Intense sounds may alter the mechanical properties of the cochlear partition

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Abstract
Morphological and structural changes have been observed in various cochlear elements following exposure to intense sounds. Whether these changes are sufficient to locally alter the mechanical properties of the cochlear partition is unknown. Here a psychophysical test for mechanical changes in the partition is developed and applied. Monaural exposures to an intense 1700‐Hz tone were preceded and followed by a binaural task in which the subject adjusted the interaural time difference of a 250‐Hz tone in order to perceive it as centered inside his head. If the local gradients of mass, stiffness, and/or coupling were altered by the exposure, then the post‐exposure settings should contain a time lead toward one ear or the other, depending upon the direction of the mechanical changes. Following exposure to an intense tone, the maximum temporary threshold shift (TTS) is often displaced upward in frequency from the exposure stimulus—an effect widely known as the half‐octave shift in TTS. Time leads toward the nonexposed ear in the post‐exposure centering task would be in accord with a mechanical‐change explanation of the half‐octave shift. Settings in this direction were observed with the most intense and longest durations of exposure used, but with less intense or shorter exposures, the post‐exposure settings were initially toward the exposed ear. Thus, the psychophysical evidence reported here supports the idea that exposure to intense sounds does produce temporary, local changes in cochlear mechanics, but it fails to provide a simple confirmation of the possibility that these mechanical changes underlie the half‐octave shift in TTS.

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