Effects of temporal fine structure on the lateralization of speech and on speech understanding in noise

Behavioral characterization of the cochlear amplifier lesion due to loss of function of stereocilin (STRC) in human subjects

Loss of function of stereocilin (STRC) is the second most common cause of inherited hearing loss. The loss of the stereocilin protein, encoded by the STRC gene, induces the loss of connection between outer hair cells and tectorial membrane. This only affects the outer hair cells (OHCs) function, involving deficits of active cochlear frequency selectivity and amplifier functions despite preservation of normal inner hair cells. Better understanding of cochlear features associated with mutation of STRC will improve our knowledge of normal cochlear function, the pathophysiology of hearing impairment, and potentially enhance hearing aid and cochlear implant signal processing. Nine subjects with homozygous or compound heterozygous loss of function mutations in STRC were included, age 7-24 years. Temporal and spectral modulation perception were measured, characterized by spectral and temporal modulation transfer functions. Speech-in-noise perception was studied with spondee identification in adaptive steady-state noise and AzBio sentences with 0 and -5 dB SNR multitalker babble. Results were compared with normal hearing (NH) and cochlear implant (CI) listeners to place STRC-/- listeners' hearing capacity in context. Spectral ripple discrimination thresholds in the STRC-/- subjects were poorer than in NH listeners (p < 0.0001) but remained better than for CI listeners (p < 0.0001). Frequency resolution appeared impaired in the STRC-/- group compared to NH listeners but did not reach statistical significance (p = 0.06). Compared to NH listeners, amplitude modulation detection thresholds in the STRC-/- group did not reach significance (p=  0.06) but were better than in CI subjects (p < 0.0001). Temporal resolution in STRC-/- subjects was similar to NH (p = 0.98) but better than in CI listeners (p = 0.04). The spondee reception threshold in the STRC-/- group was worse than NH listeners (p = 0.0008) but better than CI listeners (p = 0.0001). For AzBio sentences, performance at 0 dB SNR was similar between the STRC-/- group and the NH group, 88 % and 97 % respectively. For -5 dB SNR, the STRC-/- performance was significantly poorer than NH, 40 % and 85 % respectively, yet much better than with CI who performed at 54 % at +5 dB SNR in children and 53 % at + 10 dB SNR in adults. To our knowledge, this is the first study of the psychoacoustic performance of human subjects lacking cochlear amplification but with normal inner hair cell function. Our data demonstrate preservation of temporal resolution and a trend to impaired frequency resolution in this group without reaching statistical significance. Speech-in-noise perception compared to NH listeners was impaired as well. All measures were better than those in CI listeners. It remains to be seen if hearing aid modifications, customized for the spectral deficits in STRC-/- listeners can improve speech understanding in noise. Since cochlear implants are also limited by deficient spectral selectivity, STRC-/- hearing may provide an upper bound on what could be obtained with better temporal coding in electrical stimulation.

The role of suprathreshold auditory processing abilities in children with specific learning disorder

OBJECTIVE

To assess the suprathreshold auditory processing and speech recognition abilities in noise in children with specific learning disorder (SLD).

METHODS

A group of twenty-five children diagnosed with SLD and a control group of twenty-five neuro-typical children were included in the study. All the participants were between 6-11 years old. To evaluate suprathreshold auditory processing abilities, the participants were given the Temporal Fine Structure (TFS) Sensitivity Test and the Temporal Envelope (TE) Sensitivity Test, as well as the Consonant Identification Test, was administered to evaluate speech recognition ability in noise. In addition, the Wechsler Intelligence Scale for Children - Fourth Edition (WISC-IV) intelligence test was applied to children with SLD, and the relationship between WISC-IV intelligence test scores in different skills and suprathreshold auditory processing and speech recognition abilities in noise was investigated.

RESULTS

Significant differences were found between children diagnosed with SLD and neuro-typical children in terms of suprathreshold auditory processing tasks and speech recognition in noise. Additionally, no correlation was found between suprathreshold auditory processing tasks, speech recognition in noise, and intelligence tests.

