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 (f0DLs) in EAS CI users. Both the ripple and f0DL 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 f0DLs would be poorest with electric-only hearing and would improve in the acoustic-only 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 f0DLs, 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.
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. F0DLs were assessed via a 3-AFC task using a broadband harmonic complex with a baseline f0 = 110 Hz. The f0 of the test intervals was varied to determine the smallest change in f0 that the listener could detect.
Results showed that performance in all three measures was poorest when EAS CI users were tested using electric-hearing only, with significant improvements when tested in the acoustic-only and acoustic+electric listening modes. F0DLs, but not spectral ripple density discrimination thresholds or audiometric thresholds, significantly correlated with speech masking release. Speech masking release also significantly correlated with open-set AzBio sentence recognition in noise scores obtained from clinical records.
Results indicated that preservation of residual acoustic hearing allows for speech masking release, likely due to access to temporal fine structure cues provided by residual hearing. The significant correlation between speech masking release and sentence recognition in noise indicates that the ability to extract target speech embedded in temporally fluctuating competing speech is important for speech recognition in noise.
Funded by National Institutes of Health/National Institutes on Deafness and Other Communication Disorders (NIH/NIDCD) P50 DC000242, American Speech-Language-Hearing Foundation Student Research Grant, and American Academy of Audiology Student Investigator Research Grant.