Feature-guided attentional capture cannot be prevented by spatial filtering

Feature-based control processes guide attention towards objects with target features in visual search. While these processes are assumed to operate globally across the entire visual field, it remains controversial whether target-matching objects at task-irrelevant locations can be excluded from attentional selection, especially when spatial attention is already narrowly focused elsewhere. We investigated whether probe stimuli at irrelevant lateral locations capture attention when they precede search displays where targets are defined either by a specific feature (colour or orientation) or by a colour/orientation conjunction by measuring N2pc components (an electrophysiological marker of attentional target selection) to these probes. Reliable N2pcs were triggered by probes not only in the feature search tasks but also when participants searched for feature conjunctions, in spite of the fact that conjunction search requires focal spatial attention. Analogous N2pc results were found in the absence of any spatial uncertainty about the location of conjunctively defined targets, which always appeared at fixation. These results show that rapid attentional capture by objects with target-matching features cannot be prevented by top-down spatial filtering mechanisms, and confirm that feature-based attentional guidance processes operate in a spatially global fashion.