and older listeners with or without HI, although the musicians’ benefit decreased with increasing hearing loss. Additionally, models of the auditory periphery and midbrain were used to examine the effect of HI on F0 encoding. The model predictions reflected the worsening in F0 discrimination with increasing HI and accounted for up to 80 % of the variance in the data.

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.

Low-rate frequency modulation detection across the lifespan

Frequency modulation (FM) detection at a 2-Hz rate is thought to rely on temporal cues. For a 500-Hz carrier, this reliance is heightened by the use of dichotic presentation. Here, the modulator is inverted across ears such that binaural beats can cue FM. Detection acuity for FM is better in the dichotic than diotic condition, promoting the task as a measure of temporal fine structure (TFS) sensitivity. Older adults show less benefit of dichotic presentation than young adults, suggesting deficient TFS processing. This study extended the work to school-age children in order to gain a perspective of TFS processing across the lifespan. Children aged 4–10 yrs showed poorer FM detection in both diotic and dichotic conditions re young adults, and their performance was more similar to that of older adults. The dependence of the task on TFS processing was further tested by adding background noise. This reduced FM detection acuity but more for the dichotic than diotic condition. Overall, these results support the dependence of the FM detection task on TFS processing and demonstrate that such processing appears compromised at both ends of the age spectrum. [Work supported by NIDCD DC01507 (JHG) and DC00397 [EB].)Frequency modulation (FM) detection at a 2-Hz rate is thought to rely on temporal cues. For a 500-Hz carrier, this reliance is heightened by the use of dichotic presentation. Here, the modulator is inverted across ears such that binaural beats can cue FM. Detection acuity for FM is better in the dichotic than diotic condition, promoting the task as a measure of temporal fine structure (TFS) sensitivity. Older adults show less benefit of dichotic presentation than young adults, suggesting deficient TFS processing. This study extended the work to school-age children in order to gain a perspective of TFS processing across the lifespan. Children aged 4–10 yrs showed poorer FM detection in both diotic and dichotic conditions re young adults, and their performance was more similar to that of older adults. The dependence of the task on TFS processing was further tested by adding background noise. This reduced FM detection acuity but more for the dichotic than diotic condition. Overall, these results support the ...

Temporal Fine Structure Processing, Pitch, and Speech Perception in Adult Cochlear Implant Recipients.

The aim of the study was to investigate the link between temporal fine structure (TFS) processing, pitch, and speech perception performance in adult cochlear implant (CI) recipients, including bimodal listeners who may benefit better low-frequency (LF) temporal coding in the contralateral ear. The study participants were 43 adult CI recipients (23 unilateral, 6 bilateral, and 14 bimodal listeners). Two new LF pitch perception tests-harmonic intonation (HI) and disharmonic intonation (DI)-were used to evaluate TFS sensitivity. HI and DI were designed to estimate a difference limen for discrimination of tone changes based on harmonic or inharmonic pitch glides. Speech perception was assessed using the newly developed Italian Sentence Test with Adaptive Randomized Roving level (STARR) test where sentences relevant to everyday contexts were presented at low, medium, and high levels in a fluctuating background noise to estimate a speech reception threshold (SRT). Although TFS and STARR performances in the majority of CI recipients were much poorer than those of hearing people reported in the literature, a considerable intersubject variability was observed. For CI listeners, median just noticeable differences were 27.0 and 147.0 Hz for HI and DI, respectively. HI outcomes were significantly better than those for DI. Median STARR score was 14.8 dB. Better performers with speech reception thresholds less than 20 dB had a median score of 8.6 dB. A significant effect of age was observed for both HI/DI tests, suggesting that TFS sensitivity tended to worsen with increasing age. CI pure-tone thresholds and duration of profound deafness were significantly correlated with STARR performance. Bimodal users showed significantly better TFS and STARR performance for bimodal listening than for their CI-only condition. Median bimodal gains were 33.0 Hz for the HI test and 95.0 Hz for the DI test. DI outcomes in bimodal users revealed a significant correlation with unaided hearing thresholds for octave frequencies lower than 1000 Hz. Median STARR scores were 17.3 versus 8.1 dB for CI only and bimodal listening, respectively. STARR performance was significantly correlated with HI findings for CI listeners and with those of DI for bimodal listeners. LF pitch perception was found to be abnormal in the majority of adult CI recipients, confirming poor TFS processing of CIs. Similarly, the STARR findings reflected a common performance deterioration with the HI/DI tests, suggesting the cause probably being a lack of access to TFS information. Contralateral hearing aid users obtained a remarkable bimodal benefit for all tests. Such results highlighted the importance of TFS cues for challenging speech perception and the relevance to everyday listening conditions. HI/DI and STARR tests show promise for gaining insights into how TFS and speech perception are being limited and may guide the customization of CI program parameters and support the fine tuning of bimodal listening.

A two-path model of auditory modulation detection using temporal fine structure and envelope cues.

A model using temporal-envelope cues was previously developed to explain perceptual interference effects between amplitude modulation and frequency modulation (FM). As that model could not accurately predict FM sensitivity and the interference effects, temporal fine structure (TFS) cues were added to the model. Thus, following the initial stage of the model consisting of a linear filter bank simulating cochlear filtering, processing was split into an 'envelope path' based on envelope power cues and a 'TFS path' based on a measure of the distribution of time intervals between successive zero-crossings. This yielded independent detectability indices for envelope and TFS cues, which were optimally combined to produce a single decision statistic. Independent internal noises in the envelope and TFS paths were adjusted to match the data. Simulations indicate that TFS cues are required to account for FM data for young normal-hearing listeners and that TFS processing is impaired for both older normal-hearing and hearing-impaired listeners. The role of TFS was further assessed by relating the monaural FM sensitivity to measures of interaural phase difference, commonly assumed to rely on binaural TFS sensitivity. The model demonstrates that binaural TFS sensitivity is considerably lower than monaural TFS sensitivity. Similar to FM thresholds, interaural phase difference sensitivity declined with age and hearing loss, although higher degradations were observed in binaural temporal processing compared to monaural processing. Overall, this model provides a novel tool to explore the mechanisms involved in FM processing in the normal auditory system and the degradations in FM sensitivity with ageing and hearing loss.

Distorting temporal fine structure by phase shifting and its effects on speech intelligibility and neural phase locking

Envelope (E) and temporal fine structure (TFS) are important features of acoustic signals and their corresponding perceptual function has been investigated with various listening tasks. To further understand the underlying neural processing of TFS, experiments in humans and animals were conducted to demonstrate the effects of modifying the TFS in natural speech sentences on both speech recognition and neural coding. The TFS of natural speech sentences was modified by distorting the phase and maintaining the magnitude. Speech intelligibility was then tested for normal-hearing listeners using the intact and reconstructed sentences presented in quiet and against background noise. Sentences with modified TFS were then used to evoke neural activity in auditory neurons of the inferior colliculus in guinea pigs. Our study demonstrated that speech intelligibility in humans relied on the periodic cues of speech TFS in both quiet and noisy listening conditions. Furthermore, recordings of neural activity from the guinea pig inferior colliculus have shown that individual auditory neurons exhibit phase locking patterns to the periodic cues of speech TFS that disappear when reconstructed sounds do not show periodic patterns anymore. Thus, the periodic cues of TFS are essential for speech intelligibility and are encoded in auditory neurons by phase locking.