Preparatory Attention to Visual Features Spreads Globally.

Attention to visual features facilitates neural responses to task-relevant stimuli and also to task-irrelevant stimuli at other locations that share the attended features. This is assumed to enable guided visual search. However, such global facilitation of features has only been observed during active selection of physically presented features. Here, we investigated whether the global spread occurs during preparatory attention, before the presentation of the attended physical stimulus, which would provide empirical support for its role in guiding visual search. We presented overlapping blue and orange dot fields to the left and right of central fixation, all flickering at different frequencies. We recorded EEG while 20 human observers (17 female, 3 male) were cued to attend to a specific dot field (e.g., blue dots on the left) to detect a weak coherent motion signal. In the immediate-onset condition, dots on both sides were presented simultaneously following the cue; whereas in the delayed-onset condition, only unattended-side dots were presented first and attended-side dots appeared after an extended preparatory period. We observed the typical local and global feature-based attention effects on the frequency-specific visual-evoked responses in the immediate-onset condition. Notably, we also observed a global effect in the delayed-onset condition during the preparatory period. This spread of preparatory attention to features supports the link between global feature-based attention and guided visual search. Positive correlations between the local and global effects and between preparatory attention and active selection further suggest a common origin of a top-down control signal that enables global feature-based attention.