Psychophysical studies for two multiple-channel cochlear implant patients

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Abstract
Psychophysical studies were conducted on 2 multiple-channel cochlear implant patients to examine the nature of the hearing sensations produced by electrical stimulation of auditory nerve fibers using electrodes at different sites in the scala tympani (1 electrode at a time). Both time-invariant stimuli, whose parameter values did not vary in time, and time-varying stimuli, specified by a linear variation in parameter values, were used. A sharpness ranking study using time-invariant signals suggested that the hearing sensations produced by different electrodes varied from dull to sharp in an apical to basal direction in the scala tympani. A categorization study showed that the hearing sensations produced by 2 adjacent electrodes (1.5 mm apart) were rarely confused for a restricted range of time-invariant pulse rates. Discriminability studies by a same-different procedure for stimuli with pulse rates below 250 pps showed: relative difference limens of 6% to 12% for time-invariant pulse rates, and 9% to 13% for time-varying pulse rates; stimuli with time-varying electrode position differing in the direction of electrode trajectory were readily discriminated; and the discrimination of time-varying pulse rates deteriorated with decreases in the duration of the variation, while the discriminability of single-electrode trajectories was the same for 3 durations: 25, 50 and 10 ms. A speech processing strategy was also proposed on the basis of these results.

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