Subcortical Plasticity Following Perceptual Learning in a Pitch Discrimination Task

Practice can lead to dramatic improvements in the discrimination of auditory stimuli. In this study, we investigated changes of the frequency-following response (FFR), a subcortical component of the auditory evoked potentials, after a period of pitch discrimination training. Twenty-seven adult listeners were trained for 10 h on a pitch discrimination task using one of three different complex tone stimuli. One had a static pitch contour, one had a rising pitch contour, and one had a falling pitch contour. Behavioral measures of pitch discrimination and FFRs for all the stimuli were measured before and after the training phase for these participants, as well as for an untrained control group (n = 12). Trained participants showed significant improvements in pitch discrimination compared to the control group for all three trained stimuli. These improvements were partly specific for stimuli with the same pitch modulation (dynamic vs. static) and with the same pitch trajectory (rising vs. falling) as the trained stimulus. Also, the robustness of FFR neural phase locking to the sound envelope increased significantly more in trained participants compared to the control group for the static and rising contour, but not for the falling contour. Changes in FFR strength were partly specific for stimuli with the same pitch modulation (dynamic vs. static) of the trained stimulus. Changes in FFR strength, however, were not specific for stimuli with the same pitch trajectory (rising vs. falling) as the trained stimulus. These findings indicate that even relatively low-level processes in the mature auditory system are subject to experience-related change.     

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.     

Midbrain Synchrony to Envelope Structure Supports Behavioral Sensitivity to Single-Formant Vowel-Like Sounds in Noise

Vowels make a strong contribution to speech perception under natural conditions. Vowels are encoded in the auditory nerve primarily through neural synchrony to temporal fine structure and to envelope fluctuations rather than through average discharge rate. Neural synchrony is thought to contribute less to vowel coding in central auditory nuclei, consistent with more limited synchronization to fine structure and the emergence of average-rate coding of envelope fluctuations. However, this hypothesis is largely unexplored, especially in background noise. The present study examined coding mechanisms at the level of the midbrain that support behavioral sensitivity to simple vowel-like sounds using neurophysiological recordings and matched behavioral experiments in the budgerigar. Stimuli were harmonic tone complexes with energy concentrated at one spectral peak, or formant frequency, presented in quiet and in noise. Behavioral thresholds for formant-frequency discrimination decreased with increasing amplitude of stimulus envelope fluctuations, increased in noise, and were similar between budgerigars and humans. Multiunit recordings in awake birds showed that the midbrain encodes vowel-like sounds both through response synchrony to envelope structure and through average rate. Whereas neural discrimination thresholds based on either coding scheme were sufficient to support behavioral thresholds in quiet, only synchrony-based neural thresholds could account for behavioral thresholds in background noise. These results reveal an incomplete transformation to average-rate coding of vowel-like sounds in the midbrain. Model simulations suggest that this transformation emerges due to modulation tuning, which is shared between birds and mammals. Furthermore, the results underscore the behavioral relevance of envelope synchrony in the midbrain for detection of small differences in vowel formant frequency under real-world listening conditions.     

Correlations Between Pitch and Phoneme Perception in Cochlear Implant Users and Their Normal Hearing Peers

This study examined correlations between pitch and phoneme perception for nine cochlear implant users and nine normal hearing listeners. Pure tone frequency discrimination thresholds were measured for frequencies of 500, 1000, and 2000 Hz. Complex tone fundamental frequency (F0) discrimination thresholds were measured for F0s of 110, 220, and 440 Hz. The effects of amplitude and frequency roving were measured under the rationale that individuals who are robust to such perturbations would perform better on phoneme perception measures. Phoneme identification was measured using consonant and vowel materials in quiet, in stationary speech-shaped noise (SSN), in spectrally notched SSN, and in temporally gated SSN. Cochlear implant pure tone frequency discrimination thresholds ranged between 1.5 and 9.9 %, while cochlear implant complex tone F0 discrimination thresholds ranged between 2.6 and 28.5 %. On average, cochlear implant users had 5.3 dB of masking release for consonants and 8.4 dB of masking release for vowels when measured in temporally gated SSN compared to stationary SSN. Correlations with phoneme identification measures were generally higher for complex tone discrimination measures than for pure tone discrimination measures. Correlations with phoneme identification measures were also generally higher for pitch perception measures that included amplitude and frequency roving. The strongest correlations were observed for measures of complex tone F0 discrimination with phoneme identification in temporally gated SSN. The results of this study suggest that musical training or signal processing strategies that improve F0 discrimination should improve consonant identification in fluctuating noise.     

