Abstract
Visual search for target objects defined by specific features can be top-down controlled by attentional templates that are activated prior to search and subsequently bias selection of template-matching events in the visual field. In contrast, when targets are feature singletons that can be selected based on their physical (bottom-up) salience, no such guiding templates are required. However, it is unknown whether attentional guidance is purely reactive during singleton search, or whether participants still pro-actively prepare for an upcoming search display. We compared search preparation during feature-guided and singleton search in tasks in which observers respectively searched for a pre-defined target-colour object (red) or a colour singleton (red or green) in circular search displays with five homogeneously grey items. Search displays were presented every 1600ms. To assess template activation during search preparation, a continuous stream of circular probe displays (50ms duration) was presented every 200ms between successive search displays. These displays either contained a target-colour or a task-irrelevant distractor-colour singleton. N2pc components – electrophysiological markers of attentional capture – were measured separately for probes at different temporal positions between search displays. During feature search, only template-matching target-colour probes triggered N2pcs while distractor-colour probes did not, demonstrating that a feature-selective template was activated. In the singleton search task, both target- and distractor-colour probes triggered N2pc components, indicating that preparatory attentional templates were activated, but that these templates were colour-unspecific. A second experiment that compared singleton search to a feature search task with non-salient targets in heterogeneous search displays confirmed these findings. In sum, these results show that attentional guidance in singleton search is not purely reactive, and that participants activate attentional templates for colour discontinuities during the preparation for such search tasks.