Multichannel electrical stimulation of the auditory nerve in man. II. Channel interaction.

A multichannel cochlear implant can be an effective prosthesis only if its channels are independent of each other. Presumably independence is achieved by stimulating different populations of surviving neurons. Two types of interaction might occur between channels: electrical current field summation peripheral to stimulation of the nerves and neural-perceptual interaction following stimulation. Two psychophysical techniques to assess channel independence are discussed. In one technique a masker is presented on one channel in order to adapt the nerves responding to that channel. The forward masked threshold of a signal is then measured on all other channels and elevation of threshold is assumed to indicate overlapping neural populations. In the second procedure channel interaction is evaluated by measuring the loudness summation of stimuli presented simultaneously to two channels. The magnitude, distribution, and phasic components of the loudness summation are measures of interaction between channels. Data from two subjects suggests that monopolar stimulation produces broader interaction patterns than bipolar stimulation as a function of electrode separation. Considerable differences in the extent of channel interaction were observed between the two subjects, possibly because of the difference in the absolute current levels needed for equivalent sensation levels.