Intensity-dependent peak shift in cochlear transfer functions at the cellular level, its elimination by sound exposure, and its possible underlying mechanisms.

Our systematic study of cochlear transfer functions has confirmed earlier results that, in a normal cochlea, the cochlear AC responses at any given cochlear location do not have a fixed best frequency at which the response is maximal. The best frequency depends on sound intensity, shifting to lower frequencies as the intensity is increased. This phenomenon may account for the so called 'half-octave shift' of maximum cochlear damage relative to the frequency of the damaging sound observed in studies of auditory noise exposure. Our experimental results combined with the results of others and with our model studies bring us to the conclusion that, at low to moderate sound intensities, the dependence of the best frequency on sound intensity is due to an effect of the active feedback, which decreases as sound intensity increases. Consequently, the feedback, when present, must shift the best frequency upward.