The interaction of stimulus- and response-related processes measured by event-related lateralizations of the EEG

The present study focused on the relationship between movement- and stimulus-related asymmetries of the electroencephalogram (EEG). In seven tasks the same bilateral stimuli containing asymmetric information were presented but response requirements differed. Three functionally distinct asymmetries were found: (1) an asymmetry over the motor cortex prior to unimanual movements, (2) an asymmetry over the posterior cortex beginning about 20 ms after the start of the movement, and (3) an early increase of negativity contralateral to a relevant stimulus (200-300 ms after stimulus onset) that was maximal at temporo-parietal sites but was also visible at central sites. Although related to stimulus side, this asymmetry was modulated by response requirements: it was largely abolished with simple responses, smaller with nogo than with Go stimuli and occurred twice when a sequence of simple and choice responses was required. Therefore, the early temporo-parietal asymmetry most probably reflects an interface between sensory and movement-related processes.