ASA 125th Meeting Ottawa 1993 May

5aPP5. Diminished response to acoustic transients in aged rodents.

J. Ison

P. Bowen

J. Barlow

M. Taylor

Dept. of Psychol., Univ. of Rochester, Rochester, NY 14627

J. Walton

Univ. of Rochester, Rochester, NY 14627

The startle reflex to an intense noise burst (S2) is depressed if weak transients (S1) precede S2. Noise pips and gaps in noise carriers preceded S2 at various lead times in young and old CBA/J mice. Old mice differed from young in three ways: (1) reflex amplitudes were lower; (2) prepulse thresholds for inhibition were raised, especially for high frequency pips; and (3) inhibition appeared at later lead times. Inhibition thresholds, but not its slowed development, were remedied with a stronger S1. Comparable effects were seen in the F344 rat. Single units in inferior colliculus in the mice showed that the aged IC had lost sensitivity to weak pulses. Minimal gap thresholds (1--2 ms) did not vary with age, but the younger animals had more units with lower gap thresholds. The reduced behavioral thresholds of the weak S1 likely result from the presbycusislike sensitivity changes. The slow onset time of inhibition may reflect the reduced size of the population of transient-sensitive units. [Work supported by NIH Grant No. AG09524 and the RICHS.]