[Temporal cochlear changes evoked by prolonged exposure to low intensity noise. Electrophysiological study in guinea...

Behavioral temporal threshold shifts (TTS) have been shown in the chinchilla, to be correlated with cochlear microphonic (CM) and whole-nerve action potential (AP) losses in the sensitivity as well as in maximum voltage. The Guinea-pig's ear differs significantly both anatomically and physiologically from the chinchilla's; as a result, it does not seem to be as susceptible to TTS at non-injurious noise levels, except at those frequency bands were middle-ear impedance and other factors are more favorable. At times varying from 2 hrs to 48 hrs after termination of noise exposures of 48 hrs duration to an octave-band of noise (7-14 KHz) with a sound-pressure level of 95 dB, the CM from the first three cochlear turns as well as the whole-nerve AP were measured in Guinea pigs. Losses in sensitivity as well as in maximum voltage were found in the CM recorded from the first turn. CM recovered gradually with time, reaching normal levels at about 40 hrs after termination of noise exposures. The AP +showed similar losses. Microscopic examination of the organ of Corti showed no loss of hair cells.