Physiological illusions, such as the afterimages following bright
lights, or adapting stimuli of excessively longer alternating patterns
(contingent perceptual aftereffect), are presumed to be the effects on
the eyes or brain of excessive stimulation or interaction with
contextual or competing stimuli of a specific type - brightness, colour,
position, tile, size, movement, etc. The theory is that a stimulus
follows its individual dedicated neural path in the early stages of
visual processing, and that intense or repetitive activity in that or
interaction with active adjoining channels cause a physiological
imbalance that alters perception
The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a colour difference when viewing Mach bands. Once a receptor is active it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround.[1] Lateral inhibition has also been used to explain the Hermann grid illusion, but this has been disproved.[citation needed] More recent "empirical" approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled (e.g. Howe et al. 2005[2]).
Explanation of cognitive illusions
[edit] Perceptual organization
Reversible figure and vase
Duck-Rabbit 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.[3] Gestalt organization can be used to explain many illusions including the Duck-Rabbit illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the figure-ground illusion the figure and ground are reversible.
Kanizsa triangle
In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa 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.[3] Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa 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. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable.
[edit] Depth and motion perception
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.
Ponzo illusion
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.
[edit] Colour and brightness constancies
Simultaneous Contrast Illusion. The background is a colour gradient and
progresses from dark grey to light grey. The horizontal bar appears to
progress from light grey to dark grey, but is in fact just one colour.
In this illusion, the coloured regions appear rather different, roughly
orange and brown. In fact they are the same colour, and in identical
immediate surrounds, but the brain changes its assumption about colour
due to the global interpretation of the surrounding image. Also, the
white tiles that are shadowed are the same colour as the grey tiles
outside the shadow.
Perceptual constancies are sources of illusions. Colour constancy and brightness constancy are responsible for the fact that a familiar object will appear the same colour regardless of the amount of or colour of light reflecting from it. An illusion of colour or contrast difference can be created when the luminosity or colour of the area surrounding an unfamiliar object is changed. The contrast of the object will appear darker against a black field that reflects less light compared to a white field even though the object itself did not change in colour. Similarly, the eye will compensate for colour contrast depending on the colour cast of the surrounding area.
[edit] Object consistencies
Like colour, the brain has the ability to understand familiar objects as having a consistent shape or size. For example a door is perceived as rectangle regardless as to 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 illusion of the changing table[4] is an example of an illusion based on distortions in shape constancy.
[edit] 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 over 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, generating images of what will occur one-tenth of a second into the future. This foresight enables human to react to events in the present. This allows humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd.[5] Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality. For example, one illusion called the Hering illusion, looks like bike 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 moving forward, and thus, switches on our future-seeing abilities. Since we aren't actually moving and the figure is static, we misperceive the straight lines as curved ones.
The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a colour difference when viewing Mach bands. Once a receptor is active it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround.[1] Lateral inhibition has also been used to explain the Hermann grid illusion, but this has been disproved.[citation needed] More recent "empirical" approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled (e.g. Howe et al. 2005[2]).
Explanation of cognitive illusions
[edit] Perceptual organization
Reversible figure and vase 
Duck-Rabbit 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.[3] Gestalt organization can be used to explain many illusions including the Duck-Rabbit illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the figure-ground illusion the figure and ground are reversible.
Kanizsa triangle In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa 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.[3] Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa 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. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable.
[edit] Depth and motion perception
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.
Ponzo illusion 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.
[edit] Colour and brightness constancies
Simultaneous Contrast Illusion. The background is a colour gradient and
progresses from dark grey to light grey. The horizontal bar appears to
progress from light grey to dark grey, but is in fact just one colour. 
In this illusion, the coloured regions appear rather different, roughly
orange and brown. In fact they are the same colour, and in identical
immediate surrounds, but the brain changes its assumption about colour
due to the global interpretation of the surrounding image. Also, the
white tiles that are shadowed are the same colour as the grey tiles
outside the shadow. Perceptual constancies are sources of illusions. Colour constancy and brightness constancy are responsible for the fact that a familiar object will appear the same colour regardless of the amount of or colour of light reflecting from it. An illusion of colour or contrast difference can be created when the luminosity or colour of the area surrounding an unfamiliar object is changed. The contrast of the object will appear darker against a black field that reflects less light compared to a white field even though the object itself did not change in colour. Similarly, the eye will compensate for colour contrast depending on the colour cast of the surrounding area.
[edit] Object consistencies
Like colour, the brain has the ability to understand familiar objects as having a consistent shape or size. For example a door is perceived as rectangle regardless as to 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 illusion of the changing table[4] is an example of an illusion based on distortions in shape constancy.
[edit] 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 over 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, generating images of what will occur one-tenth of a second into the future. This foresight enables human to react to events in the present. This allows humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd.[5] Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality. For example, one illusion called the Hering illusion, looks like bike 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 moving forward, and thus, switches on our future-seeing abilities. Since we aren't actually moving and the figure is static, we misperceive the straight lines as curved ones.
No comments:
Post a Comment