CONCLUSION

Suprathreshold auditory processing and speech recognition abilities in noise were found to be affected in children with SLD. A holistic evaluation including a multidisciplinary approach that includes suprathreshold auditory processing abilities is required for children diagnosed with SLD.

Application of Signals with Rippled Spectra as a Training Approach for Speech Intelligibility Improvements in Cochlear Implant Users

In cochlear implant (CI) users, the discrimination of sound signals with rippled spectra correlates with speech discrimination. We suggest that rippled-spectrum signals could be a basis for training CI users to improve speech intelligibility. Fifteen CI users participated in the study. Ten of them used the software for training (the experimental group), and five did not (the control group). Software based on the phase reversal discrimination of rippled spectra was used. The experimental group was also tested for speech discrimination using phonetic material based on polysyllabic balanced speech material. An improvement in the discrimination of the rippled spectrum was observed in all CI users from the experimental group. There was no significant improvement in the control group. The result of the speech discrimination test showed that the percentage of recognized words increased after training in nine out of ten CI users. For five CI users who participated in the training program, the data on word recognition were also obtained earlier (at least eight months before training). The increase in the percentage of recognized words was greater after training compared to the period before training. The results allow the suggestion that sound signals with rippled spectra could be used not only for testing rehabilitation results after CI but also for training CI users to discriminate sounds with complex spectra.

Effect of gap detection threshold and localisation acuity on spatial release from masking in older adults

OBJECTIVE

The primary objective of this experiment was to measure the temporal and spatial processing capabilities of older individuals and use statistical models to identify the individual contributions of these temporal and spatial processing capabilities to spatial release from masking (SRM).

DESIGN

Repeated measures.

STUDY SAMPLE

Twenty-five older listeners with varying degrees of hearing loss participated in this experiment. SRM using the coordinate response measure, gap detection thresholds and localisation acuity for 1/3-octave-wide Gaussian noise bands centred at 500 and 4000 Hz were measured for all the listeners.

RESULTS

Older listeners had better speech recognition thresholds when target and maskers were spatially separated as compared to when they were co-located. In addition, hearing loss and localisation acuity at 500 Hz were significant predictors in a multiple regression model predicting SRM. However, gap detection thresholds did not significantly contribute to the multiple regression model predicting SRM.

CONCLUSION

Based on our data, we conclude that SRM at 30° spatial separation between the target and symmetric maskers is driven by the ability of the individuals to use interaural time difference cues.

Noise Attenuation Effects on Speech Recognition of Cochlear Implant Users Inside Helicopters

BACKGROUND: The speech recognition levels of cochlear implant (CI) users are still incompatible with ICAO hearing requirements for civil aviation pilots testing in the noisy background condition of the helicopter cockpit. In this study, we evaluated noise attenuation effects on speech recognition in the same background condition.METHODS: The study involved the evaluation of 12 Portuguese-speaking CI users with post-lingual deafness and with a pure tone average up to 35 dB HL between 500 and 2000 Hz and up to 50 dB at 3000 Hz on at least one of the ears, and of three normal hearing pilots (controls). We performed speech recognition tests using sentences, numbers, and disyllables for all participants through the VHF radio. The assessment took place inside a helicopter with engine on, using three setups: 1) with headset without the active noise cancellation; 2) activating the noise cancellation system of the headset itself; and 3) connecting the speech processor directly to the helicopter radio system.RESULTS: The headset active noise-cancellation improved only the recognition of sentences. The direct connection system compared to the headset without anti-noise attenuation significantly improved all the recognition tests. The median for numbers was 90%, but the best score for disyllables recognition was 56%.DISCUSSION: The noise attenuation resources proposed in this study improved the CI users speech recognition when exposed to the noisy helicopter cockpit. However, speech recognition of CI users still did not meet the standards of ICAO, which requires at least 80% for understanding disyllables in the speech in noise test.Caldeira JMA, Goffi-Gomez MVS, Imamura R, Bento RF. Noise attenuation effects on speech recognition of cochlear implant users inside helicopters. Aerosp Med Hum Perform. 2021; 92(11):880-885.

