How do Interaural Time and Interaural Level Differences Interact in Spatial Hearing with Cochlear Implants?

Abstract

Sound localization in the horizontal plane is based on two binaural signals: interaural level differences (ILDs) and interaural time differences (ITDs). For bilateral cochlear implant (biCI) users, sound localization and spatial hearing are major challenges, and ITD sensitivity is particularly poor, with most biCI patients relying almost exclusively on ILDs. However, our recent work demonstrates that neonatally deafened rats with synchronized biCIs can develop excellent ITD sensitivity. Here, we investigate how the sensitivities to ILDs and ITDs compare and interact in ND rats which are supplied with synchronized biCI input from the outset. Eight neonatally deafened biCI rats were trained to lateralize pulse trains at a pulse rate of 900 pps containing either congruent or incongruent ITDs {+/- 100, 80, 60, 0} μs and ILDs {+/-6, 4, 1, 0.5, 0} dB. Trials in which ITDs and ILDs varied independently from each other were used to determine the relative strength and interaction of these two spatial cues. Our biCI animals developed both excellent ILD as well as ITD sensitivities, and the two types of cues interacted additively. Importantly, very small pulse timing ITDs of ~80 μs could influence an animal’s lateralization judgment as powerfully as relatively very large electrical pulse amplitude ILDs of ~4 dB, resulting in a time-intensity-trading-ratio of 20 μs/dB. Our results show that under synchronous CI stimulation, ITDs can be so strong compared to ILDs that they would interfere even with the perception of large ILDs unless one becomes ITD-insensitive. This could explain why biCI patients with currently asynchronous CI processors develop a poor ITD sensitivity in favor of an ILD sensitivity.