Binaural Optimization of Cochlear Implants: Discarding Frequency Content Without Sacrificing Head-Shadow Benefit

Abstract

Objectives: Single-sided deafness cochlear-implant (SSD-CI) listeners and bilateral cochlear-implant (BI-CI) listeners gain near-normal levels of head-shadow benefit but limited binaural benefits. One possible reason for these limited binaural benefits is that cochlear places of stimulation tend to be mismatched between the ears. SSD-CI and BI-CI patients might benefit from a binaural fitting that reallocates frequencies to reduce interaural place mismatch. However, this approach could reduce monaural speech recognition and head-shadow benefit by excluding low- or high-frequency information from one ear. This study examined how much frequency information can be excluded from a CI signal in the poorer-hearing ear without reducing head-shadow benefits and how these outcomes are influenced by interaural asymmetry in monaural speech recognition. Design: Speech-recognition thresholds for sentences in speech-shaped noise were measured for 6 adult SSD-CI listeners, 12 BI-CI listeners, and 9 normal-hearing listeners presented with vocoder simulations. Stimuli were presented using nonindividualized in-the-ear or behind-the-ear head-related impulse-response simulations with speech presented from a 70° azimuth (poorer-hearing side) and noise from 70° (better-hearing side), thereby yielding a better signal-to-noise ratio (SNR) at the poorer-hearing ear. Head-shadow benefit was computed as the improvement in bilateral speech-recognition thresholds gained from enabling the CI in the poorer-hearing, better-SNR ear. High- or low-pass filtering was systematically applied to the head-related impulse-response–filtered stimuli presented to the poorer-hearing ear. For the SSD-CI listeners and SSD-vocoder simulations, only high-pass filtering was applied, because the CI frequency allocation would never need to be adjusted downward to frequency-match the ears. For the BI-CI listeners and BI-vocoder simulations, both low and high pass filtering were applied. The normal-hearing listeners were tested with two levels of performance to examine the effect of interaural asymmetry in monaural speech recognition (vocoder synthesis-filter slopes: 5 or 20 dB/octave). Results: Mean head-shadow benefit was smaller for the SSD-CI listeners (~7 dB) than for the BI-CI listeners (~14 dB). For SSD-CI listeners, frequencies <1236 Hz could be excluded; for BI-CI listeners, frequencies <886 or >3814 Hz could be excluded from the poorer-hearing ear without reducing head-shadow benefit. Bilateral performance showed greater immunity to filtering than monaural performance, with gradual changes in performance as a function of filter cutoff. Real and vocoder-simulated CI users with larger interaural asymmetry in monaural performance had less head-shadow benefit. Conclusions: The “exclusion frequency” ranges that could be removed without diminishing head-shadow benefit are interpreted in terms of low importance in the speech intelligibility index and a small head-shadow magnitude at low frequencies. Although groups and individuals with greater performance asymmetry gained less head-shadow benefit, the magnitudes of these factors did not predict the exclusion frequency range. Overall, these data suggest that for many SSD-CI and BI-CI listeners, the frequency allocation for the poorer-ear CI can be shifted substantially without sacrificing head-shadow benefit, at least for energetic maskers. Considering the two ears together as a single system may allow greater flexibility in discarding redundant frequency content from a CI in one ear when considering bilateral programming solutions aimed at reducing interaural frequency mismatch.

Interaural Place-of-Stimulation Mismatch Estimates Using CT Scans and Binaural Perception, But Not Pitch, Are Consistent in Cochlear-Implant Users

“…This could also be the case for BI-CI users with large mismatches. Sheffield et al (2020) found that frequencies ,1200 Hz can be discarded to match CI-ear and acoustic-ear cochlear places of stimulation without reducing the speech-in-noise benefit provided by a SSD-CI. CT-scan estimates of interaural place mismatch showed relatively close agreement with time-consuming ITD-based estimates (Figs.…”

Considerations for Fitting Cochlear Implants Bimodally and to the Single-Sided Deaf

“…150–8000 Hz) required for optimal speech intelligibility ( Landsberger et al, 2015 ). However, it has been argued that this does not need to be the case for SSD-CI users and for good bimodal CI users ( Sheffield et al, 2020 ). If these SSD-CI and bimodal CI users would be re-programmed to obtain tonotopically matched stimulation across the ears, they might lose low-frequency information on the CI side, depending on the insertion depth of the electrode array.…”

“…With head shadow being small at those frequencies, they can access the low-frequency information independent of its direction of arrival. When the CI side is the poorer performing ear, discarding low frequencies up to 1000 Hz on the CI has been shown not to compromise speech intelligibility ( Sheffield et al, 2020 ), emphasizing the value of considering the two ears as one hearing system, rather than treating the ears separately. Another approach was presented by Lambriks et al (2020) .…”

No Benefit of Deriving Cochlear-Implant Maps From Binaural Temporal-Envelope Sensitivity for Speech Perception or Spatial Hearing Under Single-Sided Deafness

“…In the most extreme cases (S4 and S8), information below 1000 Hz was removed from the stimulation. However, the removal of that low-frequency information is not expected to have a significant effect on speech understanding, because the information in that frequency range remains fully available to the AH ear, due to the lack of a substantial head-shadow effect at low frequencies (Sheffield et al 2020).…”