The timing of stimulus localisation and the Simon effect: an ERP study.

The Simon effect, an acceleration of responses at the same side that a stimulus is presented, is assumed to be the consequence of an automatic response activation evoked by the processing of the irrelevant stimulus location. This activation has been reported to decline as responses become slower. Consequently, the Simon effect decays over time. However, it remains unclear when this activation starts and what process initiates it. Up to now, the decaying hypothesis and its temporal properties have been based on indirect evidence. In the present study we tested the timing of the decay of the Simon effect more directly by combining a localisation task and a Simon task in an EEG study. It can be shown that the response activation is evoked by visual spatial processing, and that the size of the Simon effect steadily decreases as a function of the time between this localisation process and the manual response. However, this finding only holds if the encoding of relevant stimulus features follows the localisation process unequivocally.