Loudness summation, masking, and temporal interaction for sensations produced by electric stimulation of two sites in the human cochlea.

Three psychophysical studies were conducted on two multichannel cochlear implant patients. The first study investigated the amount of loudness summation as a function of the spatial separation between two bipolar electrode pairs in the cochlea. Summation was found to increase in an orderly way with the separation between the two electrode pairs. This observation suggested that loudness was related to the distribution of discharge rate of auditory neurons along the cochlea for electric stimulation, and a model of loudness summation formulated on the basis of a functional relationship between loudness and the discharge rate distribution was proposed. The second study investigated the possibility of estimating the discharge rate distribution by means of masking. The amount of masking was found to decrease in an orderly fashion with the spatial separation between the masker and probe electrode pairs. This pattern of masking is consistent with the physiological and modeling observation that the current and neural discharge rate distributions produced by an electrode pair (masker) in the cochlea are approximately bell shaped with gradually decaying borders. The third study investigated the just-discriminable changes in the temporal delay between two interleaving pulse trains delivered, respectively, to two electrode pairs in the cochlea. Discrimination performance was found to decrease with the spatial separation between the two electrode pairs.