There is considerable knowledge of the individual effects of three systems which affect transmission of signals through the peripheral auditory system: the stapedius muscle, the tensor-tympani muscle, and the medial olivocochlear efferents. Effector neurons in each of these three systems respond to sound and thereby participate in acoustic reflexes which provide feedback control of the peripheral auditory system. However, there is little knowledge of what happens when two or more of these effectors are activated at the same time. Through experiments on anesthetized cats, we propose to determine the effects of these auditory reflexes acting together. We will determine the effects of the stapedius and tensor-tympani muscles acting separately and together. These experiments will show the extent to which these muscles independently attenuate middle-ear transmission and the total attenuation of middle-ear transmission produced by both muscles acting together. In other experiments, we will determine the effects of medial olivocochlear efferents on the acoustic activation of the middle-ear muscles (MEMs). The results will indicate whether this interaction is a major one (e.g. changes of 20 dB or more) or only a minor one (e.g. changes of a few dB at most) and should provide insight into which auditory-nerve fibers carry the signal which triggers the MEM acoustic reflexes. Experiments in which medial efferents are cut at the proper place should allow us to determine whether the effects of medial efferents on MEM acoustic reflexes take place in the cochlea or in the cochlear nucleus. Knowledge of the effects of these reflexes acting in concert is essential for understanding signal processing by the peripheral auditory system and central control of the auditory periphery. This knowledge will affect the interpretation of clinical tests using middle-ear-muscle reflexes and reflex tests intended to determine the site of brainstem lesions.
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