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Subjectively, it appears to us that the main function of our visual sense is to identify objects and to evaluate relationships between them. Unless we are confronted with specially designed perceptual tasks we tend to be unaware that the segmentation of the visual world into distinct objects and shapes is, by itself, a major achievement of our visual system. The retinal image of a visual scene consists of a two-dimensional, continuous distribution of grey levels. Before the visual system can identify particular figures or objects it needs to determine which of the various luminance values belong to individual objects, or the embedding background. Some grouping has to be performed in order to associate these luminance distributions with contours, to associate particular contours with a single object, and to segregate objects with overlapping contours from each other and from the background. These operations are commonly defined as scene segmentation or figure-ground segregation. Because they are usually carried out subconsciously and do not require the direction of selective attention to particular features of the scene these operations are commonly called "preattentive visual processes' or "early visual processes' (for review and examples see: 30,31,25,4,41,42,34).

DOI: 10.1007/978-3-642-48779-8_5

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