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

Abstract

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 eleven 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 eleven 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 synchronise 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.

Reducing the Device Delay Mismatch Can Improve Sound Localization in Bimodal Cochlear Implant/Hearing-Aid Users

“…This outcome reveals a way to optimize CI systems for bimodal listeners—namely by implementation of a programmable delay element in the range of 1 to 11 ms as already suggested by Zirn et al. (2015) and Francart, Wiebe, and Wesarg (2018). As mentioned in our previous publication, this delay element should be integrated in the CI system and adjustable within the CI programming environment available to the clinician who has access to an appropriate table with τ HA values as an approximate value for the device delay mismatch to be set.…”

Spatial Speech-in-Noise Performance in Bimodal and Single-Sided Deaf Cochlear Implant Users

“…This study did not provide this compensation because we wanted to remain as close as possible to the everyday listening situation of bimodal CI listeners. The compensation is nontrivial: A major factor is the frequency dependency of the traveling wave along the basilar membrane; additionally, the sound processors of different CI and HA manufacturers have different processing delays (Francart, Wiebe, & Wesarg, 2018; Zirn et al., 2015).…”

Interaural place-of-stimulation mismatch estimates using CT scans and binaural perception, but not pitch, are consistent in cochlear-implant users

“…This made it difficult to create stimuli with equal but opposite effective ITDs as was done for the BI-CI subjects, thus preventing this approach. While procedures have been developed using auditory brainstem responses (Zirn et al, 2015) or psychophysical image centering (Francart et al, 2018) to determine this relative delay, the amount of time needed for these procedures was not feasible for the purposes of the current study because of the need to find this point for numerous reference-comparison locations. Thus, instead of requiring subjects to discriminate the perceived direction of an interaurally delayed stimulus, the current study asked subjects to detect a change in ITD in a three-interval, two-alternative forced-choice task.…”