Different binaural processing of the envelope component and the temporal fine structure component of a narrowband noise in rat inferior colliculus

Complex broadband sounds are decomposed by peripheral auditory filters into a series of relatively narrowband signals, each with a slowly varying envelope (ENV) and a rapidly fluctuating temporal fine structure (TFS). ENV and TFS information at the bilateral ears contribute differentially to auditory perception. However, whether the difference could attribute to mechanisms of binaural integration remains an open question. As a potential neural correlate, subsets of neurons in the central nucleus of the inferior colliculus (ICC) are known to integrate binaural information with binaural inhibition or binaural summation. Therefore, we recorded the frequency-following responses (FFRs) to the ENV and TFS components of narrowband noises in the ICC of anesthetized rats and examined changes in FFR amplitude and stimulus-response coherence under various sound-delivery settings. We showed that binaural FFR_ENV was predominantly elicited by contralateral inputs and inhibited by ipsilateral inputs, exhibiting a "binaural-inhibition" like property. On the other hand, binaural FFR_TFS received a balanced contribution from both sides, echoing the "binaural-summation" mechanism. What is more, binaural FFR_ENV was significantly correlated with contralateral-evoked but not ipsilateral-evoked FFR_ENV, while binaural FFR_TFS correlated with both contralateral- and ipsilateral-evoked FFR_TFS. Overall, these results suggest distinct binaural processing of ENV and TFS information at the midbrain level.

Binaural sensitivity and release from speech-on-speech masking in listeners with and without hearing loss

Listeners with sensorineural hearing loss routinely experience less spatial release from masking (SRM) in speech mixtures than listeners with normal hearing. Hearing-impaired listeners have also been shown to have degraded temporal fine structure (TFS) sensitivity, a consequence of which is degraded access to interaural time differences (ITDs) contained in the TFS. Since these "binaural TFS" cues are critical for spatial hearing, it has been hypothesized that degraded binaural TFS sensitivity accounts for the limited SRM experienced by hearing-impaired listeners. In this study, speech stimuli were noise-vocoded using carriers that were systematically decorrelated across the left and right ears, thus simulating degraded binaural TFS sensitivity. Both (1) ITD sensitivity in quiet and (2) SRM in speech mixtures spatialized using ITDs (or binaural release from masking; BRM) were measured as a function of TFS interaural decorrelation in young normal-hearing and hearing-impaired listeners. This allowed for the examination of the relationship between ITD sensitivity and BRM over a wide range of ITD thresholds. This paper found that, for a given ITD sensitivity, hearing-impaired listeners experienced less BRM than normal-hearing listeners, suggesting that binaural TFS sensitivity can account for only a modest portion of the BRM deficit in hearing-impaired listeners. However, substantial individual variability was observed.

Role of Binaural Temporal Fine Structure and Envelope Cues in Cocktail-Party Listening

While conversing in a crowded social setting, a listener is often required to follow a target speech signal amid multiple competing speech signals (the so-called "cocktail party" problem). In such situations, separation of the target speech signal in azimuth from the interfering masker signals can lead to an improvement in target intelligibility, an effect known as spatial release from masking (SRM). This study assessed the contributions of two stimulus properties that vary with separation of sound sources, binaural envelope (ENV) and temporal fine structure (TFS), to SRM in normal-hearing (NH) human listeners. Target speech was presented from the front and speech maskers were either colocated with or symmetrically separated from the target in azimuth. The target and maskers were presented either as natural speech or as "noise-vocoded" speech in which the intelligibility was conveyed only by the speech ENVs from several frequency bands; the speech TFS within each band was replaced with noise carriers. The experiments were designed to preserve the spatial cues in the speech ENVs while retaining/eliminating them from the TFS. This was achieved by using the same/different noise carriers in the two ears. A phenomenological auditory-nerve model was used to verify that the interaural correlations in TFS differed across conditions, whereas the ENVs retained a high degree of correlation, as intended. Overall, the results from this study revealed that binaural TFS cues, especially for frequency regions below 1500 Hz, are critical for achieving SRM in NH listeners. Potential implications for studying SRM in hearing-impaired listeners are discussed.

