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Visual Illusions 1077 Square on Top of 4 Disks Is Assumed Visual Illusions 1077 square on top of 4 disks is assumed. Note that just inside its edges should also be present in its the completion of the boundaries of the Kanizsa middle. This is a symbolic (logical) operation square is accompanied by a surface filling-in pro- that might be carried out in the high-level visual cess that leads to the square being perceived as cortex. Some studies have failed to find evidence brighter than the surrounding white background. for the spread of information thought to underlie In a related process, the black disks are surface perception in early retinotopic visual completed “behind” the reconstructed surface. areas, and this has been taken by some as sup- Filling-in and completion processes related to port for symbolic theories of surface perception. visible figures (the Kanizsa square) are referred Hence, further empirical work is required to test to as “modal,” while these processes are referred the traditional view as well as symbolic theories, to as amodal when related to invisible parts of and to resolve some inconsistencies in the find- figures (the parts of the disks occluded by the ings among some studies. Because the traditional Kanizsa square assumed in front by the visual view and symbolic theories are not mutually system). exclusive, elements from both theories might find Figures 1(c) and (d) illustrate the existence of support in the future. surface filling-in processes. Figure 1(c) shows that the physical gray levels in between the black bars P. De Weerd are the same. When the black bars are removed, See also Consciousness; Gestalt Approach; Object some regions in the stimulus are seen as much Perception; Object Perception: Physiology; Perceptual brighter and others as darker, an illusion referred Organization: Vision to as the Craik-O’Brien-Cornsweet illusion. The difference in brightness in Figure 1(d) is due to the luminance difference (contrast) at the edges of Further Readings the surfaces. Surfaces outlined by bright edges are Denett, D. (1991). Consciousness explained. Boston: perceived as much lighter than surfaces outlined by Little, Brown. dark edges. This supports the idea that the surface De Weerd, P. (2006). Perceptual filling-in: More than features that define the edges are interpolated to the eye can see. Progress in Brain Research, 154(1), reconstruct the percept of the surface (in this case 227–245. brightness). Boundary and surface completion pro- Komatsu, H. (2006). The neural mechanisms of filling-in. cesses are also hypothesized to underlie filling in Nature Reviews Neuroscience, 7(3), 220–231. across the blind spot caused by the absence of Pessoa, L., & De Weerd, P. (Eds.). (2003). Filling-in: retinal receptors where the optic nerve leaves From perceptual completion to skill learning. Oxford, the eye. UK: Oxford University Press. Future Research The present entry on the topic of completion and VISUAL ILLUSIONS filling-in cannot adequately reflect the full com- plexity of this research field. To do justice to Visual illusions are subjective percepts that do some of the ongoing discussion on this topic, it not match the physical reality of the world. must be mentioned that while the mechanisms of When we experience a visual illusion, we may see boundary completion are generally agreed upon, something that is not there, fail to see something the view on surface filling-in in the present entry that is there, or see something different from represents dominant concepts that are not shared what is there. Visual illusions not only demon- by all investigators. For example, it has been sug- strate the ways in which the brain fails to recre- gested that a spread of surface information from ate the physical world, but they are also useful the borders to its middle in retinotopic maps is tools to identify the neural circuits and computa- not required. Instead, a mechanism might exist tions by which the brain constructs our visual that simply “assumes” that surface properties experience. 1078 Visual Illusions The terms visual illusion and optical illusion discovered through the application of known are often used interchangeably. However, unlike physiological principles of visual processing in visual illusions, optical illusions do not result the brain. from brain processes. Instead, an optical illusion One example of this last method is the standing is the perception of a distortion that results from wave of invisibility, a type of visual masking illu- the physical properties of light, such as reflection sion in which the visibility of a central bar (the and refraction, and/or the optics of the eye. An target) is decreased by the presentation of flanking example of an optical illusion is the phenomenon bars (the masks) that flicker in alternation with in which a pencil looks bent when it is placed the target. This illusion was predicted (by Stephen upright in a glass of water, owing to the differing Macknik and Margaret Livingstone) from