Effect of Channel Envelope Synchrony on Interaural Time Difference Sensitivity in Bilateral Cochlear Implant Listeners

Objectives: For a periodic acoustic input signal, the channel envelopes coded by current bilateral cochlear implant sound processors can be asynchronous. The effect of this asynchrony on sensitivity to interaural time differences (ITDs) was assessed. Design: ITD sensitivity was measured in six bilateral cochlear implant listeners for single- and three-electrode stimuli. The three-electrode stimuli contained envelope modulations, either synchronous or asynchronous across electrodes, with delays of 1.25 up to 5.00 ms. Each individual electrode carried the same ITD. Either neighboring electrodes were chosen or a separation of four electrodes to investigate the effect of electrode distance. Results: With synchronous envelopes, no difference in ITD sensitivity was found among single-electrode, adjacent three-electrode, and spaced three-electrode stimuli. A decrease in ITD sensitivity was found with increasing across-channel envelope asynchrony, which was consistent with the use of the across-electrode aggregate stimulation pattern rather than individual information channels for ITDs. No consistent effect of electrode separation was found. Conclusions: While the binaural system was resilient to small delays between envelopes, larger delays significantly deceased ITD sensitivity, both for adjacent and further spaced electrodes.