OHC RESPONSE RECRUITMENT AND ITS CORRELATION WITH LOUDNESS RECRUITMENT

After proper noise exposure, Hensen's cells, which have been shown to follow closely the response characteristics of the outer hair cells, s uffered a loss of sensitivity at low and moderate SPLs. The lower the stimulus level, the greater was the loss. When the low-SPL loss did no t exceed about 40 dB, input-output functions showed an increased rate of amplitude growth, so that the post-exposure response caught up with its pre-exposure counterpart between 60 and 90 dB SPL, depending on t he severity of the loss. These results, together with preceding clinic al observations, led us to the conclusion that loudness recruitment oc curs at least in part at the hair cell level and is basically a local event as opposed to a pathological spread of excitation. The response recruitment we have discovered appears to result from a decreased effe ct of the active feedback when the passive cochlear mechanisms are int act. Evidence for these relationships is presented and an explanation is offered for previous experimental successes and failures in observi ng a steepening of rate-intensity functions in auditory nerve fibers a fter noise exposures or administration of ototoxic drugs.