Title : 1 Auditory Temporal Acuity

26 Cochlear implants stimulate the auditory nerve with amplitude-modulated electric pulse 27 trains. Pulse rates >2000 pulses per second (pps) have been hypothesized to enhance 28 transmission of temporal information. Recent studies, however, have shown that higher 29 pulse rates impair phase locking to sinusoidal amplitude modulation in the auditory 30 cortex and impair perceptual modulation detection. Here, we investigated the effects of 31 high pulse rates on the temporal acuity of transmission of pulse trains to the auditory 32 cortex. In anesthetized guinea pigs, signal-detection analysis was used to measure the 33 thresholds for detection of gaps in pulse trains at rates of 254, 1017, and 4069 pps and in 34 acoustic noise. Gap detection thresholds decreased by an order of magnitude with 35 increases in pulse rate from 254 to 4069 pps. Such a pulse-rate dependence likely would 36 impact speech reception through clinical speech processors. To elucidate the neural 37 mechanisms of gap detection, we measured recovery from forward masking after a 196.6 38 ms pulse train. Recovery from masking was faster at higher carrier pulse rates, and 39 masking increased linearly with current level. We fit the data with a dual-exponential 40 recovery function, consistent with a peripheral and a more central process. High-rate 41 pulse trains evoked less central masking, possibly due to adaptation of the response in the 42 auditory nerve. Neither gap detection nor forward masking varied with cortical depth, 43 indicating that these processes likely are sub-cortical. These results indicate that gap 44 detection and modulation detection are mediated by two separate neural mechanisms. 45 46