Perceptual Grouping and Color
Perceptual grouping refers to principles by means of which a set of discrete elements are partitioned into groups by the visual system, thus forming higher-order perceptual units. Perceptual grouping was first studied by Max Wertheimer, one of the fathers of Gestalt psychology . It is a core topic within the studies of perceptual organization, along with figure-ground segmentation . As regards to color, one can consider either the role of color in perceptual grouping or the effects of perceptual grouping on the perception of color. In the first case the focus is on how color can aid and affect the perceptual organization of the visual scene. In the second case the focus is on how the color appearance of a given surface is affected by the color of other surfaces with which it is perceptually grouped.
Perceptual grouping is, along with figure-ground segmentation, a core topic in the studies of perceptual organization, i.e., of those processes that structure the sensory input into coherent units (visual objects, entities, and events), thus contributing significantly to the layout of the visual scene. From such a definition, it is clear that grouping and segmentation are two faces of the same coin, as originally postulated by the fathers of Gestalt psychology Max Wertheimer, Wolfgang Köhler , and Kurt Koffka .
The Effects of Perceptual Grouping on Color
More problematic is the account of the effects of perceptual grouping on color. In an early paper, Fuchs  described situations in which perceptual grouping leads to color assimilation. Though sometimes reported by different authors (see, for instance, [6, 7]), older books dedicated to color perception [8, 9] curiously do not make mention to Fuchs’ findings on color assimilation and grouping. The issue may depend on the fact that Fuchs’ demonstrations do not always work on paper (or screen), maybe because they were mostly created by using light, shadows, projections, and reflections.
While the perceptual outcomes emerging from the observation of Fig. 3e, f are not clear, those deriving from Fig. 3a, b recall the effects of perceptual grouping on lightness: a middle gray target that is grouped with dark gray surfaces will look lighter than a middle gray target that is grouped with light gray surfaces (Fig. 3c, d) .
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