Average discharge rate of single auditory-nerve fibers in cats was measured in response to 400-msec tone bursts. For each fiber, rate versus stimulus-level functions were constructed for a number of frequencies. For tones at a fiber's characteristic frequency (CF), rate increases rapidly over a range of 20 to 30 dB above threshold. For higher stimulus levels, a range of behaviors is observed. For some fibers, rate saturates completely at higher levels; i.e., there are no further systematic increases in rate when level is increased beyond about 30 dB above threshold. For other units there is a noticeable bend in the rate-level function at 20 to 30 dB above threshold; however, rate can continue to increase gradually over another 30 to 40 dB. For frequencies above fiber CF, the slope of rate-level functions measured near the midpoint between maximum and spontaneous rates is a decreasing function of frequency. For frequencies below CF, slope is either approximately constant and equal to the slope at CF or increases to some maximum value as frequency is decreased from the CF. These properties of rate-level functions are well accounted for by a simple model consisting of a mechanical stage followed by a saturating nonlinearity (transducer stage). The input (pressure) versus output (basilar membrane displacement} functions for the mechanical stage are taken directly from the measurements of Rhode. The input-output function for the transducer is developed empirically.

Using the protocol outlined in this study, it is possible to reliably record EAP responses from most subjects and for most electrodes in Nucleus CI24M cochlear implant users. These responses are robust and recording these responses does not require that the subject sleep or remain still. Based on these results, a specific protocol is proposed for measurement of the EAP using the NRT system of the CI24M cochlear implant. Potential clinical implications of these results are discussed.