ScienceWeek

NEUROSCIENCE: ON COLOR PROCESSING IN THE BRAIN

The following points are made by R. Beau Lotto (Current Biology 2004 14:R619):

1) Understanding the morphogenesis of the brain is a challenge for neurobiologists, and investigating how nerve cells and their connections are organized into a coherent functional unit is both intrinsically interesting and medically relevant. As more is known about the visual part of the brain than any other part, it is not surprising that much work has focused on visual development. And measuring how abnormal experience affects this process has been an integral tool in demonstrating that experience can significantly alter the physiological landscape of vision [1-4].

2) From several recent studies, one can now add color vision to the list of perceptual modalities that are modulated by experience. For instance, it has been reported that the organization of the primary visual cortex is different in people lacking a functional cone system -- so-called "rod monochromats" -- who are consequently color blind. What Baseler et al.[5] discovered was that the area of cortex that normally receives information only from the central part of the retina -- the foveola -- is activated by the rod system in rod monochromats. Because there are no rods in the foveola in either these or normal individuals, this central activation could only arise from a reorganization of the primary visual cortex resulting from their altered visual experience.

3) Another piece of evidence for the plasticity of the color processing involves the perception of unique yellow. While many different color terms are typically used to describe color experience that vary from culture to culture, all sensations of color reduce to one of four perceptual categories (five if you include greyness): redness, yellowness, greenness and blueness. Thus, for any individual with normal color vision, there should be a specific wavelength that causes a "unique" sensation of yellow, that is, a color percept that contains neither redness nor greenness (the same of course should be true for perceptions of red, green and blue). Recent results have shown that the identity of this wavelength remains fairly constant from individual to individual. What makes this result unexpected is the known variability in the underlying retinal architecture in humans, which one might expect would cause large variations in the physical locus of unique yellow. The reason that it does not, it seems, is because -- through experience -- the visual system compensates for the genetically determined differences in color processing.

4) Further studies also suggest a role for experience in "higher-level" aspects of color perception. Researchers have raised monkeys in fairly extreme conditions of visual deprivation, in which each animal spent their first year of life in a room illuminated for 12 hours a day by monochromatic lights. Throughout the light cycle, the illuminant would switch randomly between four monochromatic lights every minute. As a result of this unusual experience, the monkeys' color perception was degraded in two ways: first, their judgments of color similarity differed (fairly incoherently) from control animals; and second, they were unable to recognize a target surface under an illuminant they had not experienced. A similar effect on perception has also been observed in fish raised under a single chromatic light. Thus, color perception, like that of motion, form, sound and touch, is modulated by experience.

References (abridged):

1. Wiesel, T.N. and Hubel, D.H. (1965). Comparison of the effects of unilateral and bilateral eye closure on cortical unit responses in kittens. J. Neurophysiol. 28, 1029-1040

2. Shatz, C.J. and Stryker, M.P. (1978). Ocular dominance in layer IV of the cat's cortex and the effects of monocular deprivation. J. Physiol. 281, 267-283

3. White, L.E., Coppola, D.M. and Fitzpatrick, D. (2001). The contribution of sensory experience to the maturation of orientation selectivity in ferret visual cortex. Nature 411, 1049-1052

4. Katz, L.C. and Crowley, J.C. (2002). Development of cortical circuits: Lessons from ocular dominance columns. Nat. Rev. 3, 34-42

5. Baseler, H.A., Bewer, A.A., Sharpe, L.T., Morland, A.B., Jagle, H. and Wandell, B.A. (2002). Reorganization of human cortical maps caused by inherited photoreceptor abnormalities. Nat. Neurosci. 5, 364-370

Current Biology http://www.current-biology.com

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ON THE EVOLUTION OF COLOR VISION

Notes by ScienceWeek:

In the vertebrate eye, "rod cells" (rods) are one of the two main types of light-sensitive cells found in the retina. Rods provide monochromatic vision in dim light and are found chiefly in the periphery of the retina. "Cone cells" (cones) are the receptor cells responsible for color vision. Apart from their ability to discriminate wavelengths of light, the two receptors differ markedly in sensitivity: a rod can respond to a single photon, whereas more than 100 photons are required to activate a cone.

