The influence of the efferent system on acoustic overstimulation

Since the first anatomical description by Rasmussen, the efferent innervation of the ear has been extensively investigated in order to establish its role in the hearing process. It is accepted that the olivocochlear bundle (OCB) influences the activity of the afferent fibers and through it, the peripheral ear is controlled by higher centers as well as connected to the other ear. Cody and Johnstone (1982) and Rajan and Johnstone (1983) concluded that the amount of ipsilateral temporary threshold shift (TTS) was reduced by simultaneous contralateral acoustic stimulation or by destroying the contralateral cochlea. They attributed the results to the efferent system. It was our aim to investigate whether, and under which conditions, the amount of TTS would be influenced by the efferent system. The efferent innervation was eliminated by cutting the superior vestibular nerve. Two factors influenced the magnitude of TTS; the integrity of the OCB and the functional condition of the contralateral ear.