7.28 Visual Illusions, Distortions, and Hallucinations
261
additive mixtures. (If you look closely at a TV screen, you can see the phosphor
dots emit red, green and blue. Yellow is constructed.) On the other hand, a surface
which appears colored by illumination has pigments that absorb certain ranges of
light at various frequencies, leaving others to be scattered. Thus, mixtures of paints
involves absorption of light, and their combination are called subtractive mixtures.
The subtractive mixture of red and green is brown.
7.27.5 Photo-Reception Celebrates Our Vision
Within the retina’s photoreceptors, light stimulates large proteins molecules (opsins)
to change shape. This activates a number transducin molecules, which in turn
activates the enzyme cGMP phosphodiesterase. That enzyme can catalyze the
reduction of many cGMP by cleavage. The effect of the original photon is amplified
in the production of modified transducin molecules, and in the multiple catalysis
(about 1000) of cGPM break-up by each enzyme molecule. The reduced level
of cGMP causes sodium ions channels to close. The photoreceptor membrane
becomes ‘hyperpolarized’ as the charge on the inside becomes more negative (−40–
65 mV). This change in potential causes the membrane channels for the flow of
calcium ions into the cell to close. The decrease in calcium in the cell slows
the release of the neurotransmitter glutamate. Depending on the cells across the
postsynaptic gaps, each of those cells will either be stimulated or inhibited. The
results are pulses sent through the optic nerves to be analyzed by the cerebral
cortex.
7.28 Visual Illusions, Distortions, and Hallucinations
Who are you going to believe, me or your lying eyes?
— Groucho Marx
Human perceptions of the world are known to be influenced by actions of the
mind. Where our attention focuses and how we interpret visual input depends both
on our genetics and on our expectations and experiences. Moreover, the brain can
give us the impression of seeing in dreams and even when awake, especially under
sensory deprivation and other stressful conditions.
Our memories of what we saw is not the same as a digital recording of a scene.
In fact, the information that is sent to the brain about an object codes its lines,
orientation, motion, color, and so forth, rather than pixels. This information is
compared to past visual memories, in sophisticated pattern-recognition algorithms.
There is even a special area dedicated to the recognition of faces. When the
visual information is analyzed, recognizable parts that are considered important
261
additive mixtures. (If you look closely at a TV screen, you can see the phosphor
dots emit red, green and blue. Yellow is constructed.) On the other hand, a surface
which appears colored by illumination has pigments that absorb certain ranges of
light at various frequencies, leaving others to be scattered. Thus, mixtures of paints
involves absorption of light, and their combination are called subtractive mixtures.
The subtractive mixture of red and green is brown.
7.27.5 Photo-Reception Celebrates Our Vision
Within the retina’s photoreceptors, light stimulates large proteins molecules (opsins)
to change shape. This activates a number transducin molecules, which in turn
activates the enzyme cGMP phosphodiesterase. That enzyme can catalyze the
reduction of many cGMP by cleavage. The effect of the original photon is amplified
in the production of modified transducin molecules, and in the multiple catalysis
(about 1000) of cGPM break-up by each enzyme molecule. The reduced level
of cGMP causes sodium ions channels to close. The photoreceptor membrane
becomes ‘hyperpolarized’ as the charge on the inside becomes more negative (−40–
65 mV). This change in potential causes the membrane channels for the flow of
calcium ions into the cell to close. The decrease in calcium in the cell slows
the release of the neurotransmitter glutamate. Depending on the cells across the
postsynaptic gaps, each of those cells will either be stimulated or inhibited. The
results are pulses sent through the optic nerves to be analyzed by the cerebral
cortex.
7.28 Visual Illusions, Distortions, and Hallucinations
Who are you going to believe, me or your lying eyes?
— Groucho Marx
Human perceptions of the world are known to be influenced by actions of the
mind. Where our attention focuses and how we interpret visual input depends both
on our genetics and on our expectations and experiences. Moreover, the brain can
give us the impression of seeing in dreams and even when awake, especially under
sensory deprivation and other stressful conditions.
Our memories of what we saw is not the same as a digital recording of a scene.
In fact, the information that is sent to the brain about an object codes its lines,
orientation, motion, color, and so forth, rather than pixels. This information is
compared to past visual memories, in sophisticated pattern-recognition algorithms.
There is even a special area dedicated to the recognition of faces. When the
visual information is analyzed, recognizable parts that are considered important