Relationships Between the Auditory Nerve's Ability to Recover From Neural Adaptation, Cortical Encoding of and Perceptual Sensitivity to Within-channel Temporal Gaps in Postlingually Deafened Adult Cochlear Implant Users.

Abstract

As a step toward identifying the origin of the across-electrode variation in within-channel gap detection thresholds (GDTs) measured in individual cochlear implant (CI) users, this study assessed the relationships between the auditory nerve's (AN's) ability to recover from neural adaptation, cortical encoding of and perceptual sensitivity to within-channel temporal gaps in postlingually deafened adult CI users. Study participants included 11 postlingually deafened adults with Cochlear Nucleus devices, including three bilaterally implanted participants. In each of the 14 ears tested, recovery from neural adaptation of the AN was measured using electrophysiological measures of the electrically evoked compound action potential at up to four electrode locations. The two CI electrodes in each ear showing the largest difference in the speed of adaptation recovery were selected for assessing within-channel temporal GDT. GDTs were measured using both psychophysical and electrophysiological procedures. Psychophysical GDTs were evaluated using a three-alternative, forced-choice procedure, targeting 79.4% correct on the psychometric function. Electrophysiological GDTs were measured using the electrically evoked auditory event-related potential (eERP) evoked by temporal gaps embedded in electrical pulse trains (i.e., the gap-eERP). Objective GDT was defined as the shortest temporal gap that could evoke a gap-eERP. Related-samples Wilcoxon Signed Rank test was used to compare psychophysical GDTs and objective GDTs measured at all CI electrode locations. It was also used to compare psychophysical GDTs and objective GDTs measured at the two CI electrode locations with different speeds or amounts of adaptation recovery of the AN. A Kendall Rank correlation test was used to assess the correlation between GDTs measured at the same CI electrode location using psychophysical or electrophysiological procedures. Objective GDTs were significantly larger than those measured using psychophysical procedures. There was a significant correlation between objective and psychophysical GDTs. GDTs could not be predicted based on the amount or the speed of adaptation recovery of the AN. Electrophysiological measures of the eERP evoked by temporal gaps can potentially be used to assess within-channel GDT in CI users who cannot provide reliable behavioral responses. The difference in adaptation recovery of the AN is not the primary factor accounting for the across-electrode variation in GDT in individual CI users.