The use of interaural time and level difference cues by bilateral cochlear implant users

Abstract

While considerable evidence suggests that bilateral cochlear implant (CI) users' sound localization abilities rely primarily on interaural level difference (ILD) cues, and only secondarily, if at all, on interaural time difference (ITD) cues, this evidence has largely been indirect. This study used head-related transfer functions (HRTFs) to independently manipulate ITD and ILD cues and directly measure their contribution to bilateral CI users' localization abilities. The results revealed a strong reliance on ILD cues, but some CI users also made use of ITD cues. The results also suggest a complex interaction between ITD and ILD cues.

Figure 1

Scatter plot of RMS error scores obtained when testing in the soundfield and with the ITD+ILD condition of the direct connect testing. Lower scores indicate better performance. Each circle represents one participant. The diagonal line indicates equivalence between the two testing conditions. Dotted circles demarcate participants who also completed the ITD-only and ILD-only conditions.

Figure 2

Bar graphs of the RMS error scores for each participant for the three direct connect testing conditions. Lower scores indicate better performance. The dots connected with solid lines indicate the upper and lower bounds for the 99.2% confidence interval (α corrected for multiple comparisons) for random guessing corrected for the response bias of each participant. For reference, the upper and lower bounds for the 99.2% confidence interval for unbiased chance is shown as dashed lines, calculated as in Grantham et al. (2007).