Ideomotor theory posits that actions are controlled by the anticipation of their effects. In line with this theoretical framework, response-contingent action effects have been shown to influence performance in choice-reaction time tasks, both in single-task and task-switching context. Using a task-switching paradigm, the present study investigated whether task-contingent action effects influenced N - 2 repetition costs in task switching. N - 2 repetition costs are thought to be related to task-switch costs, and reflect inhibitory control in task switching. It was expected that task-contingent action effects reduce between-task interference, leading to reduced N - 2 repetition costs. An experimental group (N = 24) performed eight blocks of trials with task-contingent action effects, followed by one block with non-contingent action effects; a control group (N = 24) performed nine blocks of trials with non-contingent action effects. In line with our expectations, a three-way interaction of group, block, and task sequence was obtained, indicating differential data patterns for the two groups: In error rates, the group who had received contingent action effects throughout blocks 1-8 showed larger N - 2 repetition costs in the random block 9 than in block 8, whereas the control group showed a reversed data pattern. The RT data pattern was in the same direction, although no significant three-way interaction was obtained. Taken together, we tentatively conclude that task-contingent action effects reduce task inhibition in task switching, and we outline directions for future research on the role of action effects in multitasking performance.