SIGNIFICANCE STATEMENT

Acoustic signals received by the auditory system pass first through an array of physiologically based band-pass filters. Conceptually, at the output of each filter, there are two principal forms of temporal information: slowly varying fluctuations in the envelope (ENV) and rapidly varying fluctuations in the temporal fine structure (TFS). The importance of these two types of information in everyday listening (e.g., conversing in a noisy social situation; the "cocktail-party" problem) has not been established. This study assessed the contributions of binaural ENV and TFS cues for understanding speech in multiple-talker situations. Results suggest that, whereas the ENV cues are important for speech intelligibility, binaural TFS cues are critical for perceptually segregating the different talkers and thus for solving the cocktail party problem.

The effect of tone-vocoding on spatial release from masking for old, hearing-impaired listeners

Old, hearing-impaired listeners generally benefit little from lateral separation of multiple talkers when listening to one of them. This study aimed to determine how spatial release from masking (SRM) in such listeners is affected when the interaural time differences (ITDs) in the temporal fine structure (TFS) are manipulated by tone-vocoding (TVC) at the ears by a master hearing aid system. Word recall was compared, with and without TVC, when target and masker sentences from a closed set were played simultaneously from the front loudspeaker (co-located) and when the maskers were played 45° to the left and right of the listener (separated). For 20 hearing-impaired listeners aged 64 to 86, SRM was 3.7 dB smaller with TVC than without TVC. This difference in SRM correlated with mean audiometric thresholds below 1.5 kHz, even when monaural TFS sensitivity (discrimination of frequency-shifts in identically filtered complexes) was partialed out, suggesting that low-frequency audiometric thresholds may be a good indicator of candidacy for hearing aids that preserve ITDs. The TVC difference in SRM was not correlated with age, pure-tone ITD thresholds, nor fundamental frequency difference limens, and only with monaural TFS sensitivity before control for low-frequency audiometric thresholds.

Perception of pure tones and iterated rippled noise for normal hearing and cochlear implant users

Cochlear Implant (CI) users typically perform poorly on musical tasks, especially those based on pitch ranking and melody recognition. It was hypothesized that CI users would demonstrate deterioration in performance for a pitch ranking and a melody recognition task presented with iterated rippled noise (IRN) in comparison to pure tones (PT). In Addition, it was hypothesized that normal hearing (NH) listeners would show fewer differences in performance between IRN and PT for these two tasks. In this study, the ability of CI users and NH subjects to rank pitches and to identify melodies created with IRN and PT was assessed in free field in a sound-isolated room. CI subjects scored significantly above chance level with PT stimuli in both tasks. With IRN stimuli their performance was around chance level. NH subjects scored significantly above chance level in both tasks and with all stimuli. NH subjects performed significantly better than CI subjects in both tasks. These results illustrate the difficulties of CI subjects to rank pitches and to identify melodies.

Temporal-envelope reconstruction for hearing-impaired listeners

Recent studies suggest that normal-hearing listeners maintain robust speech intelligibility despite severe degradations of amplitude-modulation (AM) cues, by using temporal-envelope information recovered from broadband frequency-modulation (FM) speech cues at the output of cochlear filters. This study aimed to assess whether cochlear damage affects this capacity to reconstruct temporal-envelope information from FM. This was achieved by measuring the ability of 40 normal-hearing listeners and 41 listeners with mild-to-moderate hearing loss to identify syllables processed to degrade AM cues while leaving FM cues intact within three broad frequency bands spanning the range 65-3,645 Hz. Stimuli were presented at 65 dB SPL for both normal-hearing listeners and hearing-impaired listeners. They were presented as such or amplified using a modified half-gain rule for hearing-impaired listeners. Hearing-impaired listeners showed significantly poorer identification scores than normal-hearing listeners at both presentation levels. However, the deficit shown by hearing-impaired listeners for amplified stimuli was relatively modest. Overall, hearing-impaired data and the results of a simulation study were consistent with a poorer-than-normal ability to reconstruct temporal-envelope information resulting from a broadening of cochlear filters by a factor ranging from 2 to 4. These results suggest that mild-to-moderate cochlear hearing loss has only a modest detrimental effect on peripheral, temporal-envelope reconstruction mechanisms.

