ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

2pPP65. Masking and aging: III. Human and gerbil neural input/output functions.

John H. Mills

Lois J. Matthews

Richard A. Schmiedt

Flint A. Boettcher

Judy R. Dubno

Jayne B. Ahlstrom

Dept. of Otolaryngol. and Commun. Sci., Medical Univ. of South Carolina, 171 Ashley Ave., Charleston, SC 29425-2242

To account for higher masked ABR thresholds, input--output (I/O) functions of the ABR were examined. For both aging humans and gerbils, amplitudes of the ABR were smaller and slopes of ABR I/O functions were less steep than those of young subjects. Smaller amplitudes could indicate abnormal function of the brainstem or an altered input to the brain stem from the periphery. Analysis of I/O functions of the compound action potential (CAP) of the auditory nerve of aging gerbils also showed a decrease in slope. Decreases in the slope of the CAP can be attributed to a loss of 10%--30% of spiral ganglion cells, to a reduction in synchronous neural activity of the remaining nerve fibers, and to a reduction of the endocochlear potential. Thus, our hypothesis is that the unusual amounts of masking observed in the ABR of both aging humans and gerbils can be accounted for by age-related alterations in the auditory periphery. It is also our hypothesis that behavioral measurements of normal quiet and masked thresholds in aging humans do not necessarily reflect an intact auditory periphery. [Work supported by NIDCD.]