The watercolor illusion, also referred to as the water-color effect, is an optical illusion in which a white area takes on a pale tint of a thin, bright, intensely colored polygon surrounding it if the coloured polygon is itself surrounded by a thin, darker border (Figures 1 and 2). The inner and outer borders of watercolor illusion objects often are of complementary colours (Figure 2). [3] The watercolor illusion is best when the inner and outer contours have chromaticities in opposite directions in color space. The most common complementary pair is orange and purple. [4] The watercolor illusion is dependent on the combination of luminance and color contrast of the contour lines in order to have the color spreading effect occur.
Baingio Pinna discovered the watercolor illusion in 1987, reporting it in Italian. [5] Jack Broerse and Robert P. O'Shea independently discovered it in 1995, [2] reporting it in English, although they called it "spread colour", relating it to neon colour spreading. [6] Broerse, Tony Vladusich, and O’Shea, demonstrated the phenomenon in 1999 (Figure 1). [1] Pinna, Gavin Brelstaff, and Lothar Spillmann published the first account of the phenomenon in English in 2001, giving it its current name. [7] Since the discovery many experiments have been performed and analyzed to understand the concept of perception of the illusion compared to various Gestalt factors and the neural processes that create the illusion.
The watercolor illusion has had much debate over whether it can be described by Gestalt psychology. Watercolor illusion has been considered a case of the Gestalt principles by some because of the similarity principles that describe the figure-ground (perception). According to the similarity principles (principles of grouping), [8] elements are grouped together based on its color, brightness, size and shape. There are seven Gestalt factors that the Watercolor Illusion filling of the figure-ground organization were compared to: proximity, good continuation, closure, symmetry, convexity, amodal completion, and past experience. These seven factors were tested in a series of experiments by Pinna, Werner, and Spillman to determine the strength of each factor compared to the illusion.
The first experiment tested the watercolor effect versus proximity to determine the figure-ground segregation. According to the Gestalt factor of proximity, closer elements are more likely to be grouped together. The stimuli had different spacing between the set of contour/flank lines. Each stimuli used had a vastly different response, but the watercolor illusion held true even in the wide spaces of the illusion. In some cases, the figure-ground areas were reversed as the filling-in of the orange flank was stronger than the filling in of the purple.
The second experiment tested the watercolor effect versus good continuation. In good continuation, the smooth continuation areas tend to be grouped together. With different variations of a square-wave pattern and basic contours with fringes, the good continuation of the stimuli was studied. It was determined that the uniform watercolor illusion is seen in only closed space.
The third experiment studied the watercolor illusion stimulus against the idea of closure and surroundness. According to the closure principle, piece creating a closed figure are grouped together. When one region encompasses another region completely, the surrounding region is perceived as ground, and the feature that is perceived as figure according to the surroundness principle. When the four purple rectangles were surrounded by a larger rectangle, the large rectangle was rarely perceived as figure while the four rectangles were seen as figures. When orange contours bordered the inside of the large rectangle, but outside the four smaller rectangles, the larger rectangle was perceived as figure while the small rectangles were perceived as holes. This showed that the closure and surroundness were weaker than the watercolor illusion.
The fourth experiment was watercolor effect versus symmetry. Parallel contours are grouped together according to the Gestalt principle of symmetry. Parallel wavy lines (rivers) were spaced apart with the purple contours on the inside and orange on the outside. Opposite of the principle, the rivers were not perceived as filed in, but the interspaces between the rivers were perceived to be filled in, or as figure in this case.
The fifth experiment was watercolor illusion compared to convexity. According to the “law of the inside” the concave regions of the stimulus should be perceived as ground and the convex ones perceived as figure. The stimuli used had different sets of concave and convex arcs alternating between two horizontal lines. The concave regions were typically perceived as figure whether the purple was flanked by red or orange fringes. However, as the curvature was increased the effect was decreased when the red fringes were used.
The sixth experiment was amodal completion compared to the watercolor illusion. Amodal completion is not a classical principle of figure-ground segregation that helps explain the perception of an object’s hidden regions. This applies to both figure and ground in the organization. From the experiments, amodal completion does not hold true when the watercolor illusion reversed the perceived segregation of components.
The seventh experiment was to determine if the observer would see the color spread effect if the stimulus was of a common object. From this, it was determined that spaces with prior knowledge (familiar words, shapes, etc. ) are more likely to be grouped together. [9]
Several experiments were performed to determine the necessary criteria to have the watercolor illusion viewed. The outcome of the experiments affected the properties that were defined by the coloration effect (below).
