A Behavioral Study of Temporal Processing and Visual Persistence in Young and Aged Rats

Young rats were presented with light flash prepulses varying in duration from 1 to 128 ms, with light offset or light onset fixed at 70 ms prior to an acoustic startle stimulus (Experiment 1A), and, with single or paired 1-ms flashes, the 2nd (or only) flash given 100 to 500 ms before the startle, and 1 ms to 400 ms interflash intervals (Experiment 1B). Older rats (10 and 20 months old) received the same single and double flashes but with the maximum interflash interval extended to 1,500 ms (Experiment 2). Reflex inhibition increased with increased duration from 1 to 8 ms and decreased as light onset progressively exceeded 100 ms. Inhibition for both single and double flashes also declined for onset lead times beyond 100 ms, then increased for a double flash once the interflash interval exceeded 100 ms in young and middle-aged rats and 1,500 ms in the oldest rats. Peak inhibition was much reduced in the oldest rats at short lead times but was greater than that of younger rats at long lead times. These data suggest that aged rats process visual stimuli more slowly than younger rats and show poorer temporal acuity coupled with greater visual persistence.