THE LOCUS OF INTERFERENCE IN THE SIMON EFFECT - AN ERP STUDY

The Simon effect was studied using ERPs and the lateralized readiness potential (LRP). In Experiment 1, visual stimuli requiring left or rig ht hand responses were presented at either side of fixation. In Experi ment 2, the empty side was occupied by a distracter. Spatial S-R incom patibility delayed P300 latency in both experiments, as predicted by p erceptual interference accounts of the Simon effect. However, the LRP results were not conclusive. The lateralized presentation of visual st imuli (Experiment 1) or targets (Experiment 2) produced a posterior ER P hemispheric asymmetry that spread to central electrodes and obscured the early development of the LRP. Experiment 3 avoided the problem by presenting the stimuli in the vertical meridian, and the LRP showed s igns of incorrect response preparation in incompatible trials, in agre ement with response-interference models. Both perceptual and response interference may be present in the Simon effect.