Exponential onset and recovery of temporary threshold shift after loud sound: evidence for long-term inactivation of mechano-electrical transduction channels

The onset and recovery of temporary threshold shift (TTS) in one human subject (the author) has been studied during and after pure-tone overstimulation lasting between minutes and days. Under the conditions of these experiments the time courses appeared reproducible, and thresholds always recovered to normal within 3 days. The onset and recovery followed a multiple-exponential time course, with the time constants for the onset being 6.5 and 800 min, and the recovery time constants being 30, 240 and 800 min. The observed time courses were consistent with data previously reported in humans, and with the view that the threshold elevation was due to an inactivation and reactivation of the stretch-activated channels at the apex of the outer hair cells of the cochlea. The time constants of the multi-exponential onset and recovery do not appear to depend on the duration of the overstimulation, but the exponential coefficients do. A simple kinetic model of the onset and recovery is described (for more detail see Patuzzi (1998)). It is suggested that the rapid recovery in the first 5 min after exposure is due to a short-lived disruption of the synapses between the inner hair cells and the primary afferent neurones. Intermittent exposures were found to produce much less TTS than continuous tones, and this reduction was found to be inconsistent with the Equal Energy Hypothesis, in that the TTS produced by intermittent tones was much less than predicted using the Equal Energy model, and the recovery time course was also different from that expected from a shorter exposure to a continuous tone of equal energy.