ASA 124th Meeting New Orleans 1992 October

4aPP8. Cochlear noise damage: Empirical evidence for the origin of the ``half-octave shift.''

Jozef J. Zwislocki

Minsheng Zhang

Inst. for Sensory Res., Syracuse Univ., Syracuse, NY 13244

The phenomenon of ``half-octave shift'' between the frequency of the damaging sound and the frequency of the greatest sensitivity loss, whether measured behaviorally or neurally, is well known. A more detailed analysis of the data suggests that, for small to moderate losses, the greatest morphological damage corresponds to a shift of about one octave and that the shift of maximum sensitivity loss tends to be smaller. It is now possible to account for these relationships on the basis of intensity series of hair cell transfer functions obtained before and immediately after exposure to damaging noise. Before exposure, the response maximum gradually moves toward lower frequencies, as SPL is increased. After exposure, it is shifted toward lower frequencies even at relatively low SPLs, and its dependence on SPL is reduced, both effects becoming more prominent with increasing sensitivity loss. In the presence of severe loss, the shift reaches one octave, and its dependence on SPL is abolished altogether. [Work supported by NIDCD.]