Morphometric classification and spatial organization of spiral ganglion neurons in the human cochlea: consequences for single fiber response to electrical stimulation

The unique, unmyelinated perikarya of spiral ganglion cells (SGCs) in the human cochlea are often arranged in functional units covered by common satellite glial cells. This micro anatomical peculiarity presents a crucial barrier for an action potential (AP) travelling from the sensory receptors to the brain. Confocal microscopy was used to acquire systematically volumetric data on perikarya and corresponding nuclei in their full dimension along the cochlea of two individuals.

Four populations of SGCs within the human inner ear of two different specimens were identified using agglomerative hierarchical clustering, contrary to the present distinction of two groups of SGCs. Furthermore, we found evidence of a spatial arrangement of perikarya and their accordant nuclei along the cochlea spiral. In this arrangement, the most uniform sizes of cell bodies are located in the middle turn, which represents the majority of phonational frequencies.

Since single-cell recordings from other mammalians may not be representative to humans and human SGCs are not accessible for physiological measurements, computer simulation has been used to quantify the effect of varying soma size on single neuron response to electrical micro stimulation. Results show that temporal parameters of the spiking pattern are affected by the size of the cell body. Cathodic stimulation was found to induce stronger variations of spikes while also leading to the lowest thresholds and longest latencies. Therefore, anodic stimulation leads to a more uniform excitation profile among SGCs with different cell body size.

The influence of non-spatial factors on measures of spatial release from masking

This study tested the hypothesis that the reduction in spatial release from masking (SRM) resulting from sensorineural hearing loss in competing speech mixtures is influenced by the characteristics of the interfering speech. A frontal speech target was presented simultaneously with two intelligible or two time-reversed (unintelligible) speech maskers that were either colocated with the target or were symmetrically separated from the target in the horizontal plane. The difference in SRM between listeners with hearing impairment and listeners with normal hearing was substantially larger for the forward maskers (deficit of 5.8 dB) than for the reversed maskers (deficit of 1.6 dB). This was driven by the fact that all listeners, regardless of hearing abilities, performed similarly (and poorly) in the colocated condition with intelligible maskers. The same conditions were then tested in listeners with normal hearing using headphone stimuli that were degraded by noise vocoding. Reducing the number of available spectral channels systematically reduced the measured SRM, and again, more so for forward (reduction of 3.8 dB) than for reversed speech maskers (reduction of 1.8 dB). The results suggest that non-spatial factors can strongly influence both the magnitude of SRM and the apparent deficit in SRM for listeners with impaired hearing.

Current research on music perception in cochlear implant users

The authors present a comprehensive review of the state of music perception with cochlear implant (CI) users. They discuss methods of assessment and results of studies of the aspects of music perception, melody, timbre, rhythm, and so forth in individuals with cochlear implants. They discuss neural mechanisms of music perception and the anticipation of broader acceptance of standardized tests for music perception in CI users.

Cochlear implants: current status and future potential

This article reviews the current status of cochlear implantation in both adults and children, including expanding candidacy groups, bilateral implantation, advances in speech processing software, internal and external device hardware, surgical techniques and outcomes. Promising advances, novel therapies and evolving concepts are also highlighted in terms of their future impact on clinical outcomes.

Can basic auditory and cognitive measures predict hearing-impaired listeners' localization and spatial speech recognition abilities?

