THE SIMON EFFECT AND ITS REVERSAL STUDIED WITH EVENT-RELATED POTENTIALS

The Simon effect (slower RT when stimulus and response locations do no t match, stimulus location being irrelevant) and its reversal. were in vestigated using P300 latency and motor-related potentials. Stimuli we re one of two colors, and response keys were two buttons with color la bels that changed randomly in every trial. Depending on the stimulus-r esponse (S-R) mapping instructions, subjects pressed the same-color ke y or the alternate-color key, Behavioral measures showed a Simon effec t in same- and a reverse Simon effect in alternate-color mapping contr adicting the display-control arrangement correspondence and the S-S co mpatibility, as explanations for the reverse Simon effect. P300 latenc ies followed the same pattern as RT, i.e. they were influenced by the S-R mapping, suggesting that this measure does not index the duration of the stimulus evaluation, but response selection processes. The moto r-related potentials demonstrated response activation based on stimulu s location, also supporting response- interference interpretations of the Simon effect.