The first experiment was done to determine the distance over which the color spreading effect occurred. 25 stimuli of different dimension were hand–drawn with magic markers: purple for the outside border and orange for the inner fringe. The stimuli were presented 50 cm away from the observer with no time limit on determining the color spread. It was determined that the color spreading reported from the experiment decreased with increasing length of the shorter axis. The determined threshold was a height of 45 degrees of visual angle for the height of the surface.
The second experiment was to determine the duration of the exposed stimuli to see the illusion. Again, the stimuli were shown to the observer 50 cm away in an ambient lighted room. An electromagnetic shutter was in front of one eye. The smallest interval for the electromagnetic shutter was 100 ms, and the continuous color could be seen at the time duration. It was determined that the watercolor illusion could be perceived instantaneously in the given conditions.
The third experiment was to determine the optimal line thickness. Various thicknesses of the border and fringe were drawn for the stimulus pattern. The observers compared the strength and uniformity of the illusion between the various stimuli. The color spreading was perceived the strongest when the contour and fringe subtended a visual angle of 6 arcminutes; The strength of the illusion was discovered to decrease as the thickness of lines increased.
The fourth experiment was evaluating the waviness of the lines used compared to the strength of the illusion. The stimulus patterns varied in frequency of sinusoidal waves. The results showed increase of strength with increasing spatial frequency of the sinusoidal waves. The effects are the strongest with the wave patterns, however, the watercolor illusion is still strong for a stimulus with straight borders.
The fifth experiment tested inducing color. The colors were drawn with magic marker with pairs of red, green, blue, and yellow lines. Red and blue pairs produced the strongest effects while green and yellow produced the weakest illusion effect. All combinations of two colors produced a clearly visible spreading effect.
The sixth experiment tested the contrast between the different color lines to see which produced the most striking effect. Pinna first discovered the watercolor illusion with high contrast lines (a black outer line and lighter fringe). When the luminance between the two lines is different, the color spreading effect is the strongest. As the luminance between the two lines becomes closer, the spreading effect grows weaker but is still present.
Limitations of the watercolor illusion were studied. The color-spreading effect occurs on colored backgrounds in addition to the white or gray. In the case of a colored background, the watercolor illusion color spread does not mix with the background color but does get superimposed onto the colored background. For lighting conditions, the color spreading effect declines as the illumination in the room increases. It is the strongest at medium illumination. Lastly, the watercolor illusion persists even when dotted lines are used for the purple contour and orange fringe instead of continuous lines. [10]
The coloration effect is one of the phenomenal effects of watercolor illusions. Pinna and Reeves, (2006) identified thirteen properties of the coloration effect through experiments of the water color illusion. All of the main properties can be seen with any pair of complementary (opposite in color space) contour lines. However, from the experiment described above, purple and orange have been found to produce the strongest effects. The thirteen properties are: the stimulus (1) is uniform, (2) solid and (3) can be perceived on a white, black or, colored background (the background does not affect the experiment as the color spread effect is superimposed on it without any mixing). (4) The orange hue is best observed if the object contains wiggly lines, though it is perceived with straight or dotted lines. (5) The effect can work with all basic colors, though (6) the effect is shown better when the lines have a high luminance contrast. (7) The line with the less luminance to the background will always produce the coloration effect. For example the light orange line spreads to produce the color spreading effect. (8) If the lines were reversed and the orange was on the outside than the orange hue would appear to be penetrating outward of the object while the purple line would be on the inside. Some other properties are the following: (9) the coloration extends about 45 visual degrees (found in experiments listed above); (10) the coloration is complete by 100 millisecond the smallest possible measurable unit due to the experiment equipment; (11) the line width that produces the best coloration effect is 6 arc minutes; (12) the color also spreads in directions other than the line; (13) and lastly it can induce a complementary color when one of the lines is achromatic and the other is chromatic.
