The location of preceding stimuli affects selective processing in a sustained attention situation.

Event-related potentials (ERPs) were recorded to visual and auditory stimuli in a situation where subjects were required to attend selectively to the left or right side for an entire experimental block and to detect occasional target stimuli at attended locations. Stimuli were presented randomly at attended and unattended locations. In exp. 1, visual and auditory stimuli were presented in separate blocks, while in exp. 2, they were presented together and subjects had to detect visual targets at attended locations. Stimuli at attended positions elicited enlarged sensory-evoked potentials and an enhanced negativity at midline electrodes as compared with unattended stimuli. The latter effect was, however, modulated by the location of the preceding stimulus. At frontocentral electrodes, it was larger for stimuli that were preceded by 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 stimulus. When the predecessor is at a to-be-attended location, it has to processed more intensively which may interfere with the processing of the next stimulus.