Computing auditory fatigue of the whole nerve action potential

Fatigue at the level of the VIII nerve is often calculated through the measurement of the whole nerve action potential (AP). The losses have been quantified by calculating the dB shift required to record a criterion AP voltage or by determining the ratio of the pre- and post-exposure AP voltages at a constant probe intensity. The issue raised here is whether the two methods produce similar AP losses. Fifteen rats were exposed to a 6000-Hz pure tone at either 100, 105, or 110 dB SPL for five minutes. Input-output functions were determined at five probe frequencies prior to and following exposure. When the probe intensity (i.e., AP voltage) was low, AP losses appeared greater when the AP fatigue was computed at a constant intensity. This trend was reversed at higher probe levels. These results are explained on the basis of the growth behavior of the AP.