the refraction indices of air and water. An example of responses of visual neurons to flashing objects of a classical visual illusion is the Ebbinghaus illu- varying durations. The Standing Wave of sion, named for its creator, Hermann Ebbinghaus. Invisibility illusion demonstrates that a set of If two identical circles are placed side by side, one masks can render a target perpetually invisible, surrounded by large circles and the other sur- even though the masks do not overlap the target rounded by small circles, the first central circle spatially or temporally. The invisibility of the tar- will look smaller than the second one (see Action get results from the adjacent masks suppressing and Vision, Figure 2a). The Ebbinghaus illusion the neural responses normally evoked by the onset cannot be explained by the physical properties of and the termination of the target. the visual stimulus or by the optics of the eye. Instead, it is due to neural processes that compare a visual object with its context. Categories of Visual Illusions Only a fraction of the visual illusions known Some attempts have been made to classify visual today have been developed within the framework illusions into general categories with varying of the visual sciences. Visual artists have often used degrees of success. One substantial obstacle to their insights regarding perception to create visual classifications or taxonomies of visual illusions is illusions in their artwork. Historically, long before that some visual illusions that seem similar may be visual science existed as a formal discipline, artists due to disparate neural processes, whereas other had devised a series of techniques to “trick” the visual illusions that are phenomenologically differ- brain into thinking that a flat canvas was three- ent may be related at a neural level. Taking these dimensional or that a series of brushstrokes was in shortcomings into account, some representative fact a still life. Thus, the visual arts have sometimes categories and examples of visual illusions follow. preceded the visual sciences in the discovery of When known, their underlying neural bases are fundamental vision principles. In this sense, art, also discussed. However, the reader should keep in illusions, and visual science have always been mind that the neural underpinnings of many visual implicitly linked. This entry describes various types illusions—especially those discovered recently— of visual illusions. are not understood. What follows is by no means an exhaustive list. How to Make Visual Illusions Adaptation Illusions Some visual illusions are developed intentionally by applying known visual principles to stimuli The first documented visual illusion was described patterns and/or experimenting with variations of in Aristotle’s Parva Naturalia. This illusion, later existing illusions. Other illusions are discovered known as the “waterfall illusion,” can be observed completely by chance: An attentive observer may while looking at a waterfall, river, or other flowing simply notice something strange about the way water. Watch the flowing water for a while (a minute that the world looks and try to understand and or more works best), and then quickly shift your replicate the underlying conditions leading to center of gaze to the stationary objects next to the the unusual percept. Finally, illusions may be water (for instance, the rocks to the side of the Visual Illusions 1079 waterfall). The stationary objects will appear to a gray circle will take on a reddish hue when placed flow in the opposite direction to that of the water. against a green background, and a greenish hue The illusion occurs because neurons that detect when placed against a red background. This local motion in a specific direction (for instance, down- contrast effect is based on retinal lateral inhibitory ward motion if you stare at a waterfall) become processes. Other context-dependent color illusions, adapted (that is, less active) in response to steady such as the “Rubik cube” created by R. Beau Lotto stimulation. Neurons that have not been adapted and Dale Purves, are more difficult to explain by (such as the neurons that detect upward motion) local lateral inhibition at the level of the retina, and are more active in comparison, despite having been may thus reflect a more central origin. at rest. The differential responses of both neuronal Benham’s disk, or Benham’s top, was discov- populations produce the illusion of the stationary ered in 1894 by C. E. Benham, a toymaker. A spin- rocks to the side of the waterfall flowing upward ning top with a certain pattern of black and white for a few seconds. lines appears to take on colors as it rotates. This illusion has been studied by vision scientists for Brightness Illusions over 100 years, and it continues to inspire novel research. The underlying neural processes are not Some visual illusions change the apparent well understood, but current theories point toward brightness of objects.
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