Stapedius reflex thresholds in relation to tails of auditory nerve fiber frequency tuning curves.

The acoustic stapedius reflex (ASR) threshold on non-anesthetized rabbits was compared to some measures of the single auditory nerve fiber activity of rabbits. The observations were made on normal-hearing animals, with some additional data from noise-exposed individuals. The results showed that the ASR threshold was reached at a sound level above saturation of discharge rate for individual neurons at their characteristic frequency (CF) in normal animals. It was found, on the other hand, that the ASR threshold measured across frequencies from 0.25 to 12.0 kHz were at a level similar to that of the tails of the frequency tuning curves (FTCs). Cochlear lesions-induced changes in FTC tail levels were paralleled by changes in ASR threshold levels. The raise of ASR threshold was, however, somewhat larger than the raise of the tails which might be explained by the significant relative decrease in the total number of units found in the frequency region corresponding to the lesion. There was also a decrease in the high spontaneous rate (SR) compared to the low and medium SR fibers for higher frequencies. It is concluded that the FTC tails can be a major eighth-nerve correlate to ASR activation.