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7 Light in Biology and Medicine
are imprinted onto subsets of neurons, together with connections to past associated
memories, such as sounds, feels, smells, past pleasures and pain, etc. Stimulation of
these subsets of neurons evokes some memory of the original visions.
When visual information is partial, ambiguous or fuzzy, then the brain makes
inferences. For example, in print with missing parts to characters, the brain can
subconsciously fill in the missing parts while the person reads on. If an image
contains more than one interpretation, the subconscious brain doing the visual
analysis makes an assumption of which interpretation to adopt. A drawing of the
edges of a transparent cube has two interpretations about what faces of the cube are
in front. Only one is ‘seen’ at a time.
Cases in which the appearance of an object differs from the actual object are
called optical illusions. Visual illusions, such as mirages, are generated by unexpected external physical conditions. Some illusions are physiological, caused by a
breakdown of the mechanisms for vision. Some are psychological, caused by the
brain’s faulty interpretation and interpolation, or by a purposeful misinterpretation.
Sometimes, a brain may cause a sensation of vision without any corresponding input
from the eyes. Dreams and hallucinations are examples.
A visual hallucination occurs if the brain generates the perception of an external
visual stimulus when none exists. Such an event can be generated by brain sensory
deprivation and by pathological conditions in the brain, such as blood imbalance,
drugs, lesions, and impending seizures.
Bright light can cause rods and cones to be depleted of photochemical agents,
causing a subsequent darkening in locations where the light was bright. Quick
recovery from this fatigue is expected if the light has not damaged the retina.
If the cause of an optical illusion is due to the brain’s analysis, then the effect is
called a cognitive illusion. The cube drawing is an example of a cognitive ambiguous
illusion. If associated lines and shapes are misinterpreted, then a cognitive distortion
illusion has occurred. If the brain juxtaposes or reinterprets objects in a scene into a
physically impossible configuration, this is a cognitive paradox illusion. The brain
can also add color information where there was none, producing cognitive chromatic
illusion. (This was discovered by Edwin Land, the inventor of the Polaroid instant
camera.)
Most cognitive illusions can be understood as the brain trying to make sense
of what is seen, so that subsequent analysis or action is possible. In Fig. 7.24, the
Escher staircase drawing puzzles the brain when incongruity is perceived. The ‘old
woman/young woman’ shows that the brain makes just one interpretation of a visual
input, although it can flip between two or more.
In some cases of cognitive illusions, there may be physiological hyperfunction.
For example, in grapheme color synesthia, black and white letters on paper are seen
tinged with different colors for different letters. One plausible explanation is ‘crosstalk’ between networks of the brain. This idea could also explain why those with
synesthia are disproportionately intelligent.
7 Light in Biology and Medicine
are imprinted onto subsets of neurons, together with connections to past associated
memories, such as sounds, feels, smells, past pleasures and pain, etc. Stimulation of
these subsets of neurons evokes some memory of the original visions.
When visual information is partial, ambiguous or fuzzy, then the brain makes
inferences. For example, in print with missing parts to characters, the brain can
subconsciously fill in the missing parts while the person reads on. If an image
contains more than one interpretation, the subconscious brain doing the visual
analysis makes an assumption of which interpretation to adopt. A drawing of the
edges of a transparent cube has two interpretations about what faces of the cube are
in front. Only one is ‘seen’ at a time.
Cases in which the appearance of an object differs from the actual object are
called optical illusions. Visual illusions, such as mirages, are generated by unexpected external physical conditions. Some illusions are physiological, caused by a
breakdown of the mechanisms for vision. Some are psychological, caused by the
brain’s faulty interpretation and interpolation, or by a purposeful misinterpretation.
Sometimes, a brain may cause a sensation of vision without any corresponding input
from the eyes. Dreams and hallucinations are examples.
A visual hallucination occurs if the brain generates the perception of an external
visual stimulus when none exists. Such an event can be generated by brain sensory
deprivation and by pathological conditions in the brain, such as blood imbalance,
drugs, lesions, and impending seizures.
Bright light can cause rods and cones to be depleted of photochemical agents,
causing a subsequent darkening in locations where the light was bright. Quick
recovery from this fatigue is expected if the light has not damaged the retina.
If the cause of an optical illusion is due to the brain’s analysis, then the effect is
called a cognitive illusion. The cube drawing is an example of a cognitive ambiguous
illusion. If associated lines and shapes are misinterpreted, then a cognitive distortion
illusion has occurred. If the brain juxtaposes or reinterprets objects in a scene into a
physically impossible configuration, this is a cognitive paradox illusion. The brain
can also add color information where there was none, producing cognitive chromatic
illusion. (This was discovered by Edwin Land, the inventor of the Polaroid instant
camera.)
Most cognitive illusions can be understood as the brain trying to make sense
of what is seen, so that subsequent analysis or action is possible. In Fig. 7.24, the
Escher staircase drawing puzzles the brain when incongruity is perceived. The ‘old
woman/young woman’ shows that the brain makes just one interpretation of a visual
input, although it can flip between two or more.
In some cases of cognitive illusions, there may be physiological hyperfunction.
For example, in grapheme color synesthia, black and white letters on paper are seen
tinged with different colors for different letters. One plausible explanation is ‘crosstalk’ between networks of the brain. This idea could also explain why those with
synesthia are disproportionately intelligent.