Effects of endogenous and exogenous attention on visual processing: an Inhibition of Return study.

We investigate early (P1) and late (P3) modulations of event-related potentials produced by endogenous (expected vs. unexpected location trials) and exogenous (cued vs. uncued location trials) orienting of spatial attention. A 75% informative peripheral cue was presented 1000 ms before the target in order to study Inhibition of Return (IOR), a mechanism that produces slower responses to peripherally cued versus uncued locations. Endogenous attention produced its effects more strongly at later stages of processing, while IOR (an index of exogenous orienting) was found to modulate both early and late stages of processing. The amplitude of P1 was reduced for cued versus uncued location trials, even when endogenous attention was maintained at the cued location. This result indicates that the perceptual effects of IOR are not eliminated by endogenous attention, suggesting that the IOR mechanism produces a perceptual decrement on the processing of stimuli at the cued location that cannot be counteracted by endogenous attention.