The figure-ground effect makes an object appear solid and opaque. [11] The object appears as if it can be either the background or the figure. Through switching the two contour lines, reverses the perception of the stimuli. For example, if the purple contour fringed by orange perceives a figure, when the colored contours are reversed, ground is perceived. In other words, the color spreading effect determines how the figure-ground effect is perceived. In most cases, the color spread tint is perceived as figure and the surrounding area is perceived as ground. As in the coloration effect, the watercolor illusion can show a figure-ground effect on white, black and colored backgrounds. [12] The object-hole effect happens when the object of the watercolor illusion has a hole inside of it. The hole appears 3D and can help define what is figure and what is background. [12] This can be difficult in determining if the boundaries of the hole belong to the background or the watercolor region because it appears that both options can be true. This effect can also be increased by increasing the number of lines. [12]
Coloration and figure-ground are two of the effects that can be observed within the watercolor illusion. Dissociation implies that coloration can be observed without the figure-ground effect, and that the figure-ground effect can be observed without color spreading into the inside edges. Border ownership assignment mechanisms (the consistency of color border and the asymmetric shape of edge) determine the figure-ground effects while the surface color from visual cortex lead to the color illusion. [13] Coloration without the figure-ground effect can be acquired by using equal luminance adjacent contours that show a flat and reversible figure-ground organization. This coloration is dependent on the luminance and colors of the inducing contour lines. In the figure-ground effect, the asymmetric luminance profile between the two lines gives a 3-D perspective with the lower contrast side appearing to bulge Figure-ground effect without coloration, the result is a figure that appears flat (does not appear 3-D). The coloration is most likely absent because of a blending of the two colored lines and not a high enough contrast between the two colors. [12]
The coloration and figural effects from above come from parallel processes occurring in the brain. The two stages are the feature processing stage and the parallel boundary processing stage. At the feature processing stage, the area around the lines produces small interactions between the lines which leads to the color spreading. The parallel boundary processing stage organizes the geometrical structure of the stimulus into the color spreading of the watercolor illusion. However, the neural mechanisms are more complex as reducing the geometric structure of the stimulus changes the appearance and strength of the watercolor illusion.
According to Pinna and Grossberg, the FAÇADE model better models the brain activity that occurs when the watercolor illusion is viewed. The FAÇADE model illustrates that the boundary contour system (BCS) and feature contour system (FCS) in parallel. The BCS does boundary grouping and the FCS does the surface filling in. These two processes occur within the regions V1 (primary visual cortex) through V4 (V2 through V4 are three extrastriate visual cortical areas). The FAÇADE model is weakened when the edges are low contrast to each other.
Another modeling of neural processes of the brain that occur when the watercolor illusion stimulus is seen is the LAMINART model. This model demonstrates that stimulus is processed in the layers 6 and 4 of the cortical areas V1 and V2. The LAMINART model betters explains spatial competition which occurs when the boundary is weakened (e.g. dotted line). Also, the LAMINART model does not completely reduces the illusion visualization if the stimulus is low-contrast. [11]
Applications of the watercolor illusion capitalize on the limitation of the human visual system. The watercolor effect can be used by artists or illustrators who want to create the effect that the illusion gives off. If they want to create a light hue of a color they can take advantage of this effect and do not have to use any color to make it look like the object is filled in. Another possible application is in computer graphics rendering. If a certain tint or light color wants to fill a small space, the watercolor illusion can be applied to reduce rendering time as well amount of color used.
In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.
Gestalt psychology, gestaltism, or configurationism is a school of psychology that emerged in the early twentieth century in Austria and Germany as a theory of perception that was a rejection of basic principles of Wilhelm Wundt's and Edward Titchener's elementalist and structuralist psychology.
The Müller-Lyer illusion is an optical illusion consisting of three stylized arrows. When viewers are asked to place a mark on the figure at the midpoint, they tend to place it more towards the "tail" end. The illusion was devised by Franz Carl Müller-Lyer (1857–1916), a German sociologist, in 1889.
The term phi phenomenon is used in a narrow sense for an apparent motion that is observed if two nearby optical stimuli are presented in alternation with a relatively high frequency. In contrast to beta movement, seen at lower frequencies, the stimuli themselves do not appear to move. Instead, a diffuse, amorphous shadowlike something seems to jump in front of the stimuli and occlude them temporarily. This shadow seems to have nearly the color of the background. Max Wertheimer first described this form of apparent movement in his habilitation thesis, published 1912, marking the birth of Gestalt psychology.
The term beta movement is used for the optical illusion of apparent motion in which the very short projection of one figure and a subsequent very short projection of a more or less similar figure in a different location are experienced as one figure moving.
Figure-ground contrast, in the context of map design, is a property of a map in which the map image can be partitioned into a single feature or type of feature that is considered as an object of attention, with the remainder of the map being relegated to the background, outside the current focus of attention. It is thus based on the concept of figure–ground from Gestalt psychology. For example, in a street map with strong figure-ground contrast, the reader would be able to isolate and focus attention on individual features, like a given street, park, or lake, as well as layers of related features, like the street network.
Ambiguous images or reversible figures are visual forms that create ambiguity by exploiting graphical similarities and other properties of visual system interpretation between two or more distinct image forms. These are famous for inducing the phenomenon of multistable perception. Multistable perception is the occurrence of an image being able to provide multiple, although stable, perceptions.
