The Simon effect and its reversal studied with event-related potentials.

The Simon effect (slower RT when stimulus and response locations do not match, stimulus location being irrelevant) and its reversal, were investigated using P300 latency and motor-related potentials. Stimuli were one of two colors, and response keys were two buttons with color labels that changed randomly in every trial. Depending on the stimulus-response (S-R) mapping instructions, subjects pressed the same-color key or the alternate-color key. Behavioral measures showed a Simon effect in same- and a reverse Simon effect in alternate-color mapping contradicting the display-control arrangement correspondence and the S-S compatibility, as explanations for the reverse Simon effect. P300 latencies 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 motor-related potentials demonstrated response activation based on stimulus location, also supporting response- interference interpretations of the Simon effect.