Searching for stimulus-driven shifts of attention.

Several types of dynamic cues (e.g., abrupt onsets, motion) draw attention in visual search tasks even when they are irrelevant. Although these stimuli appear to capture attention in a stimulus-driven fashion, typical visual search tasks might induce an intentional strategy to focus on dynamic events. Because observers can only begin their search when the search display suddenly appears, they might orient to any dynamic display change (Folk, Remington, & Johnston, 1992; Gibson & Kelsey, 1998). If so, the appearance of capture might result from task-induced biases rather than from the properties of the stimulus. In fact, such biases can even create the appearance of stimulus-driven capture by stimuli that typically do not capture attention (Gibson & Kelsey, 1998). The possibility of task-induced, top-down biases plagues the interpretation of all previous studies claiming stimulus-driven attention capture by dynamic stimuli. In two experiments, we attempt to eliminate potential task-induced biases by removing any need to monitor for display changes. In the first experiment, search displays did not change on most trials. In the second experiment, although new search displays appeared on each trial, we ensured that observers never saw the changes, by making them during large saccades. In both cases, dynamic events still received search priority, suggesting that some dynamic stimuli capture attention in a stimulus-driven fashion.