Interaural place-of-stimulation mismatch estimates using CT scans and binaural perception, but not pitch, are consistent in cochlear-implant users

Abstract

ABSTRACTBilateral cochlear implants (BI-CIs) or a CI for single-sided deafness (SSD; one normally functioning acoustic ear) can partially restore spatial-hearing abilities including sound localization and speech understanding when there are competing sounds. However for these populations, frequency information is not explicitly aligned across the ears, resulting in interaural place-of-stimulation mismatch. This diminishes spatial-hearing abilities because binaural encoding occurs in interaurally frequency-matched neurons. This study examined whether plasticity – the reorganization of central neural pathways over time – can compensate for peripheral interaural place mismatch. We hypothesized differential plasticity across two systems: none for binaural processing but adaptation toward the frequencies delivered by the specific electrodes for sequential pitch perception. Interaural place mismatch was evaluated in 43 human subjects (20 BI-CI and 23 SSD-CI, both sexes) using interaural-time-difference (ITD) discrimination (simultaneous bilateral stimulation), place-pitch ranking (sequential bilateral stimulation), and physical electrode- location estimates from computed-tomography (CT) scans. On average, CT scans revealed relatively little BI-CI interaural place mismatch (26° insertion-angle mismatch), but relatively large SSD-CI mismatch, particularly at the apical end of the array (166° for an electrode tuned to 300 Hz, decreasing to 14° at 7000 Hz). ITD and CT measurements were in agreement, suggesting little binaural-system plasticity to mismatch. The pitch measurements did not agree with the binaural and CT measurements, suggesting plasticity for pitch encoding or procedural biases. The combined results show that binaural processing may be optimized by using CT-scan information, but not pitch measurements, to program the CI frequency allocation to reduce interaural place mismatch.SIGNIFICANCE STATEMENTPlacement of electrode arrays in users of cochlear implants (CIs; bionic auditory prostheses that partially restore hearing) does not align the frequency information to acoustic neural encoding across the ears. This interaural place-of-stimulation mismatch diminishes spatial hearing abilities. This study shows that for experienced adult CI users with two CIs or with one CI and one normal-hearing ear, the best possible binaural sensitivity occurs when the same cochlear location is stimulated in both ears. This means that binaural brainstem pathways do not experience “plasticity” to compensate for interaural place mismatch – i.e., they do not reorganize to respond to input from different cochlear places. Therefore, explicit correction of interaural place mismatch by a clinician is necessary to derive maximum spatial-hearing benefits.