Neurophysiological experiments demonstrate that the retina performs filtering which identifies contrast spatially and temporally. For instance, center surround cells at the retinal processing stage are triggered by local spatial changes in intensity (contrast) . In psychological tests, humans detect high contrast objects more readily than, say, objects with a similar colour to their background. A significant change in spatial intensity is referred to as an edge, boundary or a contour. Further research has shown that response to spatial contrast also varies with time . A moving edge, for example, triggers a strong response at the retinal level . Thus, contrast or intensity changes over space and time are important in vision and this has lead to the development of edge detectors and motion detectors.
Another property in estimating perceptual importance is symmetry . The precise definition of symmetry in the context of attentional mechanisms is different from the intuitive concept of symmetry. Symmetry, here, represents the symmetric enclosure or the approximate encirclement of a region by contours. The appropriate arrangement of edges which face each other to surround a region attracts the human eye to that region. Furthermore, the concept of enclosure is different from the mathematical sense of perfect closure since humans will still perceive a sense of enclosure despite gaps in boundaries that surround the region .
Scale is also a feature which determines a region's relative importance in a scene . It is progressively easier to detect a foreground object if it occupies a greater and greater area in our field of view. Generally, as an object is enlarged, it increases in relative importance.