Improving Interaural Time Difference Sensitivity Using Short Inter-pulse Intervals with Amplitude-Modulated Pulse Trains in Bilateral Cochlear Implants.

Interaural time differences (ITDs) at low frequencies are important for sound localization and spatial speech unmasking. These ITD cues are not encoded in commonly used envelope-based stimulation strategies for cochlear implants (CIs) using high pulse rates. However, ITD sensitivity can be improved by adding extra pulses with short inter-pulse intervals (SIPIs) in unmodulated high-rate trains. Here, we investigated whether this improvement also applies to amplitude-modulated (AM) high-rate pulse trains. To this end, we systematically varied the temporal position of SIPI pulses within the envelope cycle (SIPI phase), the fundamental frequency (F0) of AM (125 Hz and 250 Hz), and AM depth (from 0.1 to 0.9). Stimuli were presented at an interaurally place-matched electrode pair at a reference pulse rate of 1000 pulses/s. Participants performed an ITD-based left/right discrimination task. SIPI insertion resulted in improved ITD sensitivity throughout the range of modulation depths and for both male and female F0s. The improvements were largest for insertion at and around the envelope peak. These results are promising for conveying salient ITD cues at high pulse rates commonly used to encode speech information.