The effects on performance of unattended stimulus–response
spatial relationships in choice reaction time tasks (i.e.,
the Simon effect) have been attributed to automatic activation
of the response ipsilateral to stimulus location. We tested
this assumption using the lateralized readiness potential
(LRP). The response key labels changed randomly from trial
to trial and were presented either 400 ms before (immediate-reaction
trials) or 400 ms after (delayed-reaction trials) stimulus.
The critical test for the automatic activation hypothesis
was on delayed-reaction trials, in which LRP deflections
were expected in the interval between stimulus and response-key
labels. Contrary to this prediction, there were no LRP
signs of response activation within that interval.