This study aimed to clarify the basic auditory and cognitive processes that affect listeners' performance on two spatial listening tasks: sound localization and speech recognition in spatially complex, multi-talker situations. Twenty-three elderly listeners with mild-to-moderate sensorineural hearing impairments were tested on the two spatial listening tasks, a measure of monaural spectral ripple discrimination, a measure of binaural temporal fine structure (TFS) sensitivity, and two (visual) cognitive measures indexing working memory and attention. All auditory test stimuli were spectrally shaped to restore (partial) audibility for each listener on each listening task. Eight younger normal-hearing listeners served as a control group. Data analyses revealed that the chosen auditory and cognitive measures could predict neither sound localization accuracy nor speech recognition when the target and maskers were separated along the front-back dimension. When the competing talkers were separated along the left-right dimension, however, speech recognition performance was significantly correlated with the attentional measure. Furthermore, supplementary analyses indicated additional effects of binaural TFS sensitivity and average low-frequency hearing thresholds. Altogether, these results are in support of the notion that both bottom-up and top-down deficits are responsible for the impaired functioning of elderly hearing-impaired listeners in cocktail party-like situations.

Spectro-temporal envelope changes caused by temporal fine structure modification

The study of speech from which the temporal fine structure (TFS) has been removed has become an important research area. Common procedures for removing TFS include noise and tone vocoders. In the noise vocoder, bands of noise are modulated by the envelope of the speech within each band, and in the tone vocoder the carrier is a sinusoid at the center of each frequency band. Five different procedures for removing TFS are evaluated in this paper: the noise vocoder, a low-noise noise approach in which the noise envelope is replaced by the speech envelope in each frequency band, phase randomization within each band, the tone vocoder, and sinusoidal modeling with random phase. The effects of TFS modification on the speech envelope are evaluated using an index based on the envelope time-frequency modulation. The results show that for all of the TFS techniques implemented in this study, there is a substantial loss in the accuracy of reproduction of the envelope time-frequency modulation. The tone vocoder gives the best accuracy, followed by the procedure that replaces the noise envelope with the speech envelope in each band.

Relationship between behavioral and physiological spectral-ripple discrimination

Previous studies have found a significant correlation between spectral-ripple discrimination and speech and music perception in cochlear implant (CI) users. This relationship could be of use to clinicians and scientists who are interested in using spectral-ripple stimuli in the assessment and habilitation of CI users. However, previous psychoacoustic tasks used to assess spectral discrimination are not suitable for all populations, and it would be beneficial to develop methods that could be used to test all age ranges, including pediatric implant users. Additionally, it is important to understand how ripple stimuli are processed in the central auditory system and how their neural representation contributes to behavioral performance. For this reason, we developed a single-interval, yes/no paradigm that could potentially be used both behaviorally and electrophysiologically to estimate spectral-ripple threshold. In experiment 1, behavioral thresholds obtained using the single-interval method were compared to thresholds obtained using a previously established three-alternative forced-choice method. A significant correlation was found (r = 0.84, p = 0.0002) in 14 adult CI users. The spectral-ripple threshold obtained using the new method also correlated with speech perception in quiet and noise. In experiment 2, the effect of the number of vocoder-processing channels on the behavioral and physiological threshold in normal-hearing listeners was determined. Behavioral thresholds, using the new single-interval method, as well as cortical P1-N1-P2 responses changed as a function of the number of channels. Better behavioral and physiological performance (i.e., better discrimination ability at higher ripple densities) was observed as more channels added. In experiment 3, the relationship between behavioral and physiological data was examined. Amplitudes of the P1-N1-P2 "change" responses were significantly correlated with d' values from the single-interval behavioral procedure. Results suggest that the single-interval procedure with spectral-ripple phase inversion in ongoing stimuli is a valid approach for measuring behavioral or physiological spectral resolution.