Electrophysiological and Psychophysical Measures of Temporal Pitch Sensitivity in Normal-hearing Listeners

To obtain combined behavioural and electrophysiological measures of pitch perception, we presented harmonic complexes, bandpass filtered to contain only high-numbered harmonics, to normal-hearing listeners. These stimuli resemble bandlimited pulse trains and convey pitch using a purely temporal code. A core set of conditions consisted of six stimuli with baseline pulse rates of 94, 188 and 280 pps, filtered into a HIGH (3365–4755 Hz) or VHIGH (7800–10,800 Hz) region, alternating with a 36% higher pulse rate. Brainstem and cortical processing were measured using the frequency following response (FFR) and auditory change complex (ACC), respectively. Behavioural rate change difference limens (DLs) were measured by requiring participants to discriminate between a stimulus that changed rate twice (up-down or down-up) during its 750-ms presentation from a constant-rate pulse train. FFRs revealed robust brainstem phase locking whose amplitude decreased with increasing rate. Moderate-sized but reliable ACCs were obtained in response to changes in purely temporal pitch and, like the psychophysical DLs, did not depend consistently on the direction of rate change or on the pulse rate for baseline rates between 94 and 280 pps. ACCs were larger and DLs lower for stimuli in the HIGH than in the VHGH region. We argue that the ACC may be a useful surrogate for behavioural measures of rate discrimination, both for normal-hearing listeners and for cochlear-implant users. We also showed that rate DLs increased markedly when the baseline rate was reduced to 48 pps, and compared the behavioural and electrophysiological findings to recent cat data obtained with similar stimuli and methods.

     

Binaural pitch perception in normal-hearing and hearing-impaired listeners

The effects of hearing impairment on the perception of binaural-pitch stimuli were investigated. Several experiments were performed with normal-hearing and hearing-impaired listeners, including detection and discrimination of binaural pitch, and melody recognition using different types of binaural pitches. For the normal-hearing listeners, all types of binaural pitches could be perceived immediately and were musical. The hearing-impaired listeners could be divided into three groups based on their results: (a) some perceived all types of binaural pitches, but with decreased salience or musicality compared to normal-hearing listeners; (b) some could only perceive the strongest pitch types; (c) some were unable to perceive any binaural pitch at all. The performance of the listeners was not correlated with audibility. Additional experiments investigated the correlation between performance in binaural-pitch perception and performance in measures of spectral and temporal resolution. Reduced frequency discrimination appeared to be linked to poorer melody recognition skills. Reduced frequency selectivity was also found to impede the perception of binaural-pitch stimuli. Overall, binaural-pitch stimuli might be very useful tools within clinical diagnostics for detecting specific deficiencies in the auditory system.     

Distortion products and their influence on representation of pitch-relevant information in the human brainstem for unresolved harmonic complex tones

Pitch experiments aimed at evaluating temporal pitch mechanism(s) often utilize complex sounds with only unresolved harmonic components, and a low-pass noise masker to eliminate the potential contribution of audible distortion products to the pitch percept. Herein we examine how: (i) masker induced reduction of neural distortion products (difference tone: DT; and cubic difference tone: CDT) alters the representation of pitch relevant information in the brainstem; and (ii) the pitch salience is altered when distortion products are reduced and/or eliminated. Scalp recorded brainstem frequency following responses (FFR) were recorded in normal hearing individuals using a complex tone with only unresolved harmonics presented in quiet, and in the presence of a low-pass masker at SNRs of?+15,?+5, and?-5?dB. Difference limen for F0 discrimination (F0 DL) was obtained in quiet and in the presence of low-pass noise. Magnitude of DT components (with the exception of components at F0 and 2F0), and the CDT components decreased with increasing masker level. Neural pitch strength decreased with increasing masker level for both the envelope-related (FFR(ENV)) and spectral-related (FFR(SPEC)) phase-locked activity. Finally, F0 DLs increased with decreasing SNRs suggesting poorer F0 discrimination with reduction of the distortion products. Collectively, these findings support the notion that both DT and CDT, as reflected in the FFR(ENV) and FFR(SPEC), respectively, influence both the brainstem representation of pitch relevant information and the pitch salience of the complex sounds.     

