Refractory period and habituation of acoustic startle response in rats.

The time course of the recovery cycle (refractory period) of the acoustic startle response in rats was determined. The refractory effect was shown to be highly dependent upon the interval between two startle stimuli but independent of the intensity of the stimuli at a given interval, provided that the intensity of both stimuli was the same. A model based on summation of refractory effects to repetitive stimulation did not predict habituation to repetitive stimulation. The interaction between refractory effect and habituation was discussed, and it was suggested that short interstimulus intervals reduce the effective intensity of the habituation stimulus so that the effects of frequency and intensity become confounded in habituation studies where the interval between habituation stimuli invades the refractory period of the response being studied. INTRODUCTION The acoustic startle reaction is a brief short-latency sequence of generalized muscular contractions to the onset of a loud noise (Dodge & Louttit, 1926; Fleshler, 1965; Landis & Hunt, 1939). It occurs in a wide variety of species and shows pronounced decrement in response magnitude to a repetitive stimulus delivered at interstimulus intervals (ISIs) of from seconds to minutes (Cohen, 1929; Prosser & Hunter, 1936). For this reason, it has proven to be a useful response system for studying elementary forms of behavioral plasticity such as habituation and sensitization (Davis & Wagner, 1968; Groves & Thompson, 1970; Moyer, 1963; Prosser & Hunter, 1936). Continuous auditory background stimulation has been shown to facilitate the amplitude of the acoustic startle response (Hoffman & Fleshler, 1963; Ison & Hammond, 1971) whereas pulsed background stimulation produces a pronounced decrease in the amplitude of the reaction (Hoffman & Fleshler, 1963). The explanation of the latter phenomenon has been that the acoustic startle response is refractory for some period of time following its occurrence (Cohen, 1929; Dodge & Louttit, 1926). Thus,if one startle stimulus precedes another within a restricted time period, the second stimulus will produce a relatively smaller response (Dodge & Louttit, 1926; Hoffman & Searle, 1965; Ison & Hammond, 1971). This effect, which has been called variously “prepulse inhibition” or “refractory period,” is dependent upon the time between the two startle stimuli, with shorter intervals producing more marked refractoriness. Although it has been shown that the degree of refractory effect is dependent upon the intensity of the first stimulus (Hoffman & Searle, 1965; Ison & Hammond, 1971), a higher intensity producing greater decrement in the second response, whether the refractory period is dependent upon intensity when the intensity of both stimuli is held constant, has not been reported. Another question of recent concern has been the role of the refractory period of the acoustic startle response, as well as other response systems, in the progressive decrement in response amplitude (habituation) which results from repetition of the acoustic stimulus. The hypothesis that response refractory period might account for habituation was first suggested by Cohen (1932) for habituation of the eye blink to acoustic stimulation inhumans and, later, as part of a general theory of habituation by Ratner (1970). The present experiments were designed to determine (a) the effects of absolute stimulus intensity and ISI on the refractory period of the acoustic startle response in the rat and (b) the possible contribution of refractory period to habituation of the acoustic startle J.Comp. Physiological Psychology 1973, Vol. 83, No. 3, 492-498