A look at neural integration in the human auditory system through the stapedius muscle reflex.

Ipsilateral and contralateral stapedius muscle contractions were studied as functions of the sound pressure level (SPL) and duration of 2-kHz tone bursts. The study complemented a preceding study in which temporal summation of stapedius muscle contractions produced by pairs of short tone bursts was determined and analyzed. The muscle contractions were determined indirectly by measuring changes in the acoustic impedance they produced at the tympanic membrane. The data for the stapedius muscle contraction as a function of tone-burst duration were derived from another study and analyzed in part with the help of the SPL functions obtained in the present study. According to the experimental results, the stapedius muscle contraction produced by contralateral stimulation follows a compressive power function paralleling both the cochlear output and loudness functions. The ipsilateral contraction follows an expansive power function. Mathematical analysis showed that the muscle tension due to contralateral stimulation increases with tone duration approximately according to the characteristic of a linear integrator having an exponentially decaying memory with a time constant that increases with SPL from approximately 200 to 370 msec. The simple relationship appears to be possible because of mutual cancellation of neural-processing characteristics preceding and following the temporal integration.