White's illusion is a brightness illusion in which certain stripes of a black-and-white grating are replaced by gray rectangles. Both of the gray bars of A and B have the same color, luminance, and opacity. The brightness of the gray rectangles appears to be closer to the brightness of the top and bottom bordering stripes. This is opposite to any explanation based on lateral inhibition; hence it cannot explain the illusion. A similar illusion occurs when the horizontal stripes have different colors; this is known as the Munker–White illusion or the Munker illusion, based on the Bezold effect.
The Ehrenstein illusion is an optical illusion of brightness or colour perception. The visual phenomena was studied by the German psychologist Walter H. Ehrenstein (1899–1961) who originally wanted to modify the theory behind the Hermann grid illusion. In the discovery of the optical illusion, Ehrenstein found that grating patterns of straight lines that stop at a certain point appear to have a brighter centre, compared to the background.
The Abney effect or the purity-on-hue effect describes the perceived hue shift that occurs when white light is added to a monochromatic light source.
Rubin's vase is a famous example of ambiguous or bi-stable two-dimensional forms developed around 1915 by the Danish psychologist Edgar Rubin.
Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century, but they are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 Scientific American paper marked the resurgence of interest in illusory contours for vision scientists.
In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis. Classical demonstrations of perceptual filling-in involve filling in at the blind spot in monocular vision, and images stabilized on the retina either by means of special lenses, or under certain conditions of steady fixation. For example, naturally in monocular vision at the physiological blind spot, the percept is not a hole in the visual field, but the content is “filled-in” based on information from the surrounding visual field. When a textured stimulus is presented centered on but extending beyond the region of the blind spot, a continuous texture is perceived. This partially inferred percept is paradoxically considered more reliable than a percept based on external input..
The Chubb illusion is an optical illusion or error in visual perception in which the apparent contrast of an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed. These visual illusions are of particular interest to researchers because they may provide valuable insights in regard to the workings of human visual systems.
The principles of grouping are a set of principles in psychology, first proposed by Gestalt psychologists to account for the observation that humans naturally perceive objects as organized patterns and objects, a principle known as Prägnanz. Gestalt psychologists argued that these principles exist because the mind has an innate disposition to perceive patterns in the stimulus based on certain rules. These principles are organized into five categories: Proximity, Similarity, Continuity, Closure, and Connectedness.
Neon color spreading is an optical illusion in the category of transparency effects, characterized by fluid borders between the edges of a colored object and the background in the presence of black lines. The illusion was first documented in 1971 and was eventually rediscovered in 1975 by Van Tuijl.
A phantom contour is a type of illusory contour. Most illusory contours are seen in still images, such as the Kanizsa triangle and the Ehrenstein illusion. A phantom contour, however, is perceived in the presence of moving or flickering images with contrast reversal. The rapid, continuous alternation between opposing, but correlated, adjacent images creates the perception of a contour that is not physically present in the still images. Quaid et al. have also authored a PhD thesis on the phantom contour illusion and its spatiotemporal limits which maps out limits and proposes mechanisms for its perception centering around magnocellularly driven visual area MT.
Due to the effect of a spatial context or temporal context, the perceived orientation of a test line or grating pattern can appear tilted away from its physical orientation. The tilt illusion (TI) is the phenomenon that the perceived orientation of a test line or grating is altered by the presence of surrounding lines or grating with a different orientation. And the tilt aftereffect (TAE) is the phenomenon that the perceived orientation is changed after prolonged inspection of another oriented line or grating.
Hans Wallach was a German-American experimental psychologist whose research focused on perception and learning. Although he was trained in the Gestalt psychology tradition, much of his later work explored the adaptability of perceptual systems based on the perceiver's experience, whereas most Gestalt theorists emphasized inherent qualities of stimuli and downplayed the role of experience. Wallach's studies of achromatic surface color laid the groundwork for subsequent theories of lightness constancy, and his work on sound localization elucidated the perceptual processing that underlies stereophonic sound. He was a member of the National Academy of Sciences, a Guggenheim Fellow, and recipient of the Howard Crosby Warren Medal of the Society of Experimental Psychologists.
Amodal completion is the ability to see an entire object despite parts of it being covered by another object in front of it. It is one of the many functions of the visual system which aid in both seeing and understanding objects encountered on an everyday basis. This mechanism allows the world to be perceived as though it is made of coherent wholes. For example, when the sun sets over the horizon it is still perceived as a full circle, despite occlusion causing it to appear as a semi-circle. Another example of this is a cat behind a picket fence. Amodal completion allows the cats to be seen as a full animal continuing behind each picket of the fence. Essentially amodal completion allows for sensory stimulation from any parts of an occluded object we can not directly see.