Bilateral cochlear implant (CI) listeners currently use stimulation strategies that encode
interaural time differences (ITD) in the temporal envelope. However, the strategies do not transmit ITD in the fine structure, because of the constant phase in the electric pulse train. To determine the utility of encoding ITD in the fine structure, ITD-based lateralization was investigated with four CI listeners and four normal hearing (NH) subjects who listened to a simulation of electric stimulation.
Lateralization discrimination was tested at different pulse rates for various combinations of
independently controlled fine structure ITD and envelope ITD. Results for electric hearing show that the fine structure ITD had the strongest impact on lateralization at lower pulse rates, with significant effects for pulse rates up to 800 pulses per second. At higher pulse rates, lateralization discrimination depended solely on the envelope ITD. The data suggest that bilateral CI listeners benefit from transmitting fine structure ITD at lower pulse rates. However, there were strong inter-individual differences: the better performing CI listeners performed comparably to the NH listeners.
The result that bilateral CI listeners benefit from transmitting fine structure ITD at lower pulse rates is relevant to future CI stimulation strategies that encode fine timing cues. It is expected that appropriate encoding of these cues improves sound localization abilities and speech understanding in noise.
Majdak, P., Laback, B., Baumgartner., W.D. (2006). Effects of interaural time differences in fine structure and envelope on lateral discrimination in electrical hearing, J. Acoust. Soc. Am. 120, 2190-201.