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In a two-stimulus two-response choice reaction time (RT) task in which Ss made stimulus predictions, the probability of a correct prediction was manipulated between Ss. The magnitude of the difference in RT to correctly and incorrectly predicted stimuli (i.e., the prediction outcome effect) was an increasing function of the probability of a correct prediction. This finding was primarily due to a reliable decrease in RT to correctly predicted stimuli as the probability of a correct prediction increased, since RT to incorrectly predicted stimuli was not affected by prediction outcome probability. These results were interpreted as partially supporting a continuous expectancy notion which involves facilitory and inhibitory mechanisms which are differentially influenced by the probability of a correct prediction.

Choice latency was significantly influenced by the outcome of 5s' predictions on current and preceding trials in a discrete three-stimulus, two-response reaction time (RT) experiment. Three mutually exclusive categories of prediction outcome (PO) were correct stimulus and response prediction (CoS-CoR), incorrect stimulus and correct response prediction (InS-CoR), and incorrect stimulus and response prediction (InS-InR), Mean RTs of the current PO category, CoS-CoR, were significantly faster when the preceding PO was CoS-CoR. Mean RTs following an InS-CoR prediction were faster than RTs following an InS-InR prediction only when the preceding PO was InS-CoR. These results were interpreted as indications of the facilitation of the stimulus-identification or the response-selection components of the choice reaction process. The implications of an interaction between preceding PO and the complexity of the S-R relationship were discussed.

In two-choice reaction time (RT) situations, the interval between the prediction and the presentation of successive stimuli was either 3, 7, or 11 sec. The 20 5s of one group (Group Stim) pressed a left-hand trigger following one stimulus and a right-hand trigger following the alternative stimulus; 5s of another group (Group Pred) identified the prediction outcome on each of the 200 trials by making one response if the prediction was correct and the other response following an incorrect prediction. The influence of the prediction-stimulus interval implicated both expectancy and memory factors in choice RT. That is, RTs were faster following correct than incorrect predictions, and this effect of prediction outcome was greatest following the shortest interval. The reliable effects of prediction outcome on RT were more prominent for Group Pred than for Group Stim.When required to predict the sequential occurrence of stimulus alternatives in a choice reaction time (RT) paradigm, 5s react markedly faster to correctly predicted than to incorrectly predicted events (e.g., Geller & Pitz, 1970;Geller, Whitman, & Karris, 1972;Keele, 1969). The present experiment was designed to determine if the temporal interval occurring between the prediction and the presentation of a stimulus would influence the effects of the prediction outcome on reaction latency. The three prediction-stimulus intervals used (3, 7, and 11 sec.) were within a range in which short-term memory (STM) for consonant trigrams was shown to decay (Peterson & Peterson, 1959). Hence, an STM model might hypothesize that the prediction outcome effect would be most apparent at the shortest interval (i.e., 3 sec.), since at this interval the memory trace of a verbalized expectancy would be strongest. Following the longest interval (i.e., 11 sec.) the effect of prediction outcome on choice RT should be least prominent, since the verbal STM for the stimulus 1 The authors thank David A. Williams for his assistance in collecting the data and Western Electric Company, Inc., for its donation of electronic supplies.2 Requests for reprints should be sent to E.