The gradual emergence of spatially selective target processing in visual search: From feature-specific to object-based attentional control.

To dissociate feature-based and object-based stages in the control of spatial attention during visual search, we employed the N2pc component as an electrophysiological marker of attentional object selection. Participants searched for a target object that was defined by a conjunction of color and shape. Some search displays contained the target or a nontarget object that matched either the target color or its shape among 3 nonmatching distractors. In other displays, the target and a partially target-matching nontarget object appeared together. N2pc results demonstrated that the initial stage of attentional object selection is controlled by local feature-specific signals. Attention is allocated in parallel and independently to objects with target-matching features during this early stage, irrespective of whether another target-matching object is simultaneously present elsewhere. From around 250 ms poststimulus, information is integrated across feature dimensions, and spatially selective attentional processing becomes object-based. These findings demonstrate that feature-based and object-based stages of attentional selectivity in visual search can be dissociated in real time.