Sridhar Kalluri

A comparison of CIC and BTE hearing aids for three-dimensional localization of speech.

Abstract Three-dimensional sound localization of speech in anechoic space was examined for eleven listeners with sensorineural hearing loss. The listeners were fitted bilaterally with CIC and BTE hearing aids having similar bandwidth capabilities. The goal was to determine whether differences in microphone placement for these two styles (CICs at the ear canal entrance; BTEs above the pinna) would influence the availability of pinna-related spectral cues and hence localization performance. While lateral and polar angle localization was unaffected by the hearing aid style, the rate of front-back reversals was lower with CICs. This pattern persisted after listeners accommodated to each set of aids for a six week period, although the overall rate of reversals declined. Performance on all measures in all conditions was considerably poorer than in a control group of listeners with normal hearing. Sumario Se evaluó la localización tridimensional de sonidos del habla en un espacio anecoide en once personas con pérdida auditiva sensorineural. A los once se les adaptaron CIC y BTE bilateralmente con capacidad de ancho de banda similar. El objetivo fue determinar si las diferencias en la colocación del micrófono con ambos tipos (CIC en el ingreso del conducto auditivo; BTE sobre el pabellón auricular) podría influenciar la disponibilidad de claves espectrales relacionadas con el pabellón y por ello, el rendimiento para la localización. Mientras que la localización del ángulo lateral y polar no fue afectada por el tipo de auxiliar, la tasa de inversiones frente-atrás fue menor con los CIC. Este patrón persistió después de que estas personas se acostumbraron a cada set de auxiliares por un período de seis semanas, a pesar de que declinó la tasa global de reversiones. El rendimiento en todas las mediciones y condiciones fue considerablemente más pobre que en el grupo control de personas con audición normal.

Acclimatization to hearing aids.

Objective: Evidence for a clinically significant effect of acclimatization to hearing aids is mixed. The aim of this study was to test for auditory acclimatization effects in new unilateral and bilateral adult hearing aid users. Hypotheses were i) there would be improvements in aided speech recognition in new hearing aid users, compared with unaided listening and with a control group of experienced hearing aid users, and ii) improvements would correlate with severity of hearing loss, hearing aid use, and cognitive capacity. Design: Speech recognition in noise was measured for a 65 and a 75 dB SPL target with the Four Alternative Auditory Feature test. Speech recognition in noise was measured within 1 week of fitting and retested at 12 weeks postfitting in new hearing aid users (16 unilateral and 16 bilateral fit). A control group of experienced hearing aid users (n = 17) was tested over a similar time scale. Cognitive capacity (reaction time and working memory) was measured, and self-reported change in performance was assessed using the Speech, Spatial and Qualities of Hearing Scale. Hearing aid use was assessed via data logging at the completion of the study. Results: Mean improvements in speech recognition of up to 4% were observed across conditions and across groups consistent with a general practice effect. On average there was no evidence of auditory acclimatization in the new hearing aid user groups in terms of improvement in aided listening conditions above that observed in unaided recognition or in the control group. There was no correlation between change in aided speech recognition and severity of hearing loss, hearing aid use, or cognitive capacity. New users reported significant improvement over time in aided performance on a self-report questionnaire compared with the control group. Conclusions: On average, there was no improvement over time in new users’ aided speech recognition relative to unaided recognition or to the control group. This does not support a robust acclimatization effect with nonlinear hearing aids. Test–retest variability may obscure small average acclimatization effects; variability was not accounted for by individual differences in severity of hearing loss, hearing aid use, or cognitive capacity. New users’ subjective report of increased benefit over time may be reflective of other aspects of adjustment to hearing aid use not examined in this study.

