Perceptual organization illusion To make sense of the world it is necessary to organize incoming sensations into information which is meaningful.
Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole. In addition, gestalt theory can be used to explain the
illusory contours in the
Kanizsa's triangle. A floating white triangle, which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements.
Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa's triangle is based in
evolutionary psychology and the fact that in order to survive it was important to see form and edges. The use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions including
impossible objects. The brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which is not there to that which is believable. The
gestalt principles of perception govern the way different objects are grouped. Good form is where the perceptual system tries to fill in the blanks in order to see simple objects rather than complex objects. Continuity is where the perceptual system tries to disambiguate which segments fit together into continuous lines. Proximity is where objects that are close together are associated. Similarity is where objects that are similar are seen as associated. Some of these elements have been successfully incorporated into quantitative models involving optimal estimation or Bayesian inference. The double-anchoring theory, a popular but recent theory of lightness illusions, states that any region belongs to one or more frameworks, created by gestalt grouping principles, and within each frame is independently anchored to both the highest luminance and the surround luminance. A spot's lightness is determined by the average of the values computed in each framework.
Monocular depth and motion perception where the vertical line is thought to be longer than the horizontal Illusions can be based on an individual's ability to see in three dimensions even though the image hitting the retina is only two dimensional. The
Ponzo illusion is an example of an illusion which uses monocular cues of depth perception to fool the eye. But even with two-dimensional images, the brain exaggerates vertical distances when compared with horizontal distances, as in the
vertical–horizontal illusion where the two lines are exactly the same length. In the Ponzo illusion the converging
parallel lines tell the brain that the image higher in the
visual field is farther away, therefore, the brain perceives the image to be larger, although the two images hitting the
retina are the same size. The optical illusion seen in a
diorama/
false perspective also exploits assumptions based on monocular cues of
depth perception. The
M.C. Escher painting
Waterfall exploits rules of depth and proximity and our understanding of the physical world to create an illusion. Like
depth perception,
motion perception is responsible for a number of sensory illusions. Film
animation is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly than smaller objects, like a car, although the larger object is actually moving faster. The
phi phenomenon is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession. The ambiguity of direction of motion due to lack of visual references for depth is shown in
the spinning dancer illusion. The spinning dancer appears to be moving clockwise or counterclockwise depending on spontaneous activity in the brain where perception is subjective. Recent studies show on the fMRI that there are spontaneous fluctuations in cortical activity while watching this illusion, particularly the parietal lobe because it is involved in perceiving movement.
Binocular illusions Illusions in
binocular vision refer to situations which are exclusive for binocular viewing.
Illusory disparities Binocular depth information is abstracted from
binocular disparities. In general this information is more trustworthy than monocular depth information. Two identical objects behind each other have the same retinal images as two similar objects next to each other. At a small distance between A and B the brain chooses to see option C,D. This results in an illusion if the real objects are present at positions A,B and not at C,D (
double-nail illusion). This illusion illustrates
binocular ghost images and has many
Variants and conflicts with tactile, motor and monocular cues (
multi-modal illusion).
Edge detection When a thin object like a razor blade is held in the midsagittal plane, then it is seen at a right angle to the viewing direction (
Midsagittal-strip illusion). This illusion suggests that the visual system detects the disparity of edges (rims) with equal contrast sign only.
Depth of surfaces When a black disc is present hovering in front of a white disc, then this can be perceived as it physically is, or as a truncated white cone. If a physical white cone with a black top is presented, then this can be perceived as it physically is, or as a black disc hovering above a white disc. In other words, the observer cannot distinguish between seeing a disc on a pin above a white background, and a white truncated cone with a black top-plane (
Ambiguous 3D-surfaces). This illusion suggests that the visual system detects the disparity (depth) of equal-sign edges and fills in the orientation of surfaces in between.
Delayed signals When viewing the swinging movement of the rain wiper of a car, and holding a grey filter or dark sunglass in front of one of the eyes, the pendulum appears to make an elliptical movement in depth. It even appears to move through the glass. (
Pulfrich illusion). This is suggests that the signals of the covered eye are processed with a delay.
Interaction with monocular depth cues When stereoimages are swapped (
pseudoscopy) binocular depth is inversed and conflicts with monocular depth cues. Perceived depth appears to correspond with the inversed disparity, but the apparent size of objects looks different. Nearby objects appear bigger and far objects appear smaller than normal.
Color and brightness constancies and progresses from dark gray to light gray. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one color. Perceptual constancies are sources of illusions.
Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The luminosity of the object will appear brighter against a black field (that reflects less light) than against a white field, even though the object itself did not change in luminosity. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area. In addition to the gestalt principles of perception, water-color illusions contribute to the formation of optical illusions. Water-color illusions consist of object-hole effects and coloration. Object-hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin-colored edge lining a darker chromatic contour. The water-color illusion describes how the human mind perceives the wholeness of an object such as top-down processing. Thus, contextual factors play into perceiving the brightness of an object.
Object Just as it perceives color and brightness constancies, the brain has the ability to understand familiar objects as having a consistent shape or size. For example, a door is perceived as a rectangle regardless of how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The
Shepard tables illusion is an example of an illusion based on distortions in shape constancy.
Future perception Researcher
Mark Changizi of
Rensselaer Polytechnic Institute in New York has a more imaginative take on optical illusions, saying that they are due to a neural lag which most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world. Scientists have known of the lag, yet they have debated how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay. Changizi asserts that the human visual system has evolved to compensate for neural delays by generating images of what will occur one-tenth of a second into the future. This foresight enables humans to react to events in the present, enabling humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd. In an interview with ABC Changizi said, "Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality." For example, an illusion called the
Hering illusion looks like bicycle spokes around a central point, with vertical lines on either side of this central, so-called vanishing point. The illusion tricks us into thinking we are looking at a perspective picture, and thus according to Changizi, switches on our future-seeing abilities. Since we are not actually moving and the figure is static, we misperceive the straight lines as curved ones. Changizi said: Evolution has seen to it that geometric drawings like this elicit in us premonitions of the near future. The converging lines toward a vanishing point (the spokes) are cues that trick our brains into thinking we are moving forward—as we would in the real world, where the door frame (a pair of vertical lines) seems to bow out as we move through it—and we try to perceive what that world will look like in the next instant. == Pathological visual illusions (distortions) ==