Speech masking release in hybrid cochlear implant users

Abstract

Improved cochlear implant (CI) designs and surgical techniques have allowed CI patients to retain acoustic hearing in the implanted ear post-operatively. These EAS (electric-acoustic stimulation) CI users listen with a combination of acoustic and electric hearing in the same ear. While electric hearing alone improves speech recognition in quiet, preserved acoustic hearing allows EAS CI users to outperform traditional CI users in speech recognition in noise and demonstrate "speech masking release," an improvement in speech recognition in temporally fluctuating noise relative to steady noise. Masking release is arguably an ecologically valid metric, as listeners often attend to target speech embedded in fluctuating competing speech. Improved speech recognition outcomes have been attributed to the spectral and temporal resolution provided by acoustic hearing. However, the relationship between spectral and temporal resolution and outcomes in EAS CI users is not clear. This study evaluated speech masking release, spectral ripple density discrimination thresholds, and fundamental frequency difference limens (f 0 DLs) in EAS CI users. Both the ripple and f 0 DL tasks are thought to measure underlying spectral resolution and temporal fine structure. EAS CI subjects underwent testing in three listening modes: acoustic-only, electric-only, and acoustic+electric. Comparisons across listening modes allowed the benefit provided by acoustic hearing to be quantified. It was hypothesized that speech masking release, spectral ripple density discrimination thresholds, and f 0 DLs would be poorest with electric-only hearing and would improve in the acousticonly and acoustic+electric listening modes. This would reflect the benefit of preserved acoustic hearing. It was also hypothesized that speech masking release would correlate with spectral ripple density discrimination thresholds and f 0 DLs, reflecting the roles of spectral and temporal fine structure cues. Lastly, it was hypothesized that EAS CI users with more residual hearing (lower audiometric thresholds) would perform better on all three tasks. v Speech masking release was evaluated using a 12-alternative-forced-choice (AFC) spondee recognition in noise task. The noise was a two-talker and a ten-talker babble presented at-5 dB SNR, and masking release was quantified as the difference in spondee recognition in two-talker babble relative to ten-talker babble. Spectral ripple density discrimination thresholds were assessed in a 3-AFC task using a broadband stimulus that contained spectral peaks and valleys logarithmically spaced on the frequency axis. The spacing between spectral peaks (ripple density) was varied to determine the threshold at which listeners could no longer resolve the individual spectral peaks. F 0 DLs were assessed via a 3-AFC task using a broadband harmonic complex with a baseline f 0 = 110 Hz. The f 0 of the test intervals was varied to determine the smallest change in f 0 that the listener could detect. Results showed that performance in all three measure...