Rate and Temporal Coding of Regular and Irregular Pulse Trains in Auditory Midbrain of Normal-Hearing and Cochlear-Implanted Rabbits

Although pitch is closely related to temporal periodicity, stimuli with a degree of temporal irregularity can evoke a pitch sensation in human listeners. However, the neural mechanisms underlying pitch perception for irregular sounds are poorly understood. Here, we recorded responses of single units in the inferior colliculus (IC) of normal hearing (NH) rabbits to acoustic pulse trains with different amounts of random jitter in the inter-pulse intervals and compared with responses to electric pulse trains delivered through a cochlear implant (CI) in a different group of rabbits. In both NH and CI animals, many IC neurons demonstrated tuning of firing rate to the average pulse rate (APR) that was robust against temporal jitter, although jitter tended to increase the firing rates for APRs ≥ 1280 Hz. Strength and limiting frequency of spike synchronization to stimulus pulses were also comparable between periodic and irregular pulse trains, although there was a slight increase in synchronization at high APRs with CI stimulation. There were clear differences between CI and NH animals in both the range of APRs over which firing rate tuning was observed and the prevalence of synchronized responses. These results suggest that the pitches of regular and irregular pulse trains are coded differently by IC neurons depending on the APR, the degree of irregularity, and the mode of stimulation. In particular, the temporal pitch produced by periodic pulse trains lacking spectral cues may be based on a rate code rather than a temporal code at higher APRs.     

Rate discrimination,gap detection and ranking of temporal pitch in cochlear implant users

Cochlear implant (CI) users have poor temporal pitch perception, as revealed by two key outcomes of rate discrimination tests: (i) rate discrimination thresholds (RDTs) are typically larger than the corresponding frequency difference limen for pure tones in normal hearing listeners, and (ii) above a few hundred pulses per second (i.e. the “upper limit” of pitch), CI users cannot discriminate further increases in pulse rate. Both RDTs at low rates and the upper limit of pitch vary across listeners and across electrodes in a given listener. Here, we compare across-electrode and across-subject variation in these two measures with the variation in performance on another temporal processing task, gap detection, in order to explore the limitations of temporal processing in CI users. RDTs were obtained for 4–5 electrodes in each of 10 Advanced Bionics CI users using two interleaved adaptive tracks, corresponding to standard rates of 100 and 400 pps. Gap detection was measured using the adaptive procedure and stimuli described by Bierer et al. (JARO 16:273-284, 2015), and for the same electrodes and listeners as for the rate discrimination measures. Pitch ranking was also performed using a mid-point comparison technique. There was a marginal across-electrode correlation between gap detection and rate discrimination at 400 pps, but neither measure correlated with rate discrimination at 100 pps. Similarly, there was a highly significant across-subject correlation between gap detection and rate discrimination at 400, but not 100 pps, and these two correlations differed significantly from each other. Estimates of low-rate sensitivity and of the upper limit of pitch, obtained from the pitch ranking experiment, correlated well with rate discrimination for the 100- and 400-pps standards, respectively. The results are consistent with the upper limit of rate discrimination sharing a common basis with gap detection. There was no evidence that this limitation also applied to rate discrimination at lower rates.     

Perception of Across-Frequency Asynchrony by Listeners with Cochlear Hearing Loss

Cochlear hearing loss is often associated with broader tuning of the cochlear filters. Cochlear response latencies are dependent on the filter bandwidths, so hearing loss may affect the relationship between latencies across different characteristic frequencies. This prediction was tested by investigating the perception of synchrony between two tones exciting different regions of the cochlea in listeners with hearing loss. Subjective judgments of synchrony were compared with thresholds for asynchrony discrimination in a three-alternative forced-choice task. In contrast to earlier data from normal-hearing (NH) listeners, the synchronous-response functions obtained from the hearing-impaired (HI) listeners differed in patterns of symmetry and often had a very low peak (i.e., maximum proportion of “synchronous” responses). Also in contrast to data from NH listeners, the quantitative and qualitative correspondence between the data from the subjective and the forced-choice tasks was often poor. The results do not provide strong evidence for the influence of changes in cochlear mechanics on the perception of synchrony in HI listeners, and it remains possible that age, independent of hearing loss, plays an important role in temporal synchrony and asynchrony perception.     

