Interaural Time Difference Perception with a Cochlear Implant and a Normal Ear

Currently there is a growing population of cochlear-implant (CI) users with (near) normal hearing in the non-implanted ear. This configuration is often called SSD (single-sided deafness) CI. The goal of the CI is often to improve spatial perception, so the question raises to what extent SSD CI listeners are sensitive to interaural time differences (ITDs). In a controlled lab setup, sensitivity to ITDs was investigated in 11 SSD CI listeners. The stimuli were 100-pps pulse trains on the CI side and band-limited click trains on the acoustic side. After determining level balance and the delay needed to achieve synchronous stimulation of the two ears, the just noticeable difference in ITD was measured using an adaptive procedure. Seven out of 11 listeners were sensitive to ITDs, with a median just noticeable difference of 438 μs. Out of the four listeners who were not sensitive to ITD, one listener reported binaural fusion, and three listeners reported no binaural fusion. To enable ITD sensitivity, a frequency-dependent delay of the electrical stimulus was required to synchronize the electric and acoustic signals at the level of the auditory nerve. Using subjective fusion measures and refined by ITD sensitivity, it was possible to match a CI electrode to an acoustic frequency range. This shows the feasibility of these measures for the allocation of acoustic frequency ranges to electrodes when fitting a CI to a subject with (near) normal hearing in the contralateral ear.