Latency and amplitude changes in the acoustic startle reflex of the rat produced by variation in auditory prestimulation

Two experiments investigated the effect of an auditory prestimulus (S₁) on the amplitude and latency of the startle reflex to an intense tone burst (S₂). In the first, with a fixed S₁ intensity, with very short (<15 msec) ISIs reflex amplitudes were increased and latencies reduced by S₁ presence and with longer intervals (40 msec) amplitudes were reduced and latencies increased. Inhibition was apparent on the amplitude measure at lag times ineffective in altering the latency measure. In the second, with a fixed and long S₁ duration reflex amplitudes increased with an increase in S₁ intensity up to moderate (≈75 db) levels, but further increases in S₁ produced a reduction in amplitudes. However any increase in S₁ intensity over the minimum value tested (60 db) resulted in an increment in reflex latency. These experiments reveal the presence of three separable influences of auditory prestimuli on the acoustic startle reflex in the rat.