Human frequency-following response: representation of pitch contours in Chinese tones

Auditory nerve single-unit population studies have demonstrated that phase-locking plays a dominant role in the neural encoding of both the spectrum and voice pitch of speech sounds. Phase-locked neural activity underlying the scalp-recorded human frequency-following response (FFR) has also been shown to encode certain spectral features of steady-state and time-variant speech sounds as well as pitch of several complex sounds that produce time-invariant pitch percepts. By extension, it was hypothesized that the human FFR may preserve pitch-relevant information for speech sounds that elicit time-variant as well as steady-state pitch percepts. FFRs were elicited in response to the four lexical tones of Mandarin Chinese as well as to a complex auditory stimulus which was spectrally different but equivalent in fundamental frequency (f0) contour to one of the Chinese tones. Autocorrelation-based pitch extraction measures revealed that the FFR does indeed preserve pitch-relevant information for all stimuli. Phase-locked interpeak intervals closely followed f0. Spectrally different stimuli that were equivalent in F0 similarly showed robust interpeak intervals that followed f0. These FFR findings support the viability of early, population-based 'predominant interval' representations of pitch in the auditory brainstem that are based on temporal patterns of phase-locked neural activity.     

Ménière's disease: effects of glycerol on tasks involving temporal processing

The present study examined the effect of glycerol ingestion on aspects of auditory performance in subjects having Ménière's disease. It was hypothesized that Ménière's disease may be associated with abnormal firing in the auditory nerve and that this should result in a decreased ability to code the auditory temporal fine structure. Psychoacoustical measures of interaural time discrimination and quasi frequency modulation rate discrimination were used as measures of temporal coding, and performance on these tasks was examined both before and after glycerol ingestion. Pre- and postglycerol measures of speech recognition and audiometric thresholds were also obtained. In agreement with previous results, glycerol-related changes in audiometric thresholds were modest or absent, but improvements in speech recognition were relatively reliable. Improvements in interaural time discrimination and quasi frequency modulation rate discrimination were also observed. The results provide limited support for the hypothesis that Ménière's disease may be associated with a reduced ability to code the temporal fine structure of sound.     

Envelope Coding in Auditory Nerve Fibers Following Noise-Induced Hearing Loss

Recent perceptual studies suggest that listeners with sensorineural hearing loss (SNHL) have a reduced ability to use temporal fine-structure cues, whereas the effects of SNHL on temporal envelope cues are generally thought to be minimal. Several perceptual studies suggest that envelope coding may actually be enhanced following SNHL and that this effect may actually degrade listening in modulated maskers (e.g., competing talkers). The present study examined physiological effects of SNHL on envelope coding in auditory nerve (AN) fibers in relation to fine-structure coding. Responses were compared between anesthetized chinchillas with normal hearing and those with a mild–moderate noise-induced hearing loss. Temporal envelope coding of narrowband-modulated stimuli (sinusoidally amplitude-modulated tones and single-formant stimuli) was quantified with several neural metrics. The relative strength of envelope and fine-structure coding was compared using shuffled correlogram analyses. On average, the strength of envelope coding was enhanced in noise-exposed AN fibers. A high degree of enhanced envelope coding was observed in AN fibers with high thresholds and very steep rate-level functions, which were likely associated with severe outer and inner hair cell damage. Degradation in fine-structure coding was observed in that the transition between AN fibers coding primarily fine structure or envelope occurred at lower characteristic frequencies following SNHL. This relative fine-structure degradation occurred despite no degradation in the fundamental ability of AN fibers to encode fine structure and did not depend on reduced frequency selectivity. Overall, these data suggest the need to consider the relative effects of SNHL on envelope and fine-structure coding in evaluating perceptual deficits in temporal processing of complex stimuli.     

Human frequency-following response: representation of tonal sweeps

Auditory nerve single-unit population studies have demonstrated that phase locking plays a dominant role in the neural encoding of steady-state speech sounds. Recently, we have reported that the phase-locked activity underlying the human frequency-following response (FFR) could also encode the first two formants of several tonal approximations of steady-state vowels. Since auditory nerve single-unit population studies have also demonstrated that phase locking is used to represent time-varying speech-like sounds, it was reasoned that the phase-locked neural activity underlying the human FFR, likewise, is dynamic enough to represent time-varying sounds. FFRs to a rising and a falling tone were obtained from 8 normal-hearing adults at 95, 85, 75 and 65 dB nHL. Results clearly demonstrated that the human FFR does indeed follow the trajectory of the rising and falling tones. Also, amplitude changes in the FFR supported the view that neural phase locking decreases with increasing frequency. Finally, the relatively smaller FFR amplitude for the falling tone compared to its rising counterpart lends further support to the notion that rising tones produce greater neural synchrony than falling tones. These results indicate that the human FFR may be used to evaluate encoding of time-varying speech sounds like diphthongs and certain consonant-vowel syllables.     

