The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search ("pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints-first,that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal ratherthanon "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction ofshapefrom shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.We use three-dimensional (3-D) depth perception to find our way around the world and to manipulate objects that we encounter. Although the retinal image is two-dimensional, somehow the brain is able to use the information from this image to yield an experience of solidity and depth.Of the numerous mechanisms used by the visual system to recover the third dimension, the ability to use shading is probably phylogenetically one of the most primitive. One reason for believing this is that in the natural world, animals have often evolved the principle of countershading to conceal their shapes from predators; they have pale bellies that serve to neutralize the effects of the sun shining from above (Thayer, 1909). The prevalence of countershading in a variety of animals (including fishes) suggests that shading must be a very important source of information about 3-D shapes.Although artists have long recognized the importance of shading, there have been few studies of how the human visual system actually extracts and uses this information. Since the time when Leonardo da Vinci first thought about this problem, there have been only a small ha...

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The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search C'pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints-first, that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.We use three-dimensional (3-D) depth perception to find our way around the world and to manipulate objects that we encounter. Although the retinal image is two-dimensional, somehow the brain is able to use the information from this image to yield an experience of solidity and depth.Of the numerous mechanisms used by the visual system to recover the third dimension, the ability to use shading is probably phylogenetically one of the most primitive. One reason for believing this is that in the natural world, animals have often evolved the principle of countershading to conceal their shapes from predators; they have pale bellies that serve to neutralize the effects of the sun shining from above (Thayer, 1909). The prevalence of countershading in a variety of animals (including fishes) suggests that shading must be a very important source of information about 3-D shapes.Although artists have long recognized the importance of shading, there have been few studies of how the human visual system actually extracts and uses this information. Since the time when Leonardo da Vinci first

When confronted with moving images, the visual system often must decide whether the motion signals arise from a single object or from multiple objects. A special case of this problem arises when two independently moving gratings are superimposed. The gratings tend to cohere and move unambiguously in a single direction (pattern motion) instead of moving independently (component motion). Here we report that the tendency to see pattern motion depends very strongly on the luminance of the intersections (that is, to regions where the gratings overlap) relative to that of the gratings in a way that closely parallels the physics of transparency. When the luminance of these regions is chosen appropriately, pattern motion is destroyed and replaced by the appearance of two transparent gratings moving independently. The observations imply that motion detecting mechanisms in the visual system must have access to tacit 'knowledge' of the physics of transparency and that this knowledge can be used to segment the scene into different objects. The same knowledge could, in principle, be used to avoid confusing shadows with real object boundaries.

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