Combination rules for inhibitory stimuli.

Startle reactions are inhibited if their elicitation is preceded by weak neutral stimuli at lead times of fractions of a second, or by intense startle-eliciting stimuli at lead times of up to several seconds. In three experiments (a) two weak stimuli, (b) one weak stimulus and one intense stimulus, or (c) two strong stimuli were given in combination. In all three conditions the combination provided more response inhibition than did either stimulus alone. Conditions 1 and 2 gave combination effects in which the inhibited proportion of the response was described by the rule for compounding the probabilities of independent events, indicating that the stimuli had noncompetitive independent access to the inhibitory machinery. The rule over-estimated the effect of the compound in Condition c, suggesting the presence of some other summative process, perhaps involving the prolongation of the refractory period.