Decay and recovery of the acoustic stapedius reflex in humans.

The time course of decay and recovery of ipsilateral and contralateral stapedius reflex responses to 2 000 Hz pure tone stimulation was studied in 10 normal-hearing subjects. Reflex responses were found to follow a closely similar time course with respect to both decay and recovery in simultaneous bilateral recordings. The similarity is compatible with the assumption that decay and recovery originate in the afferent auditory system, and not in the muscles. Recovery was 50% complete 250 ms after the end of the stimulus and most subjects had reached their initial amplitude after 1-3 s. The individual correlation between decay and recovery was negative but weak, which is interpreted as showing that these processes have a tendency to balance each other, but are based partly on different mechanisms. The implications of the present results for diagnosis of disorders of the lower auditory system as well as for the evaluation of the protective role of the stapedius reflex against noise damage are pointed out.