Does electrode placement affect the interaural-time-difference acuity in bilateral cochlear-implant listeners?

Abstract

Postlingually deafened bilateral cochlear-implant (CI) listeners can show limited interaural-time-difference (ITD) sensitivity, even when tested using highly controlled time-synchronized research processors. It is assumed that ITD discrimination requires interaural frequency-matched inputs. However, current bilateral CI programming procedures do not account for potential interaural place-of-stimulation mismatch. This study investigated the magnitude of interaural place-of-stimulation mismatch and its effects on ITD sensitivity in bilateral CI listeners. Ten bilateral CI listeners were tested on a two-interval left-right ITD discrimination task. Loudness balanced, 300-ms, 100 or 200 pulse-per-second, constant-amplitude, monopolar pulse trains were delivered to single-electrode pairs using time-synchronized research processors. ITD just noticeable differences (JNDs) were measured for five reference electrodes evenly distributed along the array in one ear, and for at least five comparison electrodes, generating ITD tuning curves. The interaural mismatch was estimated using both the ITD tuning curves and differences in the angular insertion depth from computed-tomography (CT) scans. Data showed that ITD tuning curves were relatively broad when compared to the amount of physical interaural place-of-stimulation mismatch from the CT scans. This suggests that most bilateral CI listeners do not experience appreciably reduced binaural sensitivity due to differences in electrode placement.