THE LOCATION OF PRECEDING STIMULI AFFECTS SELECTIVE PROCESSING IN A SUSTAINED ATTENTION SITUATION

Event-related potentials (ERPs) were recorded to visual and auditory s timuli in a situation where subjects were required to attend selective ly to the left or right side for an entire experimental block and to d etect occasional target stimuli at attended locations. Stimuli were pr esented randomly at attended and unattended locations. In exp. 1, visu al and auditory stimuli were presented in separate blocks, while in ex p. 2, they were presented together and subjects had to detect visual t argets at attended locations. Stimuli at attended positions elicited e nlarged sensory-evoked potentials and an enhanced negativity at midlin e electrodes as compared with unattended stimuli. The latter effect wa s, however, modulated by the location of the preceding stimulus. At fr ontocentral electrodes, it was larger for stimuli that were preceded b y stimuli at the contralateral side as compared with stimuli preceded by stimuli at the same location. It is argued that this effect may be due to a different amount of processing required for the preceding sti mulus. When the predecessor is at a to-be-attended location, it has to be processed more intensively which may interfere with the processing of the next stimulus.