When a visual stimulus is randomly displayed either to the left or to the right of the fixation point, manual response is faster when both the response key and the stimulus are located at the same side (compatible condition) than at opposite sides (incompatible condition). In a spatial compatibility task, a difference of 30-40 ms is found between the manual reaction times (MRT) of compatible and incompatible conditions. In the Simon effect, though the criterion to response selection is primarily stimulus shape (or color), the stimulus position also influences the MRT, either increasing it in the compatible condition or delaying it in the incompatible condition. The Simon effect can be inverted if the subject has recently performed incompatible tasks, showing that short-term memory processes elicited by the incompatible associations can modulate stimulus-response relationships based on long-term memory.