Effects of middle-ear muscle contraction on transmission of sound to the inner ear (cat): Some direct measurements

Abstract

Measurement of sound pressure in scala vestibuli was used to examine the effects of middle-ear muscle contraction on transmission of tone bursts so intense (e.g., 80 to 140 dB SPL at the tympanic membrane) that conventional techniques using cochlear electric potentials are inapplicable. The muscles were stimulated electrically in order to elicit stable, well-controlled muscular contractions. Stapedius contraction appears to dominate the measured effects on transmission, although a contribution from the tensor tympani is not excluded. Stimulating electrode locations and stimulus levels were adjusted to produce displacement amplitudes of stapedius contraction within the range measured by direct optical observation of acoustically elicited stapedius contraction in decerebrate preparations. The electrically induced contractions attenuated tone bursts by as much as 35 dB at 500 Hz and 20 dB at 6000 Hz. The effect was approximately linear in that tone bursts of differing amplitudes received essentially the same attenuation for a given level of muscular contraction. [Work supported in part by NIH and the Research Fund of the American Otological Society. Decerebrate surgical preparation of cat developed by J. J. Guinan and E. M. Marr.]