Event:

Interaural time differences (ITDs) are are important for horizontal sound localization and for spatial release from speech masking, particularly with speech maskers. While envelope-based stimulation strategies for cochlear implants (CIs) do not convey salient ITD cues in bilateral implantees, recent socalled fine-structure coding strategies attempt to convey salient ITD cues at apical electrodes using low stimulation rates. This approach is based on the idea that CI listeners are most sensitive to ITD cues if the stimulus contains low-rate information, either in the carrier or in the envelope. This talk will first give an overview of the state-of-the-art on this topic and then present two new approaches towards improving CI listeners’ access to ITD cues.

In order to convey ITD cues even at electrodes using high stimulation rates, which are important for effective speech coding, we followed a different approach which was inspired by previous psychophysical and physiological studies (Laback and Majdak, 2008; Hancock et al., 2012). We show that introducing extra pulses upon a periodic high-rate pulse train (1000 pulses/s), forming short inter-pulse intervals (SIPI), largely enhances ITD sensitivity to performance levels observed for low-rate pulse trains. We also explored the parameter space of SIPI pulses imposed on pulse trains with speech-like temporal envelopes to identify configurations providing maximal enhancement of ITD sensitivity.

In another study we explored a hypothesis attempting to explain the overall low ITD sensitivity of CI listeners even under optimal laboratory conditions (direct stimulation and low stimulation rate) as compared to normal hearing listeners under optimal conditions. According to the hypothesis, chronic CI stimulation lacking salient ITD cues that are consistent with spatial information from interaural level differences (ILDs) and from vision, results in decreased perceptual weighting of ITD cues and, thus, decreased ITD sensitivity. A seven-day perceptual learning experiment with 20 normal-hearing listeners, involving visual and proprioceptive feedback that reinforced either ITD or ILD cues (in either of two listener groups), supported the idea that re-weighting of binaural cues is possible. The results appear promising with respect to the goal of a training-based restoration of ITD sensitivity following chronic ITD-cue deprivation