Endogenous and exogenous control of visual selection.

Among the most fundamental issues of visual attention research is the extent to which visual selection is controlled by properties of the stimulus or by the intentions, goals, and beliefs of the observer. Before selective attention operates, preattentive processes perform some basic analyses segmenting the visual field into functional perceptual units. The crucial question is whether the allocation of attention to these perceptual units is under the endogenous control of the observer (intentions, goals, beliefs) or under the exogenous control of attention in tasks in which subjects search for a particular 'basic' feature (eg search for a unique colour, shape, or brightness). In the present review it is suggested that selectivity in these types of search tasks is dependent on the relative saliency of the stimulus attributes. It is concluded that the visual system automatically calculates differences in basic features (eg difference in shape, colour, or brightness) and that visual information occupying the position of the highest saliency across stimulus dimensions is exogenously passed on to the 'central presentation' that is responsible for further stimulus analysis. Alternative explanations of the present findings and tentative speculations resulting from the present approach are discussed.