Saccadic selectivity in complex visual search displays.

Visual search is a fundamental and routine task of everyday life. Studying visual search promises to shed light on the basic attentional mechanisms that facilitate visual processing. To investigate visual attention during search processes, numerous studies measured the selectivity of observers' saccadic eye movements for local display features. These experiments almost entirely relied on simple, artificial displays with discrete search items and features. The present study employed complex search displays and targets to examine task-driven (top-down) visual guidance by low-level features under more natural conditions. Significant guidance by local intensity, contrast, spatial frequency, and orientation was found, and its properties such as magnitude and resolution were analyzed across dimensions. Moreover, feature-ratio effects were detected, which correspond to distractor-ratio effects in simple search displays. These results point out the limitations of current purely stimulus-driven (bottom-up) models of attention during scene perception.