Rapid Changes of Attentional Priorities in Visual Search: Tracking Covert Switches of Preparatory Attentional Templates in Real Time.

Attentional selectivity focuses on what is currently relevant. Relevance changes frequently in everyday life, triggering rapid reassignments of attentional priorities. Such reassignments are often not associated with behavioral changes and are thus difficult to assess objectively. Here, we measured rapid, covert switches between preparatory task settings (attentional templates) in visual search, as they occurred in real time. Participants searched for color-defined targets in search displays that appeared unpredictably either early (after 700 msec) or late (after 1500 msec) on each trial. In Experiment 1, early and late targets were defined by different colors. Participants first had to activate a template for the early target color and then switch to a template for the late target color if no early search display appeared. In Experiment 2, cues signaled whether the initial target template had to be maintained or changed. Template activation states were tracked with N2pc components to rapid sequences of irrelevant probes matching either the early or late target color. A template for the early target color was active from about 300 msec before the expected arrival of early search displays, followed by a template switch. Switches based on endogenous temporal expectations emerged more gradually in time than switches in response to external cues. Presenting cues in Experiment 2 triggered a temporary search template deactivation even when the target color remained unchanged, indicating that template maintenance is subject to an attentional blink. Results demonstrate that rapid switches between attentional templates in visual search can be tracked with high temporal precision.