Modeling effects of precursor duration on behavioral estimates of cochlear gain.

Physiological data show that preceding sound can reduce cochlear amplifier gain via the medial olivocochlear reflex (MOCR). Our laboratory has used forward masking as a tool to look for evidence of this effect psychophysically, which has led us to reassess mechanisms of forward masking. The traditional temporal window model (TWM) proposes that masking occurs via an excitatory mechanism that integrates within a temporal window. We propose that gain reduction may also contribute to forward masking. In our stimulus paradigm, growth of masking is measured with an off-frequency forward masker to estimate the basilar membrane input/output (I/O) function. The gain of the I/O function is reduced when an on-frequency precursor is introduced, consistent with a gain reduction hypothesis. Recently we explored the time course of this estimated gain reduction by examining the effect of precursor duration (Roverud and Strickland ). In that study, thresholds initially increased with increasing precursor duration, then decreased for longer durations. This result is not consistent with solely excitatory masking, but may reflect gain reduction by the MOCR. If the precursor is long enough, it could be influenced by the gain reduction it elicited. In the present study, we examine the effect of precursor duration with an on-frequency precursor and an off-frequency precursor. If intense enough, an off-frequency precursor may reduce gain at the signal frequency place. However, assuming it has no gain at the signal place, it would not be influenced by the reduction in gain, regardless of its duration. We developed a modified TWM that includes time-varying gain reduction by the precursor, resulting in an adapting I/O function. Results are modeled with the standard TWM and the TWM with gain reduction.