Exploring handedness asymmetries in the Simon effect.

Spatially corresponding stimulus-response pairings usually produce shorter reaction times (RTs) than do non-corresponding pairings, even when the spatial dimension of the stimulus is irrelevant to the task. This "Simon effect" for visual stimuli and manual responses is often larger for the stimulus location on the side to which the person's dominant hand is operating. The present study aimed at replicating and examining the nature of this asymmetry. To determine whether the Simon effect asymmetry is a function of the hand distinction or of conceptual spatial codes, performance with left and right manual key-presses was compared to that with vocal responses "left" and "right." Whether the asymmetric Simon effect pattern is restricted to spatial stimuli was tested by comparing effects obtained with left and right located squares to those found with the centered words Left and Right. The asymmetry was only replicated for the spatial stimulus-manual response Simon task, for which a second experiment showed similar results when the hands and response box were not visible during task performance. The analysis revealed a general dominant-hand RT advantage that similarly adds to both corresponding and non-corresponding trials and is rather independent from the Simon effect. This advantage yields an apparent asymmetry when the data are analyzed as a function of correspondence for each stimulus location because the corresponding and non-corresponding RTs that are compared come from different hands.