Neural encoding in the human brainstem relevant to the pitch of complex tones

Psychoacoustic studies have shown that complex tones containing resolved harmonics evoke stronger pitches than complex tones with only unresolved harmonics. Also, unresolved harmonics presented in alternating sine and cosine (ALT) phase produce a doubling of pitch. We examine here whether the temporal pattern of phase-locked neural activity reflected in the scalp recorded human frequency following response (FFR) preserves information relevant to pitch strength, and to the doubling of pitch for ALT stimuli. Results revealed stronger neural periodicity strength for resolved stimuli, although the effect of resolvability was weak compared to the effect observed behaviorally; autocorrelation functions and FFR spectra suggest a different pattern of phase-locked neural activity for ALT stimuli with resolved and unresolved harmonics consistent with the doubling of pitch observed in our behavioral estimates; and the temporal pattern of neural activity underlying pitch encoding appears to be similar at the auditory nerve (auditory nerve model response) and the rostral brainstem level (FFR). These findings suggest that the phase-locked neural activity reflected in the scalp recorded FFR preserves neural information relevant to pitch that could serve as an electrophysiological correlate of the behavioral pitch measure. The scalp recorded FFR may provide for a non-invasive analytic tool to evaluate neural encoding of complex sounds in humans.     

Neural temporal coding of low pitch. I. Human frequency-following responses to complex tones

The neural basis of low pitch was investigated in the present study by recording a brainstem potential from the scalp of human subjects during presentation of complex tones which evoke a variable sensation of pitch. The potential recorded, the frequency-following response (FFR), reflects the temporal discharge activity of auditory neurons in the upper brainstem pathway. It was used as an index of neural periodicity in order to determine the extent to which the low pitch of complex tones is encoded in the temporal discharge activity of auditory brainstem neurons. A tone composed of harmonics of a common fundamental produces a sensation of pitch equal to that of the 'missing' fundamental. Such signals generate brainstem potentials which are spectrally similar to FFR recorded in response to sinusoidal signals equal in frequency to the missing fundamental. Both types of signals generate FFR which are periodic, with a frequency similar to the perceived pitch of the stimuli. It is shown that the FFR to the missing fundamental is not the result of a distortion product by recording FFR to a complex signal in the presence of low-frequency bandpass noise. Neither is the FFR the result of neural synchronization to the waveform envelope modulation pattern. This was determined by recording FFR to inharmonic and quasi-frequency-modulated signals. It was also determined that the 'existence region' for FFR to the missing fundamental lies below 2 kHz and that the most favorable spectral region for FFR to complex tones is between 0.5 and 1.0 kHz. These results are consistent with the hypothesis that far-field-recorded FFR does reflect neural activity germane to the processing of low pitch and that such pitch-relevant activity is based on the temporal discharge patterns of neurons in the upper auditory brainstem pathway.     

Pitch Discrimination in Musicians and Non-Musicians: Effects of Harmonic Resolvability and Processing Effort

Musicians typically show enhanced pitch discrimination abilities compared to non-musicians. The present study investigated this perceptual enhancement behaviorally and objectively for resolved and unresolved complex tones to clarify whether the enhanced performance in musicians can be ascribed to increased peripheral frequency selectivity and/or to a different processing effort in performing the task. In a first experiment, pitch discrimination thresholds were obtained for harmonic complex tones with fundamental frequencies (F0s) between 100 and 500 Hz, filtered in either a low- or a high-frequency region, leading to variations in the resolvability of audible harmonics. The results showed that pitch discrimination performance in musicians was enhanced for resolved and unresolved complexes to a similar extent. Additionally, the harmonics became resolved at a similar F0 in musicians and non-musicians, suggesting similar peripheral frequency selectivity in the two groups of listeners. In a follow-up experiment, listeners’ pupil dilations were measured as an indicator of the required effort in performing the same pitch discrimination task for conditions of varying resolvability and task difficulty. Pupillometry responses indicated a lower processing effort in the musicians versus the non-musicians, although the processing demand imposed by the pitch discrimination task was individually adjusted according to the behavioral thresholds. Overall, these findings indicate that the enhanced pitch discrimination abilities in musicians are unlikely to be related to higher peripheral frequency selectivity and may suggest an enhanced pitch representation at more central stages of the auditory system in musically trained listeners.     