Simulations of cochlear-implant speech perception in modulated and unmodulated noise

Experiment 1 replicated the finding that normal-hearing listeners identify speech better in modulated than in unmodulated noise. This modulated-unmodulated difference ("MUD") has been previously shown to be reduced or absent for cochlear-implant listeners and for normal-hearing listeners presented with noise-vocoded speech. Experiments 2-3 presented normal-hearing listeners with noise-vocoded speech in unmodulated or 16-Hz-square-wave modulated noise, and investigated whether the introduction of simple binaural differences between target and masker could restore the masking release. Stimuli were presented over headphones. When the target and masker were presented to one ear, adding a copy of the masker to the other ear ("diotic configuration") aided performance but did so to a similar degree for modulated and unmodulated maskers, thereby failing to improve the modulation masking release. Presenting an uncorrelated noise to the opposite ear ("dichotic configuration") had no effect, either for modulated or unmodulated maskers, consistent with the improved performance in the diotic configuration being due to interaural decorrelation processing. For noise-vocoded speech, the provision of simple spatial differences did not allow listeners to take greater advantage of the dips present in a modulated masker.

Aging alters the perception and physiological representation of frequency: evidence from human frequency-following response recordings

Older adults, even with clinically normal hearing sensitivity, have auditory perceptual deficits relative to their younger counterparts. This difficulty may in part, be related to a decline in the neural representation of frequency. The purpose of this study was to examine the effect of age on behavioral and physiological measures of frequency representation. Thirty two adults (ages 22-77), with hearing thresholds 25 dB HL at octave frequencies 0.25-8.0 kHz, participated in this experiment. Frequency discrimination difference limens (FDLs) were obtained at 500 and 1000 Hz using a two-interval, two-alternative forced choice procedure. Linear regression analyses showed significant declines in FDLs at both frequencies as age increased. Frequency-following responses (FFRs) were elicited by 500 and 1000 Hz tonebursts, as well as at frequencies within and outside those FDLs. Linear regression of FFR phase coherence and FFR amplitude at frequencies at and slightly below 1000 Hz showed significant decreases as age increased. Therefore, pitch discrimination, as measured by FDLs, and neural representation of frequency, as reflected by FFR, declined as age increased. Although perception and neural representation concurrently declined, one was not predictive of the other.

Effects of training on recognition of musical instruments presented through cochlear implant simulations

BACKGROUND

The simulation of the CI (cochlear implant) signal presents a degraded representation of each musical instrument, which makes recognition difficult.

PURPOSE

To examine the efficiency and effectiveness of three types of training on recognition of musical instruments as presented through simulations of the sounds transmitted through a CI.

RESEARCH DESIGN

Participants were randomly assigned to one of three training conditions: repeated exposure, feedback, and direct instruction.

STUDY SAMPLE

Sixty-six adults with normal hearing.

INTERVENTION

Each participant completed three training sessions per week, over a five-week time period, in which they listened to the CI simulations of eight different musical instruments.

DATA COLLECTION AND ANALYSIS

Analyses on percent of instruments identified correctly showed statistically significant differences between recognition accuracy of the three training conditions (p < .01).

RESULTS

those assigned to the direct instruction group showed the greatest improvement over the five-week training period as well as sustained improvement after training. The feedback group achieved the next highest level of recognition accuracy. The repeated exposure group showed modest improvement during the first three-week time period, but no subsequent improvements.

CONCLUSIONS

These results indicate that different types of training are differentially effective with regard to improving recognition of musical instruments presented through a degraded signal, which has practical implications for the auditory rehabilitation of persons who use cochlear implants.

Tuning in the spatial dimension: evidence from a masked speech identification task

Spatial release from masking was studied in a three-talker soundfield listening experiment. The target talker was presented at 0 degrees azimuth and the maskers were either colocated or symmetrically positioned around the target, with a different masker talker on each side. The symmetric placement greatly reduced any "better ear" listening advantage. When the maskers were separated from the target by +/-15 degrees , the average spatial release from masking was 8 dB. Wider separations increased the release to more than 12 dB. This large effect was eliminated when binaural cues and perceived spatial separation were degraded by covering one ear with an earplug and earmuff. Increasing reverberation in the room increased the target-to-masker ratio (TM) for the separated, but not colocated, conditions reducing the release from masking, although a significant advantage of spatial separation remained. Time reversing the masker speech improved performance in both the colocated and spatially separated cases but lowered TM the most for the colocated condition, also resulting in a reduction in the spatial release from masking. Overall, the spatial tuning observed appears to depend on the presence of interaural differences that improve the perceptual segregation of sources and facilitate the focus of attention at a point in space.

