Electrode interactions measured by loudness summation in cochlear implant listeners

Abstract

Electrode interactions in a multichannel cochlear implant have been assumed to be a key factor affecting speech recognition performance. This study investigates whether loudness summation can estimate the degree of interaction between electrodes and whether the electrode interaction can be modeled mathematically. Thresholds and uncomfortable loudness levels in a two-electrode stimulation were measured in cochlear implantees, where the distance between electrodes was varied systematically. In two-electrode simultaneous stimulation, for two totally dependent electrodes, threshold was reached when the sum of current levels on the two electrodes equaled the threshold of either electrode. On the other hand, for two independent electrodes, threshold was not reached until the current level exceeded the threshold of either electrode. These data are consistent with linear summation of electrical fields. In the suprathreshold measure, loudness was due to the summation of electrical field strength, or loudness sensation, or both. A model has been developed to account for both the effect of electrical field summation in the cochlear and the effect of loudness summation in the sensation. Electrode interaction data will be related to speech recognition performance in these subjects. [Work supported by NIH.]