Combined Effects of Noise and Reverberation on Sound Localization for Listeners With Normal Hearing and Bilateral Cochlear Implants

Challenging Times for Cochlear Implant Users - Effect of Face Masks on Audiovisual Speech Understanding during the COVID-19 Pandemic

Unhindered auditory and visual signals are essential for a sufficient speech understanding of cochlear implant (CI) users. Face masks are an important hygiene measurement against the COVID-19 virus but disrupt these signals. This study determinates the extent and the mechanisms of speech intelligibility alteration in CI users caused by different face masks. The audiovisual German matrix sentence test was used to determine speech reception thresholds (SRT) in noise in different conditions (audiovisual, audio-only, speechreading and masked audiovisual using two different face masks). Thirty-seven CI users and ten normal-hearing listeners (NH) were included. CI users showed a reduction in speech reception threshold of 5.0 dB due to surgical mask and 6.5 dB due to FFP2 mask compared to the audiovisual condition without mask. The greater proportion of reduction in SRT by mask could be accounted for by the loss of the visual signal (up to 4.5 dB). The effect of each mask was significantly larger in CI users who exclusively hear with their CI (surgical: 7.8 dB, p = 0.005 and FFP2: 8.7 dB, p = 0.01) compared to NH (surgical: 3.8 dB and FFP2: 5.1 dB). This study confirms that CI users who exclusively rely on their CI for hearing are particularly susceptible. Therefore, visual signals should be made accessible for communication whenever possible, especially when communicating with CI users.

Sound Localization of Listeners With Normal Hearing, Impaired Hearing, Hearing Aids, Bone-Anchored Hearing Instruments, and Cochlear Implants: A Review

PURPOSE

This review article reviews the contemporary studies of localization ability for different populations in different listening environments and provides possible future research directions.

CONCLUSIONS

The ability to accurately localize a sound source relying on three cues (interaural time difference, interaural level difference, and spectral cues) is important for communication, learning, and safety. Confounding effects including noise and reverberation, which exist in common listening environments, mask or alter localization cues and negatively affect localization performance. Hearing loss, a common public health issue, also affects localization accuracy. Although hearing devices have been developed to provide excellent audibility of speech signals, less attention has been paid to preserving and replicating crucial localization cues. Unique challenges are faced by users of various hearing devices, including hearing aids, bone-anchored hearing instruments, and cochlear implants. Hearing aids have failed to consistently improve localization performance and, in some cases, significantly impair sound localization. Bone-conduction hearing instruments show little to no benefit for sound localization performance in most cases, although some improvement is seen in binaural users. Although cochlear implants provide great hearing benefit to individuals with severe-to-profound sensorineural hearing loss, cochlear implant users have significant difficulty localizing sound, even with two implants. However, technologies in each of these areas are advancing to reduce interference with desired sound signals and preserve localization cues to help users achieve better hearing and sound localization in real-life environments.

Impact of Reverberation on Speech Perception and Sound Localization Accuracy in Cochlear Implant Users With Single-Sided Deafness

OBJECTIVES

The aim of this study was to compare the impact of reverberation on sound localization accuracy and speech perception in noise between subjects with single-sided deafness using a cochlear implant (SSD-CI) and a normal-hearing control group.

METHODS

Nine SSD-CI subjects and 21 normal-hearing subjects participated in the study. In Experiment 1, the sound localization accuracy was measured with and without reverberation. In Experiment 2, speech reception thresholds were determined with four asymmetrically arranged noise sources in free-field and in reverberation. For the realization of reverberation, a room simulation system comprising of 128 loudspeakers was used.

RESULTS

No significant impact of reverberation was found on the sound localization accuracy of the SSD-CI subjects (free-field: 12.6°, reverberation: 11.9°), whereas the normal-hearing subjects performed significantly worse in reverberation (free-field: 1.8°, reverberation: 3.3°).Both subject groups experienced significantly deteriorated speech reception thresholds due to reverberation (pSSD-CI = 0.008, pNH < 0.001). Mean speech reception thresholds in the SSD-CI subjects were -9.7 dB SNR (free-field) and -4.2 dB SNR (reverberation) and a median individual decrease of 5.7 dB SNR in reverberation. Mean speech reception thresholds in the normal-hearing group were -14.0 dB SNR (free-field) and -10.3 dB SNR (reverberation).

CONCLUSION

A strong deterioration of sound localization accuracy due to reverberation did not occur. However, although partial restoration of binaural hearing is assumed in SSD-CI users, the impact of reverberation on speech perception in noise is much stronger compared to normal-hearing.

Normative Study of the Binaural Interaction Component of the Human Auditory Brainstem Response as a Function of Interaural Time Differences

OBJECTIVES

The binaural interaction component (BIC) of the auditory brainstem response (ABR) is obtained by subtracting the sum of the monaural right and left ear ABRs from the binaurally evoked ABR. The result is a small but prominent negative peak (herein called "DN1"), indicating a smaller binaural than summed ABR, which occurs around the latency of wave V or its roll-off slope. The BIC has been proposed to have diagnostic value as a biomarker of binaural processing abilities; however, there have been conflicting reports regarding the reliability of BIC measures in human subjects. The objectives of the current study were to: (1) examine prevalence of BIC across a large group of normal-hearing young adults; (2) determine effects of interaural time differences (ITDs) on BIC; and (3) examine any relationship between BIC and behavioral ITD discrimination acuity.

DESIGN

Subjects were 40 normal-hearing adults (20 males and 20 females), aged 21 to 48 years, with no history of otologic or neurologic disorders. Midline ABRs were recorded from electrodes at high forehead (Fz) referenced to the nape of the neck (near the seventh cervical vertebra), with Fpz (low forehead) as the ground. ABRs were also recorded with a conventional earlobe reference for comparison to midline results. Stimuli were 90 dB peSPL biphasic clicks. For BIC measurements, stimuli were presented in a block as interleaved right monaural, left monaural, and binaural stimuli with 2000+ presentations per condition. Four measurements were averaged for a total of 8000+ stimuli per analyzed waveform. BIC was measured for ITD = 0 (simultaneous bilateral) and for ITDs of ±500 and ±750 µs. Subjects separately performed a lateralization task, using the same stimuli, to determine ITD discrimination thresholds.

RESULTS

An identifiable BIC DN1 was obtained in 39 of 40 subjects at ITD = 0 µs in at least one of two measurement sessions, but was seen in lesser numbers of subjects in a single session or as ITD increased. BIC was most often seen when a subject was relaxed or sleeping, and less often when they fidgeted or reported neck tension, suggesting myogenic activity as a possible factor in disrupting BIC measurements. Mean BIC latencies systematically increased with increasing ITD, and mean BIC amplitudes tended to decrease. However, across subjects, there was no significant relationship between the amplitude or latency of the BIC and behavioral ITD thresholds.

CONCLUSIONS

Consistent with previous studies, measurement of the BIC was time consuming and a BIC was sometimes difficult to obtain in awake normal-hearing subjects. The BIC will thus continue to be of limited clinical utility unless stimulus parameters and measurement techniques can be identified that produce a more robust response. Nonetheless, modulation of BIC characteristics by ITD supports the concept that the ABR BIC indexes aspects of binaural brainstem processing and thus may prove useful in selected research applications, e.g. in the examination of populations expected to have aberrant binaural signal processing ability.