SIMON EFFECT USING MOVEMENT-RELATED BRAIN POTENTIALS

The present study was aimed to diagnosis the locus of interference in the Simon effect (before or during the response-preparation stage) usi ng movement-related brain potentials (the lateralized readiness potent ial, LRP). Arrowheads pointing left or right, were presented at either side of fixation and subjects pressed a key to indicate the direction of the arrow. The RTs showed the typical Simon effect and the spatial S-R incompatibility produced an early motor activation of the incorre ct response. These results are in agreement with models which posit an automatic activation of the ipsilateral response (e.g., Craft and Sim on, 1970; Kornblum et al., 1990) and in contradiction with perceptual interference models such as Hasbroucq and Guiard (1991) or Chen-Hui an d Proctor (1994). However, we doubt that the LRP between 100 and 200 m s poststimulus is indexing response preparation, at least in this expe riment. The reason is that lateralized presentation of visual stimuli produces amplitude asymetries in the same temporal range (N100) at par ietal and occipital sites, and we have gather preliminary evidence sup porting that these interhemispheric amplitude differences might contam inated the ERPs recorded at central sites.