Stop before you leap: changing eye and hand movements requires stopping.

The search-step paradigm addresses the processes involved in changing movement plans, usually saccadic eye-movements. Subjects move their eyes to a target (T1) among distractors, but when the target steps to a new location (T2), subjects are instructed to move their eyes directly from fixation to the new location. We ask whether moving to T2 requires a separate stop process that inhibits the movement to T1. It need not. The movement plan for the second response may inhibit the first response. To distinguish these hypotheses, we decoupled the offset of T1 from the onset of T2. If the second movement is sufficient to inhibit the first, then the probability of responding to T1 should depend only on T2 onset. If a separate stop process is required, then the probability of responding to T1 should depend only on T1 offset, which acts as a stop signal. We tested these hypotheses in manual and saccadic search-step tasks and found that the probability of responding to T1 depended most strongly on T1 offset, supporting the hypothesis that changing from one movement plan to another involves a separate stop process that inhibits the first plan.