Forward masking of the compound action potential : thresholds for the detection of the N1 peak

A two-interval forced-choice method was developed that provides a rapid and objective computerized measurement of the threshold for detection of the N1 peak of the compound action potential (CAP) recorded in response to a probe tone. The CAP was recorded at the round window of anesthetized chinchillas and several gerbils. An adaptive threshold-tracking procedure was verified by comparing measured thresholds to those obtained from neurometric functions, which plot the proportion of correct detections of the probe as a function of probe intensity. The adaptive procedure was applied in a forward masking paradigm to study the growth of masking of the CAP as a function of masker intensity. Results indicate that growth of masking of the CAP more closely corresponds to that observed psychophysically, than does forward masking observed in the response of a single neuron. Implications for neural encoding mechanisms are discussed.