Richard L. Gregory contructs a fascinating "Peeriodic Table of Illusions":
"Let's concentrate on perception: it is tricky enough. I've tried to classify illusions in a way that shows the principles underlying them, starting with physical causes, moving on to physiological disturbances of neural signals, and finally to cognitive processes - where the brain tries to make sense of sensory signals, not always successfully.
The distinction between physiological and cognitive is not straightforward. It's rather like the distinction between how a machine works and what it does. For example, a can opener needs two descriptions: the mechanism of levers and cutters, and what this does to open a can.
That distinction between physiological and cognitive has "real-world" consequences. Think of the placebo effect, which suggests close connections between the physiological and the cognitive-psychological. So different types of illusions could be significant in ways we do not yet know. That's why I have constructed my Peeriodic Table of Illusions (bad pun intended) thus: blindness, the ambiguities, instability, distortion, fiction, and paradox, plus their causes.
Starting with blindness perhaps seems odd but the many kinds of blindness and accompanying visual phenomena tell us much about perception. Blindness ranges from the physiological, with no sensation of light and colour (congenitally or from injury or disease) to various mind blindnesses, such as agnosia, when light, colour, movement and form are perceived but the object seen lacks meaning.
Another form is change blindness, where a person fails to notice big differences in a picture or scene - sometimes even when someone in that scene has been substituted.
Next come the ambiguities. Confounded ambiguity illusions depend on a failure to properly distinguish between two objects, in poor light or because of our ageing senses. Differences in the brightness of regions of an object or scene help us see detail; limited light makes the visual brain choose whether neural activity is due to the presence of light or to neural "noise". Both neural noise and light fluctuate randomly so to see anything reliably we need significantly more photons."
"One of the most famous kinds of instability illusion is created by the use of those repeated patterns so typical of 1960s Op Art, which make the picture appear to be moving. Again, the causes are controversial. One view is that these patterns stimulate brain regions in the V5 area to produce sensations of movement. Or it may be that there is motion at the retina from eye tremor, and from the lens trying to focus the image, which may stimulate the movement systems - especially from high-contrast repeated lines.
Distortion illusions are arguably the most controversial since they most concern the distinction between illusions created by receiving neural signals (reception), where things can go wrong physiologically, and illusions of misreading signals (perception), where things can go wrong cognitively; back to the physiological versus the cognitive again."
The best part is the evolutionary illusion created by nature:
"One "fiction" concerns the blind spot on the retina, where the optic nerve is. One of nature's most amazing illusions is that we don't see this region as a black hole in visual space. The brain generally "fills in", using surrounding colours and patterns.One of nature's most amazing illusions is that we don't see our blind spot."
"Let's concentrate on perception: it is tricky enough. I've tried to classify illusions in a way that shows the principles underlying them, starting with physical causes, moving on to physiological disturbances of neural signals, and finally to cognitive processes - where the brain tries to make sense of sensory signals, not always successfully.
The distinction between physiological and cognitive is not straightforward. It's rather like the distinction between how a machine works and what it does. For example, a can opener needs two descriptions: the mechanism of levers and cutters, and what this does to open a can.
That distinction between physiological and cognitive has "real-world" consequences. Think of the placebo effect, which suggests close connections between the physiological and the cognitive-psychological. So different types of illusions could be significant in ways we do not yet know. That's why I have constructed my Peeriodic Table of Illusions (bad pun intended) thus: blindness, the ambiguities, instability, distortion, fiction, and paradox, plus their causes.
Starting with blindness perhaps seems odd but the many kinds of blindness and accompanying visual phenomena tell us much about perception. Blindness ranges from the physiological, with no sensation of light and colour (congenitally or from injury or disease) to various mind blindnesses, such as agnosia, when light, colour, movement and form are perceived but the object seen lacks meaning.
Another form is change blindness, where a person fails to notice big differences in a picture or scene - sometimes even when someone in that scene has been substituted.
Next come the ambiguities. Confounded ambiguity illusions depend on a failure to properly distinguish between two objects, in poor light or because of our ageing senses. Differences in the brightness of regions of an object or scene help us see detail; limited light makes the visual brain choose whether neural activity is due to the presence of light or to neural "noise". Both neural noise and light fluctuate randomly so to see anything reliably we need significantly more photons."
"One of the most famous kinds of instability illusion is created by the use of those repeated patterns so typical of 1960s Op Art, which make the picture appear to be moving. Again, the causes are controversial. One view is that these patterns stimulate brain regions in the V5 area to produce sensations of movement. Or it may be that there is motion at the retina from eye tremor, and from the lens trying to focus the image, which may stimulate the movement systems - especially from high-contrast repeated lines.
Distortion illusions are arguably the most controversial since they most concern the distinction between illusions created by receiving neural signals (reception), where things can go wrong physiologically, and illusions of misreading signals (perception), where things can go wrong cognitively; back to the physiological versus the cognitive again."
The best part is the evolutionary illusion created by nature:
"One "fiction" concerns the blind spot on the retina, where the optic nerve is. One of nature's most amazing illusions is that we don't see this region as a black hole in visual space. The brain generally "fills in", using surrounding colours and patterns.One of nature's most amazing illusions is that we don't see our blind spot."
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