Color vision, the ability to discriminate light on the basis of wavelength composition, is found in humans, in other primates, and in certain species of birds, fishes, reptiles, and insects. These animals have visual receptors that respond differentially to the various wavelengths of visible light. Each type of receptor is especially sensitive to light of a particular wavelength composition. Evidence indicates that primates, including humans, possess three classes of cone photoreceptors that differ in the photopigments they contain and in their neural connections. In humans, two of these, the R and G cones, are sensitive to all wavelengths of the visible spectrum from 380 to 700 nanometers. The B cones, whose sensitivity peaks at about 440 nm, are not appreciably excited by wavelengths longer than 540 nm. The perception of blueness and yellowness depends upon the level of excitation of B cones in relation to that of R and G cones. No two wavelengths of light can produce equal excitations in all three kinds of cones. It follows that, provided they are sufficiently different to be discriminable, no two wavelengths can give rise to identical sensations.

The following points are made by Kit Wolf (Current Biology 2002 12:R253):

1) Most mammals are dichromats and can only distinguish between two dimensions of color: bright versus dark and blue versus yellow (1). In contrast, humans are trichromats, our extra class of photoreceptor enabling us to discriminate between reds and greens which would otherwise appear identical. However, this ostensibly modest improvement in our visual capabilities has hidden costs: the increased sparsity of each type-specific cone matrix may theoretically reduce visual spatial acuity, and color-anomalous ("color-blind") humans, whose visual world is akin to that of dichromats, can sometimes see features camouflaged by red-green patterns that trichromats cannot detect (2). Nonetheless, trichromacy is highly conserved in those few primate species that have evolved it. Of over 3200 old-world monkeys and apes surveyed, inherited color-anomalous vision has only ever been found in three closely related individuals [3,4], though on an evolutionary timescale such transmissible deficits are likely to have arisen spontaneously many times over. What tips the evolutionary balance so decisively in favour of trichromats?

2) Several explanations for the evolution of color vision have been put forward (5). Color might serve as a cue for object recognition; animals may use color to assess the health of other members of their species; and color could aid image segmentation. But the hypothesis that has attracted the most attention is that trichromacy evolved as an aid to frugivory (the eating of fruits). This notion is particularly attractive, as many fruits gradually turn yellow, red or orange during ripening. These colors are strikingly visible to trichromats, but dichromats have difficulty distinguishing them from a dappled background of green leaves (5). Furthermore, fruit is an important component of most modern primate diets, and fossil and physiological evidence suggests that this was also true of early primates. Parraga et al. [2002] have recently demonstrated that the spatial characteristics of human red-green vision are better matched to scenes containing fruit than they are to natural scenes chosen at random.

3) In summary: Trichromatic vision may have evolved as an aid to frugivory. This hypothesis is supported by the recent demonstration that the spatial characteristics of pictures containing fruit are particularly well matched to the abilities of the human visual system.

References (abridged):

1. Jacobs G.H. (1993) The distribution and nature of color vision among the mammals. Biol. Rev., 68:413-471.

2. Morgan M.J., Adam A. and Mollon J.D. (1992) Dichromates detect color-camouflaged objects that are not detected by trichromates. Proc. Roy. Soc. Lond. Series B-Biol. Sci., 248:291-295.

3. Jacobs G.H. and Williams G.A. (2001) The prevalence of defective color vision in Old World monkeys and apes. Color Res. Appl., 26:S123-S127.

4. Onishi A., Koike S., Ida M., Imai H., Shichida Y., Takenaka O., Hanazawa A., Konatsu H., Mikami A. and Goto S. et al. (1999) Vision-Dichromatism in macaque monkeys. Nature, 402:139-140.

