Spatial organization and the appearances thereof in early vision

The perception of the lightness of surfaces has been shown to be affected by information about the spatial configuration of those surfaces and their illuminants. For example, two surfaces of equal luminance can appear to be of very different lightness if one of the two appears to lie in a shadow. How are we to understand the character of the processes that integrate such spatial configuration information so as to yield the eventual appearance of lightness? This paper makes some simple observations about the vocabulary of appearance used in these contexts, and proposes that the end results can be called "phenomenal" in a traditional sense of that word. Processes whose products are phenomenal are next distinguished from processes characterized in other terms: (a) processes of perceptual grouping; (b) processes of perceptual organization; and (c) attentional (as opposed to preattentive) processes. These four categories are conceptually and empirically distinct. In particular, the paper reviews some evidence that appearances as of contours, occlusion, and amodally completed shapes can occur preattentively. Some implications for understanding gestalt grouping processes are briefly discussed. It is not difficult to show that experimental psychologists use various notions of perceptual appearance--of how perceptible items look, or appear, or seem to the subject perceiving them. Examples are rife in the lightness constancy literature. Here for example is Adelson (1993) describing the "customary terminology" (his Fig. 1 is to the right): ... lightness refers to the apparent reflectance of a surface in a scene, whereas brightness refers to the apparent luminance of a patch in the image itself. ... In Fig. 1, patches a and c are obviously brighter than patch b because they are seen to have higher luminance on the page. Patch c also appears lighter than patch b, in that the three-dimensional (3D) physical surface represented by c seems to be painted a lighter shade of gray than b. On the other hand, patches a and b seem to have the same lightness, as they appear to represent surfaces painted the same shade. (Adelson 1993, 2042) It may not be entirely clear what "apparent reflectance" is, but it is clear that lightness is not the same as the physical reflectance of that surface. A major task for accounts of lightness processing is to explain why and how lightness differs from the physical properties characterizing the stimuli. This paper will focus on examples in which information about the spatial layout of the surfaces has a pronounced effect on their perceived lightness; sometimes (most spectacularly) an effect that is independent of their Clark, Spatial Organization 2 luminance. Adelson's "checker shadow" illusion is a compelling example (see Fig. 3, below). Discussions of it provide other examples of the use the vocabulary of appearance: A process interpretation that is consistent with this result (but not compelled by it) is that the mechanisms that generate appearance act on the output of early image encoding mechanisms and that the action of these appearance mechanisms is most strongly revealed by the shadow stimulus configuration. To be effective at stabilizing appearance, such a mechanism would need access to information about the adapted state of the earlier image encoding mechanisms ... as well as information about lighting variation in the scene. We cannot at this point make any definitive statement about how or at what stage in the visual pathways this information begins to affect lightness processing. (Hillis & Brainard 2007, 1718) The question I want to discuss is precisely how to characterize the "mechanisms that generate appearance", in particular those that contribute spatial information to lightness processing. But first it will be helpful to produce one more example. Here are Gilchrist et al. describing the effect of grouping factors ("belongingness") on lightness: The Benary (1924) effect ... was offered by Wertheimer as evidence of the crucial role of belongingness. The gray triangle that appears to lie on top of the black cross appears lighter than the gray triangle that appears to lie on the white background, despite the fact that the two triangles have equivalent surrounds. Each triangle borders white on its hypotenuse and black on the perpendicular sides. In terms of belongingness, however, when perceptual structure is taken into account, one triangle appears to belong to the black background, and it appears lighter than the other triangle that appears to belong to the white background... (Gilchrist et al. 1999, 816) Here we have apparent spatial relations ("appears to lie on top of", "appears to lie on the white background") affecting lightness ("appears lighter than") because of the grouping ("appears to belong to") that those spatial relations induce. Relations are proposed among processes characterized in three differ­ ent ways. Some terms describe the perceived spatial relations, others the grouping processes, and a final set describe appearances ("appears lighter than") as the end result. Now the philosophers in this workshop have no special competence to decide whether the quoted propositions are true or false. But they will all want to know what they mean. How are we to understand these characterizations? What sense can we make of the relations between them?