Two stimuli are better than one: combination rules for prepulse inhibition revisited

Stimuli that precede elicitation of the startle reflex can inhibit that reflex. J. R. Ison, M. Zuckerman, and J. M. Russo (1975), measuring the startle reflex in the rat, showed that the effect of combining two inhibiting stimuli was greater than that of either alone. They also demonstrated that this combination effect could be described by using the rule for adding the probability of independent events, expressing the effect of each prestimulus as the proportion of the response inhibited by that stimulus. The purpose of this research was to extend those findings using the startle blink in humans. In two experiments, participants encountered trials in which two stimuli (p1 and p2) were given in succession, ending with a blink-eliciting noise (S). In Experiment 1 (N=18) p1 was a synchronous tone and a word (appearing on a computer screen), and p2 was a blink-eliciting noise identical to S. In Experiment 2 (N=30), p1 was a soft tone and p2 was a word. For both experiments, two stimuli were more inhibiting than one, and the effect of combining p1 and p2 was accurately described by the Ison et al. combination rule.