STIMULUS FEATURES AFFECTING PSYCHOPHYSICAL DETECTION THRESHOLDS FOR ELECTRICAL-STIMULATION OF THE COCHLEA .2. FREQUENCY AND INTERPULSE INTERVAL

Psychophysical detection thresholds for electrical stimulation of the cochlea were measured in nonhuman primates (macaques) as part of a series of experiments exploring the stimulus features affecting detection. The monkeys were trained psychophysically using operant conditioning. One ear was treated with neomycin to destroy hair cells, and implanted with electrodes in the scala tympani and/or the cochlear wall. In experiment 1, detection thresholds were measured for trains of fixed-duration pulses and for sinusoids. For long-duration pulses (1 to 2 ms/phase), thresholds decreased as a function of frequency (pulse rate), reaching a minimum at a frequency between 125 and 210 pps, then increased as frequency was further increased. For shorter duration pulses, thresholds usually decreased monotonically as a function of frequency but sometimes showed a slight increase as a function of frequency near the highest frequencies tested. Typically slopes of the threshold versus frequency functions for fixed-duration pulses were equal to or less than slopes of threshold versus frequency functions for sinusoidal signals, where frequency and phase duration covaried. Additional observations on two of the cases were made in experiments 2 and 3. In experiment 2, thresholds for pairs of pulses were measured as a function of inter-pulse interval. Thresholds decreased as a function of interpulse interval up to intervals of 2 to 4 ms and then increased slightly. In experiment 3, thresholds were measured as a function of stimulus duration at two frequencies. Thresholds decreased as a function of stimulus duration at a greater rate for the lower frequency pulse train, so that the difference between the detection thresholds for the two signals increased as a function of stimulus duration. These experiments suggest that stimulus frequency (i.e., repetition rate with phase duration fixed) is a significant contributor to the detection of electrical signals. This contribution may be due to a combination of effects of (a) increasing the number of neural responses to the stimulus as frequency is increased, and (b) temporal interactions of those responses, as interpulse intervals become small, due to refractory periods or other periods of altered excitability that follow a pulse.