Interactions of Interaural Time and Level Differences in Spatial Hearing with Cochlear Implants

Normally hearing humans can localize sound sources quite accurately, with minimum audible angles as small as 1°. To achieve this, our auditory pathways combine information from multiple acoustic cues, including interaural time and interaural level differences (ITDs and ILDs). Patients relying on cochlear implants (CIs) to hear the world cannot match normal performance. These deficits are most pronounced in patients with little or no hearing experience early in life, and they appear to result from an impaired sensitivity to ITDs, but not to ILDs. However, little is known about how ITD and ILD sensitivities develop and interact in an early deafened auditory system shortly after CI implantation. We fitted neonatally deafened rats with bilateral CIs, and, providing informative ITDs and ILDs from stimulation onset, trained them to lateralize CI stimuli. These animals were exquisitely sensitive to both ILDs and ITDs of CI stimulus pulses, and combined information from both cues in a weighted sum. Importantly, ITDs are weighted heavily in our CI rats, such that only very modest ITDs pointing in one direction can confound quite large ILDs pointing in the opposite direction. This underlines the importance of informative ITDs for maximizing the potential for spatial hearing with CI devices.