A psychophysical evaluation of spectral enhancement.

Listeners with sensorineural hearing loss have well-documented elevated hearing thresholds; reduced auditory dynamic ranges; and reduced spectral (or frequency) resolution that may reduce speech intelligibility, especially in the presence of competing sounds. Amplification and amplitude compression partially compensate for elevated thresholds and reduced dynamic ranges but do not remediate the loss in spectral resolution. Spectral-enhancement processing algorithms have been developed that putatively compensate for decreased spectral resolution by increasing the spectral contrast, or the peak-to-trough ratio, of the speech spectrum. Several implementations have been proposed, with mixed success. It is unclear whether the lack of strong success was due to specific implementation parameters or whether the concept of spectral enhancement is fundamentally flawed. The goal of this study was to resolve this ambiguity by testing the effects of spectral enhancement on detection and discrimination of simple, well-defined signals. To that end, groups of normal-hearing (NH) and hearing-impaired (HI) participants listened in 2 psychophysical experiments, including detection and frequency discrimination of narrowband noise signals in the presence of broadband noise. The NH and HI listeners showed an improved ability to detect and discriminate narrowband increments when there were spectral decrements (notches) surrounding the narrowband signals. Spectral enhancements restored increment detection thresholds to within the normal range when both energy and spectral-profile cues were available to listeners. When only spectral-profile cues were available for frequency discrimination tasks, performance improved for HI listeners, but not all HI listeners reached normal levels of discrimination. These results suggest that listeners are able to take advantage of the local improvement in signal-to-noise ratio provided by the spectral decrements.     

Combined electric and acoustic hearing performance with Zebra® speech processor: Speech reception,place, and temporal coding evaluation

Abstract

Objective

To assess the auditory performance of Digisonic^® cochlear implant users with electric stimulation (ES) and electro-acoustic stimulation (EAS) with special attention to the processing of low-frequency temporal fine structure.

Method

Six patients implanted with a Digisonic^® SP implant and showing low-frequency residual hearing were fitted with the Zebra^® speech processor providing both electric and acoustic stimulation. Assessment consisted of monosyllabic speech identification tests in quiet and in noise at different presentation levels, and a pitch discrimination task using harmonic and disharmonic intonating complex sounds ( ). These tests investigate place and time coding through pitch discrimination. All tasks were performed with ES only and with EAS.

Results

Speech results in noise showed significant improvement with EAS when compared to ES. Whereas EAS did not yield better results in the harmonic intonation test, the improvements in the disharmonic intonation test were remarkable, suggesting better coding of pitch cues requiring phase locking.

Discussion

These results suggest that patients with residual hearing in the low-frequency range still have good phase-locking capacities, allowing them to process fine temporal information. ES relies mainly on place coding but provides poor low-frequency temporal coding, whereas EAS also provides temporal coding in the low-frequency range. Patients with residual phase-locking capacities can make use of these cues.     

Sensory deprivation due to otitis media episodes in early childhood and its effect at later age: A psychoacoustic and speech perception measure

BackgroundPast research has reported that children with repeated occurrences of otitis media at an early age have a negative impact on speech perception at a later age. The present study necessitates documenting the temporal and spectral processing on speech perception in noise from normal and atypical groups.ObjectivesThe present study evaluated the relation between speech perception in noise and temporal; and spectral processing abilities in children with normal and atypical groups.MethodsThe study included two experiments. In the first experiment, temporal resolution and frequency discrimination of listeners with normal group and three subgroups of atypical groups (had a history of OM) a) less than four episodes b) four to nine episodes and c) More than nine episodes during their chronological age of 6 months to 2 years) were evaluated using measures of temporal modulation transfer function and frequency discrimination test. In the second experiment, SNR 50 was evaluated on each group of study participants. All participants had normal hearing and middle ear status during the course of testing.ResultsDemonstrated that children with atypical group had significantly poorer modulation detection threshold, peak sensitivity and bandwidth; and frequency discrimination to each F0 than normal hearing listeners. Furthermore, there was a significant correlation seen between measures of temporal resolution; frequency discrimination and speech perception in noise. It infers atypical groups have significant impairment in extracting envelope as well as fine structure cues from the signal.ConclusionThe results supported the idea that episodes of OM before 2 years of agecan produce periods of sensory deprivation that alters the temporal and spectral skills which in turn has negative consequences on speech perception in noise.