Time uncertainty and occurence uncertainty of the stimulus in a simple reaction time task

Abstract

Relatively little attention has been paid to the effect of the so-called 'catch trials' on the level of performance in reaction-time experiments. The first investigator to approach systematically this important problem, which is significant also outside the sphere of reaction-time studies, was Gordon (1967), who demonstrated that reaction speed in a simple reaction-time task is an accelerating function of stimulus probability, i.e., the probability of delivery of the stimulus at a trial. The present study aims at clarifying the same problem using a somewhat different experimental setting. Instead of only one foreperiod duration as used by Gordon, four different foreperiods were employed. Also measurements under stimulus-certainty conditions were introduced for purposes of comparison. In addition, the frequency of trials was equalized between experimental conditions involving differing stimulus probabilities, instead of Gordon's equalization of the response rate. Eight subjects participated in 12 experimental sessions. The results suggested a linear relationship between the stimulus probability and the reaction time. Finally, the concept of occurence uncertainty of the stimulus, designating the subject's uncertainty as to whether the stimulus would be presented at a trial, was compared with Klemmer's time uncertainty as a determinant of the simple reaction time, and three factors were proposed to explain the slowing down of reactions as occurence uncertainty increases.