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Differentiating wavelengths
Normal humans have three different types of cones with photo-pigments that sense three different portions of the spectrum. Each cone is tuned to perceive mostly either Long wavelengths (reddish), Middle wavelengths (greenish), or Short wavelengths (bluish), referred to as L-, M-, and S- cones. The peak sensitivities occur at light wavelengths that we call red (580 nm), green (540 nm) and blue (450 nm), provided by three different photo-pigments. Light at any wavelength in the visual spectrum range from 400 to 700 nm will excite one or more of these three types of sensors. Our mind determines the color by comparing the the different types of cones sense. In colorblindness, either one photo-pigment is missing, or two happen to be the same (through genetic anomalies). Interesting, there is a variation among "normal" people. Could the faint variations of color perceptions among normal people account for differences in aesthetic taste?
Individual cones signal the rate at which they absorb photons, without regard to photon wavelengths. Though photons of different wavelengths have different probability of absorption, the wavelength does not change the neural effect that it has once it has been absorbed. Thus, single photoreceptors transmit no information about the wavelengths of the photons that they absorb. Our ability to perceive color depends upon comparisons of the outputs of the three cone types, each with different spectral sensitivity. These comparisons are done by the neural circuitry of the retina.
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Human photo response curves. All chromophores contain the vitamin-A derivative 11-cis -retinal, with different protein opsins, resulting in absorption maxima or 410, 532, and 563 nm. The 31 nm difference in their absorption maxima of the long- and medium-wavelength chromophores is due to 7 out of 364 amino acids.
The visible brain consists of multiple functionally specialized areas which receive their input largely from V1 (yellow) and the area surrounding it known as V2 (green). These are currently the best charted visual areas, but not the only ones. Other visual areas are continually being discovered.
How is color determined? The signal from the retina is analyzed by nerve cells (retinal ganglion cells) which compare the stimulation of neighboring cones, and calculate whether the light reaching a patch of cones is more blue-or-yellow, and reddish-or-greenish. Next, the signal travels to the brain where it is divided into several pathways -- like fiberoptics branching throughout the cortex. For example, visual signals from the photo receptors, pass to retinal ganglion cells which code color information, to the parvocellular cell layers of the dorsal lateral geniculate nucleus (LGN) in the thalamus, onwards to the retinotopic mapped layers of the LGN, which is named rather prosaicically, as V1, V2, an so on.
The color pathways are relatively well charted anatomically. They involve areas V1, V2, V4 and the infero-temporal cortex in the monkey. A similar pathway is involved in the human brain; imaging studies show that V1, V4 and areas located within the fusiform gyrus in the medial temporal lobe are activated by colored stimuli.
The eye alone does not tell the story
The brain must be visually nourished at critical periods after birth, failing which it is almost indefinitely blind. Forms do not have an existence without a brain. Numerous clinical and physiological studies have shown that individuals who are born blind and to whom vision is later restored find it very difficult, if not impossible, to learn to see even a few forms and these they soon forget.
For example, in 1910, the surgeons Moreau and LePrince wrote about their successful operation on an eight year boy who had been blind since birth because of cataracts. Following the operation, they were anxious to discover how he could see. But when they removed the bandages from his physically perfect eyes, they were confused and disappointed. They waved a hand in front of the boy's eyes and asked him what he saw. The boy replied meekly, "I don't know." He only saw a vague change in brightness--he did not know it was a moving hand. Not until he was allowed to touch the hand did he exclaim, "It's moving!" Without visual input during his early development, the boy had never developed the physiological stage of visual processing which is necessary for vision. Similarly, without direct knowledge of their minds, it is impossible to know what an animal "sees." The optical stage provides the raw message, but it is the physiological stage that determines what can be seen.
