AP responses in forward-masking paradigms and their relationship to responses of auditory-nerve fibers

The amplitude of N _1peakof whole-nerve AP was measured in cats using a tone-burst probe with tonal maskers in a forward-masking paradigm. Experiments examined the effects of masker level, frequency, and duration (T_m), as well as interstimulus interval (Δt). Results are consistent with the interpretation that amplitude of N_1reflectsactivity of a limited group of fibers with CF near the probe frequency. For single fibers, decrement in discharge rate (relative to an unmasked condition) is dependent upon rate to the masker [Smith, Neurophys. 40 (1977)]. For the AP, decrement in probe-elicited amplitude should reflect the rate to the masker in those fibers excited by the probe. Thus measurements of N₁decrement versus masker frequency and level are similar to single unit rate versus level functions. Since decrement in N₁reflects amount of adaptation, increasing T_mdecreases probe response. N₁amplitude as a function of T_mthus resembles PST histograms of nerve fibers. Finally, plots of N₁, amplitude as a function of Δt are interpreted as a measure of recovery from adaptation.