Binaural unmasking with temporal envelope and fine structure in listeners with cochlear implants

Lateralization of binaural envelope cues measured with a mobile cochlear-implant research processora)

Bilateral cochlear implant (BICI) listeners do not have full access to the binaural cues that normal hearing (NH) listeners use for spatial hearing tasks such as localization. When using their unsynchronized everyday processors, BICI listeners demonstrate sensitivity to interaural level differences (ILDs) in the envelopes of sounds, but interaural time differences (ITDs) are less reliably available. It is unclear how BICI listeners use combinations of ILDs and envelope ITDs, and how much each cue contributes to perceived sound location. The CCi-MOBILE is a bilaterally synchronized research processor with the untested potential to provide spatial cues to BICI listeners. In the present study, the CCi-MOBILE was used to measure the ability of BICI listeners to perceive lateralized sound sources when single pairs of electrodes were presented amplitude-modulated stimuli with combinations of ILDs and envelope ITDs. Young NH listeners were also tested using amplitude-modulated high-frequency tones. A cue weighting analysis with six BICI and ten NH listeners revealed that ILDs contributed more than envelope ITDs to lateralization for both groups. Moreover, envelope ITDs contributed to lateralization for NH listeners but had negligible contribution for BICI listeners. These results suggest that the CCi-MOBILE is suitable for binaural testing and developing bilateral processing strategies.

Stimulation Rate and Voice Pitch Perception in Cochlear Implants

The stimulation rate in cochlear implant (CI) sound coding, or the "carrier" rate in pulses per second (pps), is known to influence pitch perception, as well as loudness perception and sound quality. Our main objective was to investigate the effects of reduced carrier rate on the loudness and pitch of coded speech samples. We describe two experiments with 16 Nucleus® CI users, where we controlled modulation characteristics and carrier rate using Spectral and Temporal Enhanced Processing (STEP), a novel experimental multichannel sound coder. We used a fixed set of threshold and comfortable stimulation levels for each subject, obtained from clinical MAPs. In the first experiment, we determined equivalence for voice pitch ranking and voice gender categorization between the Advanced Combination Encoder (ACE), a widely used clinical strategy in Nucleus® recipients, and STEP for fundamental frequencies (F0) 120-250 Hz. In the second experiment, loudness was determined as a function of the input amplitude of speech samples for carrier rates of 1000, 500, and 250 pps per channel. Then, using equally loud sound coder programs, we evaluated the effect of carrier rate on voice pitch perception. Although nearly all subjects could categorize voice gender significantly above chance, pitch ranking varied across subjects. Overall, carrier rate did not substantially affect voice pitch ranking or voice gender categorization: as long as the carrier rate was at least twice the fundamental frequency, or when stimulation pulses for the lowest, 250 pps carrier were aligned to F0 peaks. These results indicate that carrier rates as low as 250 pps per channel are sufficient to support functional voice pitch perception for those CI users sensitive to temporal pitch cues; at least when temporal modulations and pulse timings in the coder output are well controlled by novel strategies such as STEP.

Sound Localization in Single-Sided Deaf Participants Provided With a Cochlear Implant

Spatial hearing is crucial in real life but deteriorates in participants with severe sensorineural hearing loss or single-sided deafness. This ability can potentially be improved with a unilateral cochlear implant (CI). The present study investigated measures of sound localization in participants with single-sided deafness provided with a CI. Sound localization was measured separately at eight loudspeaker positions (4°, 30°, 60°, and 90°) on the CI side and on the normal-hearing side. Low- and high-frequency noise bursts were used in the tests to investigate possible differences in the processing of interaural time and level differences. Data were compared to normal-hearing adults aged between 20 and 83. In addition, the benefit of the CI in speech understanding in noise was compared to the localization ability. Fifteen out of 18 participants were able to localize signals on the CI side and on the normal-hearing side, although performance was highly variable across participants. Three participants always pointed to the normal-hearing side, irrespective of the location of the signal. The comparison with control data showed that participants had particular difficulties localizing sounds at frontal locations and on the CI side. In contrast to most previous results, participants were able to localize low-frequency signals, although they localized high-frequency signals more accurately. Speech understanding in noise was better with the CI compared to testing without CI, but only at a position where the CI also improved sound localization. Our data suggest that a CI can, to a large extent, restore localization in participants with single-sided deafness. Difficulties may remain at frontal locations and on the CI side. However, speech understanding in noise improves when wearing the CI. The treatment with a CI in these participants might provide real-world benefits, such as improved orientation in traffic and speech understanding in difficult listening situations.

