Sudden and gradual presentation of distracter objects: differential interference effects

In solving the selection-for-action problem, it is believed that attentiona l mechanisms enable dominance of target over non-target objects. However, u nder some conditions, information from non-target objects "interferes" with the action to a relevant target. We investigated the possibility that this interference may result when the irrelevant object activates a specific su bset of visuomotor pathways. Participants reached to grasp three-dimensiona l. stimuli while actively attending to a nearby flanker object. The means b y which the flanker was presented was manipulated. This relevant object was illuminated either abruptly or gradually. The parvocellular pathway in ear ly visual processing is equally activated in both conditions. The magnocell ular pathway is strongly activated by abrupt presentation and weakly activa ted with gradual presentation of the flanker object. Kinematics of the reac h-to-grasp action to the target showed signs of interference only in the su dden illumination condition. This suggests a dissociation between dorsal an d ventral cortical streams in terms of relevance for action. Our data sugge sts that this effect is not due to early visual-pathway differences, but in stead reveals a property of a transient object-based visual attention mecha nism.