Interaction versus independence of startle-modification processes in the rat.

In a series of experiments it was found that the amount of inhibition produced by a weak acoustic stimulus occurring 64 msec prior to a startle-eliciting stimulus is unaffected by a second weak acoustic stimulus occurring 4 msec prior to the startle-eliciting stimulus. Likewise, the amount of latency reduction produced by an antecedent stimulus with a 4-msec lead time is unaffected by the presence of an antecedent stimulus with a 64-msec lead time. Finally, it was found that both the amount of inhibition produced by a prestimulus with a 64-msec lead time and the amount of latency reduction produced by a prestimulus with a 4-msec lead time are independent of the intensity of the startle-eliciting stimulus. These findings suggest that the inhibition effect is independent of the latency reduction effect and that both are independent of the absolute level of activation in the neural mechanisms responsible for the overt startle response.