The Simon effect in action: planning and/or on-line control effects?

Choice reaction tasks are performed faster when stimulus location corresponds to response location (Simon Effect). This spatial stimulus–response compatibility effect affects performance at the level of action planning and execution. However, when response selection is completed before movement initiation, the Simon effect arises only at planning level. The aim of the present study was to ascertain whether when a precocious response selection is requested, the Simon Effect can be detected on the kinematics characterizing the online control phase of a non-ballistic movement. Participants were presented with red or green colored squares; they could appear on the right, left, above or below a central cross. Depending on square’s color they had to release one of two buttons (right/left), reach towards and press a corresponding lateral pad. We found kinematic evidence of Simon Effect on both action planning and on-line control. Moreover, the investigation of response conflict at the level of previous trials (i.e., n-1), a factor which might determine interference at the level of the current response, revealed a conflict adaptation process across trials. Results are discussed in terms of current theories concerned with the Simon Effect and the distinction between action planning and control.