Differential effects of a foreperiod countdown procedure on simple and choice reaction time.

Subjects responded to the onset of a red or green stimulus light by pressing a key. In a simple reaction time (RT) block, they responded to the light onset without regard to its color. In a choice RT block, they pressed a left or right key depending on the stimulus color. Within each block, half of the trials were "countdown trials,"; i.e., a sequence of lights provided precise information concerning the passage of time during a constant 2-sec foreperiod. The other half of the trials involved the identical constant foreperiod, but no countdown information was presented. Data analysis revealed significantly faster reactions on countdown trials, with choice RT shortened more than simple RT. Results reflect subjects' imperfect timekeeping ability and suggest that readiness affects the central-processing phase of information processing.