Dissociation of covert and overt spatial attention during prehension movements: selective interference effects

In four experiments, the influence of distractor objects on the temporal evolution of the reach-to-grasp movement toward a target object (an apple) was examined. In the first experiment, the distractor was another apple, which moved laterally behind the target and occasionally changed direction toward the target, thus becoming the to-be-grasped object. In the second and third experiments, the distractor was a stationary piece of fruit, which sometimes became the to-be-grasped object because of a change in illumination. The fourth experiment was a combination of the first two experiments. In all cases, selective interference effects on the transport and manipulation components were observed only when attention to the distractor was covert rather than overt. It is proposed that covert visuo-spatial attention selects information about distracting but potentially important stimuli, such that a registration of significance is accomplished without the need to process all available information.