Phase effects in forward masking of the compound action potential: a comparison of responses to stimulus and distortion frequencies

Abstract

When a complex stimulus is presented, new frequencies (distortion products, DPs) are generated within the cochlea. The most intense DPs are lower in frequency than the stimulus tones (primaries). It is not clear whether the relative phase of stimuli is encoded by neural channels tuned to the primaries or by channels tuned to the DPs. We estimated the response of auditory nerve fibres tuned to each of these channels as a function of the relative phase of harmonic stimuli. The compound action potential (CAP) evoked by probes at the primary or distortion frequencies was masked by harmonic 2-tone maskers and cochlear generated DPs. The degree of masking reflected the response to the masker of fibres tuned to the probe. Changes in relative phase of the primaries resulted in a large modulation of the response of fibres tuned to the DPs. Except for a primary frequency ratio of 1:2, the response of fibres tuned to the primaries was only shallowly modulated by changes in relative phase. However, the level of response to the DPs was much lower than the response to the stimulus tones.