Effects of temporal fine structure preservation on spatial hearing in bilateral cochlear implant users

Typically, the coding strategies of cochlear implant audio processors discard acoustic temporal fine structure information (TFS), which may be related to the poor perception of interaural time differences (ITDs) and the resulting reduced spatial hearing capabilities compared to normal-hearing individuals. This study aimed to investigate to what extent bilateral cochlear implant (BiCI) recipients can exploit ITD cues provided by a TFS preserving coding strategy (FS4) in a series of sound field spatial hearing tests. As a baseline, we assessed the sensitivity to ITDs and binaural beats of 12 BiCI subjects with a coding strategy disregarding fine structure (HDCIS) and the FS4 strategy. For 250 Hz pure-tone stimuli but not for broadband noise, the BiCI users had significantly improved ITD discrimination using the FS4 strategy. In the binaural beat detection task and the broadband sound localization, spatial discrimination, and tracking tasks, no significant differences between the two tested coding strategies were observed. These results suggest that ITD sensitivity did not generalize to broadband stimuli or sound field spatial hearing tests, suggesting that it would not be useful for real-world listening.

Participation of Acoustic and Electric Hearing in Perceiving Musical Sounds

Introduction: The objective of our study was to evaluate musical perception and its relation to the quality of life in patients with bimodal binaural auditory stimulation.

Materials and Methods: Nineteen adult patients with a cochlear implant (CI) for minimum 6 months, and moderate to severe contralateral hearing loss with a hearing aid (HA), and 21 normal hearing adults were included in this prospective, cross-sectional study. Pure-tone and speech audiometry, musical test evaluating sound perception characteristics and musical listening abilities, Munich questionnaire for musical habits, and the APHAB questionnaire were recoded. Performance in musical perception test with HA, CI, and HA + CI, and potential correlations between music test, audiometry and questionnaires were investigated.

Results: Bimodal stimulation improved musical perception in several features (sound brightness, roughness, and clarity) in comparison to unimodal hearing, but CI did not add to HA performances in texture, polyphony or musical emotion and even appeared to interfere negatively in pitch perception with HA. Musical perception performances (sound clarity, instrument recognition) appeared to be correlated to hearing-related quality of life (APHAB RV and EC subdomains) but not with speech performances suggesting that the exploration of musical perception complements speech understanding evaluation to better describe every-day life hearing handicap.

Conclusion: Testing musical sound perception provides important information on hearing performances as a complement to speech audiometry and appears to be related to hearing-related quality of life.

Binaural summation, binaural unmasking and fluctuating masker benefit in bimodal and bilateral adult cochlear implant users

OBJECTIVES

The number of bilateral adult cochlear implant (CI) users and bimodal CI users is expanding worldwide. The addition of a hearing aid (HA) in the contralateral non-implanted ear (bimodal) or a second CI (bilateral) can provide CI users with some of the benefits associated with listening with two ears. Our was to examine whether bilateral and bimodal CI users demonstrate binaural summation, binaural unmasking and a fluctuating masker benefit.

METHODS

Direct audio input was used to present stimuli to 10 bilateral and 10 bimodal Cochlear^TM CI users. Speech recognition in noise (speech reception threshold, SRT) was assessed monaurally, diotically (identical signals in both devices) and dichotically (antiphasic speech) with different masking noises (steady-state and interrupted), using the digits-in-noise test.

RESULTS

Bilateral CI users demonstrated a trend towards better SRTs with both CIs than with one CI. Bimodal CI users showed no difference between the bimodal SRT and the SRT for CI alone. No significant differences in SRT were found between the diotic and dichotic conditions for either group. Analyses of electrodograms created from bilateral stimuli demonstrated that substantial parts of the interaural speech cues were preserved in the Advanced Combination Encoder, an n-of-m channel selection speech coding strategy, used by the CI users. Speech recognition in noise was significantly better with interrupted noise than with steady-state masking noise for both bilateral and bimodal CI users.

CONCLUSION

Bilateral CI users demonstrated a trend towards binaural summation, but bimodal CI users did not. No binaural unmasking was demonstrated for either group of CI users. A large fluctuating masker benefit was found in both bilateral and bimodal CI users.