Temporal quantization deteriorates the discrimination of interaural time differences

Abstract

Cochlear implants (CIs) often replace acoustic temporal fine structure by a fixed-rate pulse train. If the pulse timing is arbitrary (that is, not based on the phase information of the acoustic signal), temporal information is quantized by the pulse period. This temporal quantization is probably imperceptible with current clinical devices. However, it could result in large temporal jitter for strategies that aim to improve bilateral and bimodal CI users' perception of interaural time differences (ITDs), such as envelope enhancement. In an experiment with 16 normal-hearing listeners, it is shown that such jitter could deteriorate ITD perception for temporal quantization that corresponds to the often-used stimulation rate of 900 pulses per second (pps): the just-noticeable difference in ITD with quantization was 177 μs as compared to 129 μs without quantization. For smaller quantization step sizes, no significant deterioration of ITD perception was found. In conclusion, the binaural system can only average out the effect of temporal quantization to some extent, such that pulse timing should be well-considered. As this psychophysical procedure was somewhat unconventional, different procedural parameters were compared by simulating a number of commonly used two-down one-up adaptive procedures in Appendix B.