Visual selective attention: a theoretical analysis.

The present paper outlines a framework which allows a consistent interpretation of data regarding visual selection in visual search tasks. It organizes and reviews visual search tasks in which the target is defined by primitive features, by conjunctions of features and when the target is categorically different from non-targets. The special role of spatial attention is reviewed and different theoretical accounts are discussed. Because visual selection depends principally on the outcome of the early parallel preattentive stage of processing, the main focus will be on this stage. It is concluded that visual selection is to a large extent determined by the physical characteristics of the stimuli present in the visual field. The early preattentive parallel process computes how different each object is from each of the other objects within a particular stimulus dimension. Attention is automatically drawn to the location having the highest activation, implying that the object at that location is automatically selected irrespective of the intentions of the subject. The model also assumes some top-down control. It is well known that attention can be voluntarily directed to nonfixated locations in visual space, varying from a uniform distribution over the visual field to a highly focused concentration. The model assumes that the endogenous direction of attention to an area in the visual field is the only top-down manner of affecting visual selection. Within the area of directed attention, no top-down control is possible: selection is completely determined by the physical properties of the stimuli.