This study investigated the locus of the interference effect in a stimulus-response compatibility task using event-related potentials (ERPs). Ten participants were instructed to respond to stimulus color with the left or right middle finger. Red or blue arrows pointed in the same direction as the response hand on congruent trials and pointed in the opposite direction on incongruent trials. Neutral trials were red or blue horizontal bars. Reaction times (RTs) to incongruent stimuli were significantly longer than RTs to congruent stimuli. The peak latency of the P300 for incongruent stimuli was significantly longer than that for congruent stimuli. In addition, onset of stimulus-locked lateralized readiness potential (LRPs) was significantly later for incongruent stimuli than for congruent stimuli. However, electromyogram (EMG)-locked LRPs for incongruent stimuli showed incorrect preparation. These results suggest that the interference effect might occur at the stage in which stimulus evaluation processes and response-related processes overlap.
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