On the Change in the Acoustic Impedance of the Ear as A Measure of Middle ear Muscle Reflex Activity

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Abstract
Middle ear muscle activity, recorded as change in the acoustic impedance of the ear, was correlated to the EMG and the change in the cochlear microphonics elicited by the middle ear muscle contractions. The experiments were performed on lightly anesthetized rabbits. The three methods were equally sensitive as measures of the middle ear muscle reflex responses within a couple of dB. The amplitudes of the impedance change and the rectified and integrated EMG were linearly related and usually reached saturation below 125 dB SPL. The change in the cochlear microphonics differed from the change in the impedance in that it did not reach a maximum below 125 dB SPL. It was also shown that the m. tensor tympani contributes very little to the total impedance change under normal condition. On the other hand, it gives distinct changes in the impedance when the stapedius muscle is paralysed. Opening the acoustic bulla was found to increase the effect of contractions of the middle ear muscles on the acoustic properties of the ear, resulting in greater changes of the CM and of the impedance.