Modeling the level-dependent latency of cochlear transient responses

Human auditory brainstem responses to tone-burst stimuli exhibit remarkable consistency across tone frequency in the dependence of their latency on stimulus level. This level-dependent latency is simulated in a nonlinear, active, transmission-line model of cochlear mechanics. In this model, feedback forces due to outer hair cell (OHC) motility counteract damping in the cochlear partition, but only when they have the proper phase relative to vibration of the reticular lamina. The role of the tectorial membrane (TM) in controlling the phase the OHC force is crucial to the success of the model. In the model, TM tuning restricts the frequency range over which damping reduction is achieved. This restriction allows OHC motility to improve cochlear frequency resolution without creating instability. The only nonlinear element in the model represents OHC mechanoelectrical transduction. Level dependence of the latency of cochlear transients has been a controversial issue and its simulation presents a significant challenge to cochlear models because of the wide range of signal-processing constraints that it imposes on the model. The present model results suggest that the level dependence of ABR tone-burst latency at high frequencies is mostly due to cochlear mechanics and at low frequencies is mostly due to adaptation.