Look left but attend right: an ERP investigation of the effects of covert attention and saccadic eye movement on visual processing

Close coupling between saccade preparation and shifts of visual attention are frequently demonstrated by more efficient processing of visual stimuli presented close to the target of an intended saccade than at other locations. However, recent evidence has individuated an ‘independent’ attentional component that can be directed away from the saccadic target (Montagnini & Castet, 2007). We tested the independence of this attentional component with event related potentials. Participants were cued to simultaneously direct covert spatial attention (Attention task) and prepare a saccadic eye movement (Saccade task) toward either the same target (Same side condition) or to targets located on opposite sides (Opposite sides condition). ERPs elicited by peripheral visual stimuli at cued and uncued locations showed that visuo-spatial attention was initially shifted towards the cued target of the Attention task in both the Same and Opposite side conditions as indexed by similar enhancement of the N1 component (150-190 ms post stimulus). Crucially, attentional effects were drastically reduced in the Opposite sides condition beyond 250 ms post-stimulus. These results suggest that in the opposite sides condition, spatial attention was first allocated to the attention task target, and only after an initial processing of the visual stimulus, at least a component of attention was shifted towards the saccade target. Thus, in the opposite sides condition, visuo-spatial attention is not directed simultaneously and in parallel towards the opposite target locations but is serially allocated to the attention task target first, then subsequently re-directed towards the saccade task target closer to movement onset. These findings support the idea of shared mechanisms between covert attention and saccade preparation.