Perception and cortical neural coding of harmonic fusion in ferrets

This study examined the perception and cortical representation of harmonic complex tones, from the perspective of the spectral fusion evoked by such sounds. Experiment 1 tested whether ferrets spontaneously distinguish harmonic from inharmonic tones. In baseline sessions, ferrets detected a pure tone terminating a sequence of inharmonic tones. After they reached proficiency, a small fraction of the inharmonic tones were replaced with harmonic tones. Some of the animals confused the harmonic tones with the pure tones at twice the false-alarm rate. Experiment 2 sought correlates of harmonic fusion in single neurons of primary auditory cortex and anterior auditory field, by comparing responses to harmonic tones with those to inharmonic tones in the awake alert ferret. The effects of spectro-temporal filtering were accounted for by using the measured spectrotemporal receptive field to predict responses and by seeking correlates of fusion in the predictability of responses. Only 12% of units sampled distinguished harmonic tones from inharmonic tones, a small percentage that is consistent with the relatively weak ability of the ferrets to spontaneously discriminate harmonic tones from inharmonic tones in Experiment 1.

Unilateral and bilateral hearing aids, spatial release from masking and auditory acclimatization

Spatial release from masking (SRM) was tested within the first week of fitting and after 12 weeks hearing aid use for unilateral and bilateral adult hearing aid users. A control group of experienced hearing aid users completed testing over a similar time frame. The main research aims were (1) to examine auditory acclimatization effects on SRM performance for unilateral and bilateral hearing aid users, (2) to examine whether hearing aid use, level of hearing loss, age or cognitive ability mediate acclimatization, and (3) to compare and contrast the outcome of unilateral versus bilateral aiding on SRM. Hearing aid users were tested with and without hearing aids, with SRM calculated as the 50% speech recognition threshold advantage when maskers and target are spatially separated at ±90° azimuth to the listener compared to a co-located condition. The conclusions were (1) on average there was no improvement over time in familiar aided listening conditions, (2) there was large test-retest variability which may overshadow small average acclimatization effects; greater improvement was associated with better cognitive ability and younger age, but not associated with hearing aid use, and (3) overall, bilateral aids facilitated better SRM performance than unilateral aids.

The Influence of Cochlear Mechanical Dysfunction, Temporal Processing Deficits, and Age on the Intelligibility of Audible Speech in Noise for Hearing-Impaired Listeners

The aim of this study was to assess the relative importance of cochlear mechanical dysfunction, temporal processing deficits, and age on the ability of hearing-impaired listeners to understand speech in noisy backgrounds. Sixty-eight listeners took part in the study. They were provided with linear, frequency-specific amplification to compensate for their audiometric losses, and intelligibility was assessed for speech-shaped noise (SSN) and a time-reversed two-talker masker (R2TM). Behavioral estimates of cochlear gain loss and residual compression were available from a previous study and were used as indicators of cochlear mechanical dysfunction. Temporal processing abilities were assessed using frequency modulation detection thresholds. Age, audiometric thresholds, and the difference between audiometric threshold and cochlear gain loss were also included in the analyses. Stepwise multiple linear regression models were used to assess the relative importance of the various factors for intelligibility. Results showed that (a) cochlear gain loss was unrelated to intelligibility, (b) residual cochlear compression was related to intelligibility in SSN but not in a R2TM, (c) temporal processing was strongly related to intelligibility in a R2TM and much less so in SSN, and (d) age per se impaired intelligibility. In summary, all factors affected intelligibility, but their relative importance varied across maskers.

Brainstem processing following unilateral and bilateral hearing-aid amplification

Following previous research suggesting hearing-aid experience may induce functional plasticity at the peripheral level of the auditory system, click-evoked auditory brainstem response was recorded at first fitting and 12 weeks after hearing-aid use by unilateral and bilateral hearing-aid users. A control group of experienced hearing-aid users was tested over a similar time scale. No significant alterations in auditory brainstem response latency or amplitude were identified in any group. This does not support the hypothesis of plastic changes in the peripheral auditory system induced by hearing-aid use for 12 weeks.

Assessment of auditory spatial awareness in complex listening environments.

