Physiological overshoot and the compound action potential

The sensitivity of the compound action potential (CAP) of the auditory nerve of the Mongolian gerbil (Meriones unguiculatus) to an added signal was studied as a function of the state of adaptation to a masker. The masker consisted of a train of tone bursts. A signal was added to one of the bursts and the threshold for detecting the signal was determined using a two-interval forced-choice technique. With a signal at the same frequency as the masker, there was little or no change in threshold with increasing adaptation to the masker, i.e., with the signal applied at masker onset or later on in the train. With a signal at a different frequency, considerably higher thresholds were obtained in the unadapted state than in the adapted state, a situation that appeared to resemble psychophysical overshoot phenomena. However, the interpretation of this result was complicated by the observation that the CAP response to a two-frequency stimulus is a sum of two slightly desynchronized waveforms arising from different cochlear regions. Synchronizing the two waveforms reduced the 'overshoot', but still resulted in higher thresholds in the unadapted state. The differences in threshold are accounted for by considering the changing signal-to-noise ratios at different states of adaptation. Possible relationships to psychophysically observed overshoot are discussed.