Beyond a unitary Simon effect: dissociating stimulus- and response-driven spatial interference across finger and leg responses

The Simon effect-faster and more accurate responding when stimulus location corresponds to response side-reflects competition between goal-directed response selection and automatic spatial coding. Although typically treated as a single performance index, this aggregate measure obscures the distinct contributions of stimulus location (left vs. right) and response side (left vs. right) to spatial interference. The present study examined how effector type shapes stimulus- and response-related spatial interference by comparing seated finger presses and standing leg taps in the Simon task (N = 32). Treating stimulus location and response side as independent analytical factors revealed four stimulus- and response-driven interference contrasts for each effector. For reaction times, all four contrasts were significant for leg responses, whereas three were significant for fingers, with the right response-driven contrast the largest (58.9 ms) and the left response-driven the smallest (27.1 ms). For errors, significant effects were observed for fingers only, in the right response-driven (6.80%) and left stimulus-driven (6.49%) contrasts. The congruency sequence effect-reduced interference following incongruent relative to congruent trials-was significant for both effectors in reaction times, with a larger numerical reduction for fingers (77.2%) than legs (43.9%), but significant only for fingers in errors, where interference was eliminated and marginally reversed following incongruent trials. The four interference contrasts decreased substantially across blocks for leg responses - both in absolute and proportional terms - but remained largely stable for fingers. These findings highlight effector-specific patterns of conflict resolution, with potential implications for cognitive-motor assessment and rehabilitation.