In the real world, listeners often need to track multiple simultaneous sources in order to maintain awareness of the relevant sounds in their environments. Thus, there is reason to believe that simple single source sound localization tasks may not accurately capture the impact that a listening device such as a hearing aid might have on a listeners level of auditory awareness. In this experiment, 10 normal hearing listeners and 20 hearing impaired listeners were tested in a task that required them to identify and localize sound sources in three different listening tasks of increasing complexity: a single-source localization task, where listeners identified and localized a single sound source presented in isolation; an added source task, where listeners identified and localized a source that was added to an existing auditory scene, and a remove source task, where listeners identified and localized a source that was removed from an existing auditory scene. Hearing impaired listeners completed these tasks with and without the use of their previously fit hearing aids. As expected, the results show that performance decreased both with increasing task complexity and with the number of competing sound sources in the acoustic scene. The results also show that the added source task was as sensitive to differences in performance across listening conditions as the standard localization task, but that it correlated with a different pattern of subjective and objective performance measures across listeners. This result suggests that a measure of complex auditory situation awareness such as the one tested here may be a useful tool for evaluating differences in performance across different types of listening devices, such as hearing aids or hearing protection devices.

Restoration of loudness summation and differential loudness growth in hearing-impaired listeners

When normal-hearing (NH) listeners compare the loudness of narrowband and wideband sounds presented at identical sound pressure levels, the wideband sound will most often be perceived as louder than the narrowband sound, a phenomenon referred to as loudness summation. Hearing-impaired (HI) listeners typically show less-than-normal loudness summation, due to reduced cochlear compressive gain and degraded frequency selectivity. In the present study, loudness summation at 1 and 3 kHz was estimated monaurally for five NH and eight HI listeners by matching the loudness of narrowband and wideband noise stimuli. The loudness summation was measured as a function both of noise bandwidth and level. The HI listeners were tested unaided and aided using three different compression systems to investigate the possibility of restoring loudness summation in these listeners. A compression system employing level-dependent compression channels yielded the most promising outcome. The present results inform the development of future loudness models and advanced compensation strategies for the hearing impaired.

Spatial release of cognitive load measured in a dual-task paradigm in normal-hearing and hearing-impaired listeners

This study investigated whether spatial separation between talkers helps reduce cognitive processing load, and how hearing impairment interacts with the cognitive load of individuals listening in multi-talker environments. A dual-task paradigm was used in which performance on a secondary task (visual tracking) served as a measure of the cognitive load imposed by a speech recognition task. Visual tracking performance was measured under four conditions in which the target and the interferers were distinguished by (1) gender and spatial location, (2) gender only, (3) spatial location only, and (4) neither gender nor spatial location. Results showed that when gender cues were available, a 15° spatial separation between talkers reduced the cognitive load of listening even though it did not provide further improvement in speech recognition (Experiment I). Compared to normal-hearing listeners, large individual variability in spatial release of cognitive load was observed among hearing-impaired listeners. Cognitive load was lower when talkers were spatially separated by 60° than when talkers were of different genders, even though speech recognition was comparable in these two conditions (Experiment II). These results suggest that a measure of cognitive load might provide valuable insight into the benefit of spatial cues in multi-talker environments.

Multichannel compression hearing aids: Effect of channel bandwidth on consonant and vowel identification by hearing-impaired listeners

Aided consonant and vowel identification was measured in 13 listeners with high-frequency sloping hearing losses. To investigate the influence of compression-channel analysis bandwidth on identification performance independent of the number of channels, performance was compared for three 17-channel compression systems that differed only in terms of their channel bandwidths. One compressor had narrow channels, one had widely overlapping channels, and the third had level-dependent channels. Measurements were done in quiet, in speech-shaped noise, and in a three-talker background. The results showed no effect of channel bandwidth, neither on consonant nor on vowel identification scores. This suggests that channel bandwidth per se has little influence on speech intelligibility when individually prescribed, frequency-varying compressive gain is provided.