Acoustic and Temporal Factors in the Evocation of Startle

Abstract

Several acoustic and temporal variables responsible for the elicitation of the rat''s startle reaction were examined by measuring the subject''s overt movement under each of a variety of stimulus conditions. When startle was repeatedly evoked, the response decreased in amplitude, but adaptation was slow and was not complete after 675 stimulus presentations. The threshold for startle evocation increased with repeated stimulation, but partial recovery occurred during interpolated rest periods. Startle reactions were amplified when testing occurred in a background to steady noise, but the effect was independent of the frequency components of the background stimulation. Moreover, although a relatively weak signal could inhibit the reaction to a subsequently presented intense signal, the effect was independent of the frequency characteristics of either signal. Relatively weak signals could either inhibit the startle reaction to a subsequently presented intense signal or reduce response latency, depending upon the temporal relationships between the 2 signals. Weak signals can activate the neural mechanisms responsible for the startle reaction, and the occurrence of acoustic startle along with its associated inhibitory and facilitory effects is dependent upon the sensation levels of the signals, but not open their frequency characteristics.