Previous behavioural and neuroscience studies have shown that the systems involved in the control of attention and action are functionally and anatomically linked. We used behavioural and event-related brain potential measures to investigate whether such links are mandatory or merely optional. Cues presented at the start of each trial instructed participants to shift attention to the left or right side and to simultaneously prepare to a finger movement with their left or right hand. In different trials, cues were followed by a central Go signal, requiring execution of the prepared manual response (motor task), or by a peripheral visual stimulus, which required a target–non-target discrimination only when presented on the cued side (attention task). Lateralised ERP components indicative of covert attention shifts were found when attention and action were directed to the same side (same side condition), but not when attention and action were directed to opposite sides (opposite sides condition). Likewise, effects of spatial attention on the processing of peripheral visual stimuli were present only when attention and action were directed to the same side, but not in the opposite sides condition. These results demonstrate that preparing a manual response on one side severely disrupts the attentional selection of visual stimuli on the other side, and suggest that it is not possible to simultaneously direct attention and action to different locations in space. They support the hypothesis that the control of spatial attention and action are implemented by shared brain circuits, and are therefore linked in a mandatory fashio
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