Speech identification based on temporal fine structure cues

The contribution of temporal fine structure (TFS) cues to consonant identification was assessed in normal-hearing listeners with two speech-processing schemes designed to remove temporal envelope (E) cues. Stimuli were processed vowel-consonant-vowel speech tokens. Derived from the analytic signal, carrier signals were extracted from the output of a bank of analysis filters. The "PM" and "FM" processing schemes estimated a phase- and frequency-modulation function, respectively, of each carrier signal and applied them to a sinusoidal carrier at the analysis-filter center frequency. In the FM scheme, processed signals were further restricted to the analysis-filter bandwidth. A third scheme retaining only E cues from each band was used for comparison. Stimuli processed with the PM and FM schemes were found to be highly intelligible (50-80% correct identification) over a variety of experimental conditions designed to affect the putative reconstruction of E cues subsequent to peripheral auditory filtering. Analysis of confusions between consonants showed that the contribution of TFS cues was greater for place than manner of articulation, whereas the converse was observed for E cues. Taken together, these results indicate that TFS cues convey important phonetic information that is not solely a consequence of E reconstruction.

Discrimination of Schroeder-phase harmonic complexes by normal-hearing and cochlear-implant listeners

The temporal fine structure (TFS) of sound contributes significantly to the perception of music and speech in noise. The evaluation of new strategies to improve TFS delivery in cochlear implants (CIs) relies upon the assessment of fine structure encoding. Most modern CI sound processing schemes do not encode within-channel TFS per se, but some TFS information is delivered through temporal envelope cues across multiple channels. Positive and negative Schroeder-phase harmonic complexes differ primarily in acoustic TFS and provide a potential test of TFS discrimination ability in CI users for current and future processing strategies. The ability to discriminate Schroeder-phase stimuli was evaluated in 24 CI users and 7 normal-hearing listeners at four fundamental frequencies: 50, 100, 200, and 400 Hz. The dependent variables were percent correct at each fundamental frequency, average score across all fundamental frequencies, and a maximum-likelihood-predicted threshold fundamental frequency for 75% correct. CI listeners scored better than chance for all fundamental frequencies tested. The 50-Hz, average, and predicted threshold scores correlated significantly with consonant-nucleus-consonant word scores. The 200-Hz score correlated with a measure of speech perception in speech-shaped noise. Pitch-direction sensitivity is predicted jointly by the 400-Hz Schroeder score and a spectral ripple discrimination task. The results demonstrate that the Schroeder test is a potentially useful measure of clinically relevant temporal processing abilities in CI users.

Spectral-ripple resolution correlates with speech reception in noise in cochlear implant users

Speech perception ability in noise is one of the most practical measures of success with a cochlear implant; however, with experience, this ability can change dramatically over time, making it a less than ideal tool for comparing performance among different processing strategies. This study examined performance on a spectral discrimination task and compared it to speech perception in noise. An adaptive procedure was used to determine the spectral-ripple density that subjects could discriminate. A closed-set, forced-choice adaptive procedure was used to determine speech reception thresholds for words in two-talker babble and in speech-shaped, steady-state noise. Spectral-ripple thresholds (ripples/octave) were significantly correlated with speech reception thresholds (dB SNR) in noise for 29 cochlear implant users (r = -0.55, p = 0.002 in two-talker babble; r = -0.62, p = 0.0004 in steady-state noise), demonstrating that better spectral resolution was associated with better speech perception in noise. A significant correlation was also found between the spectral-ripple discrimination ability and word recognition in quiet (r = 0.50, p = 0.009). In addition, test-retest reliability for spectral-ripple discrimination was good, and no learning was observed. The present study demonstrates that the spectral-ripple discrimination test, which is time efficient and nonlinguistic, would be a useful tool to evaluate cochlear implant performance with different signal processing strategies.