MAKING SENSE OF THE WORLD

GROUPING AND SEGMENTATION
The patches of light and shade that form a retinal image are produced by a world of objects. The task of the visual system is to represent these objects and their spatial relationships. An important step towards this goal is to work out which local regions of the retinal image share common physical characteristics, and which do not. These processes are known as grouping and segmentation, respectively. Many of the stimulus attributes that give visual after-effects, and are probably encoded at an early stage of cortical processing, are also important in segmentation and grouping. Figure 8.4 shows a display used in a classic study by Beck (1966), who presented his observers with three adjacent patches of texture. Their task was to decide which of the two boundaries between the three regions was most salient, or prominent. They chose the boundary between the upright and tilted Ts, even though, when presented with examples of single texture elements, they said that the reversed L was more different from the uprigh T than was the tilted T. This suggests that similarities and differences in orientation between elements of different textures, rather than their perceived similarity when presented in isolation, govern whether elements of different types are grouped or segregated. Segmentation and grouping can also be done on the basis of motion (Braddick, 1974), depth ( Julesz, 1964, 1971) and size (Mayhew & Frisby, 1978) as well as colour and luminance. In addition to the nature of the elements within a display, their spatial arrangement can also contribute to grouping. . In panel A, the equi-spaced circular dots can be grouped perceptually either in rows or in columns. The dots are all physically identical and their vertical and horizontal separations are the same, so there is no reason for one or the other possible grouping to be preferred. This ambiguity may be resolved so that the elements are grouped as columns either by reducing the vertical separation of the dots (panel B), or by changing the shapes of alternate columns of dots (panel C). The Gestalt psychologists [Gestalt psychologists a group of German psychologists (and their followers) whose support for a constructionist view of perception has been enshrined in several important principles, such as ‘the whole (in German, Gestalt) is more than the sum of the parts’] first drew attention to effects of this type and attributed them to the operation of various perceptual laws (though they were really re-describing the effects rather than explaining them).