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Christian C. Ruff, Jon Driver; Attentional preparation for stimulus competition: Psychophysical and fMRI evidence. Journal of Vision 2004;4(8):24. doi: 10.1167/4.8.24.
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Selective attention enables us to process relevant stimuli in the presence of competing information. We used psychophysics and fMRI to investigate the effects and mechanisms of attentional preparation for competition between visual hemifields. In the psychophysical study, seventeen healthy volunteers made speeded responses to targets presented to the left or right of fixation, with or without distractors presented in the contralateral hemifield. In a within-subject design, participants were either cued on a trial-by-trial basis for target side (left/right), or for both target side and presence of distractors (present/not present). Response accuracy was at 90% for all types of trials. Irrespective of cueing condition, responses were slower on bilateral distractor trials than on unilateral target-only trials. However, this distractor-related slowing was significantly reduced for bilateral trials on which participants knew about the presence of the distractor in advance. Response times to target-only trials were not different for the two cueing conditions. These results indicate that competition between contralateral visual stimuli can be reduced by attentional preparation, and is thus at least partially under voluntary control. In the fMRI study, we investigated the neuronal processes underlying this preparatory mechanism with identical stimuli. Initial analyses indicate that preparation for unilateral target-only trials elicits baseline shifts only in contralateral occipital areas involved in the subsequent processing of target stimuli, while preparation for distractor trials additionally leads to baseline shifts in the neuronal structures processing the anticipated distractor.
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