Electric Acoustic Interaction in Cochlear Implant Users with Ipsilateral Residual Hearing using Electrocochleography

Background: Cochlear implantation is increasingly being used in hearing loss treatment for subjects with residual hearing to obtain combined electric acoustic stimulation (EAS). Previous studies have observed psychoacoustic interaction between electric and acoustic stimulation in humans.

Objective: This work characterizes EAS interaction through electrocochleographic (ECochG) responses, psychoacoustic masking experiments as well as cone beam computer tomography (CBCT).

Methods: Intracochlear ECochG responses were measured for an acoustic tone (A), an electric pulse train (E) and simultaneously presented acoustic and electric stimuli (A+E). The effect of electric stimulation on an acoustic tone was determined by subtracting the E from A+E response.

Psychoacoustic threshold elevation of an acoustic probe caused by the presence of an electric masker was measured using a 3 interval-forced-choice task. Two conditions were tested, keeping the masker fixed while varying the probe and varying the masker while keeping the probe fixed.

For the two conditions five EAS users participated in the study, with individual electrode insertion angles assessed via post-operative CBCT scans.

Results: Preliminary results showed EAS interaction in both, electrophysiological and psychoacoustic measurements. A psychophysical mean threshold elevation of 2dB (max. 15.7 dB) was observed for probes more apical with respect to the masker electrode position. Electrophysiologically, a mean interaction of 0.9 dB (max. 3.9 dB) was observed.

Conclusions: From these results we conclude that some aspects of EAS interaction can be measured electrophysiologically, however the sensitivity of this measure underestimates the threshold elevations observed psychophysically.

This work was supported by the DFG Cluster of Excellence EXC-1077/1 "Hearing4all".