Interaural Time Processing When Stimulus Bandwidth Differs at the Two Ears

Abstract

Advances in the design of cochlear implants (CIs), as well as improved CI surgical techniques, have led to an increase in the number of patients who retain some residual low-frequency acoustic hearing in the implanted ear. Many of these patients also possess some hearing in the unimplanted ear. Although their low-frequency audiometric configurations will likely be asymmetrical across ears, they may nevertheless be able to process interaural time differences (ITDs) which might aid them in localizing sound sources and achieving a spatial release from masking. We recently published research (Brown and Yost 2011) showing how sensitivity to ITD differences was affected when the stimulus bandwidths were varied between the ears, to simulate asymmetrical hearing loss in the low-frequency region. We showed that ITD discrimination thresholds decreased as the bandwidth of the noise presented to one ear increased beyond that presented to the other ear. In the current experiment, we expand upon those conditions to ­further explore ITD processing in the presence of interaural spectral differences. ITD sensitivity was measured when a fixed band of noise was presented to one ear and the center frequency of a spectral band of the same width was moved upward in frequency in the other ear. The data suggest that listeners have difficulty attending to ITD differences in one spectral region when there are other spectral regions that contain conflicting or inconsistent spatial information, which is likely to be the case for many CI patients who possess bilateral residual hearing.