5. Mollon J.D. (1989) 'Tho' she kneel'd in that place where they grew..'-the uses and origins of primate color vision. J. Exp. Biol., 146:21-38.

Current Biology http://www.current-biology.com

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ON EDWIN LAND (1909-1991) AND COLOR VISION

The following points are made by N. Ribe and F. Steinle (Physics Today 2002 July):

1) The now-classic experiments on color vision begun in the 1950s by Land are not only a fine example of exploratory experimentation at the frontier between physics and biology, they also have a direct bearing on the theoretical content of Goethe's Theory of Colors. Land's research began with a simple experiment using two black-and-white transparencies of the same colored scene. The first transparency, the "long record," was taken through a filter that passed only long-wavelength light. The second, the "short record," was taken through a filter that passed only short wavelengths. The two records differed only in the lightness or darkness of corresponding points; neither had any color. The transparencies were then projected onto a screen, directly on top of one another, using a beam of light from the red part of the spectrum for the long record and a beam of incandescent light for the short record. According to the classical color theory based on the work of Newton, Thomas Young, James Clerk Maxwell, and Hermann von Helmholtz, the image on the screen could only be some shade of pink. What the observer saw, however, was an image brilliantly and diversely colored, almost like the original scene.

2) Although Land was not the first to observe such two-color projection effects, his observation initiated a program of exploratory experimentation lasting more than two decades. He began with a series of 22 variations on the two-projector experiment. Those experiments demonstrated that the unexpected or "nonclassical" colors appeared essentially instantaneously, and could not be explained by time-dependent adaptations in the eye. The experiments also showed that the colors were not substantially affected by such factors as the intensities of the ambient illumination or of the projecting beams, the angle subtended by the image, or the filters used to produce the short and long records. Land then performed a more precise series of experiments using a dual monochromator that allowed the experimenter to vary at will the wavelengths of the projecting beams and to study the range of colors observed as a function of those wavelengths.(1)

3) From the experiments, Land concluded that classical color theory was valid only for spots of light observed in totally dark surroundings and that it had only limited relevance to color perception in natural situations involving multiple objects and variable illumination. In particular, he concluded that the stimulus for the color seen at a point in an image was not, as usually supposed, the wavelength composition of the radiant energy reaching the eye from that point. His subsequent experiments were aimed at uncovering the nature of the stimulus. Most of these experiments used "Mondrians," collages of paper rectangles with different shapes and colors.

4) Land began with experiments in which colorless Mondrians in white, gray, and black were viewed through dark goggles that allowed only the eye's rod (night-vision) system to operate. By adjusting the illumination of the Mondrians, Land showed that the patches maintained a constant rank order of perceived lightness, even though a patch that appeared dark might be sending much more light to the eye than one that appeared light. This suggested to Land that the eye was able to discover lightness values independent of the flux of energy it received; the reflectance, the physical correlate of lightness, might be the color stimulus he was seeking. This idea led Land to a series of experiments in which he illuminated colored Mondrians with long-, middle-, and short-wavelength light that could be mixed in any proportion.

5) In one set of experiments, the illumination was adjusted so that, for example, a white area of one Mondrian sent to the eye exactly the same triplet of radiant energies as a green area of another Mondrian. The two areas continued to appear white and green, a dramatic demonstration that their perceived colors were independent of the flux of energy they emitted as a function of wavelength. In another set of experiments, observers were asked to choose from a standard set of 1150 color chips the one that best matched the color of a given area on an illuminated Mondrian. Land found that when a match was made, it was the reflectances of the two areas that corresponded, and not the triplets of radiant energy being sent to the eye in the three illuminating wave-bands.(2)

6) The "retinex" theory of color vision that Land developed on the basis of his experiments has two essential elements: It recognizes lightness (that is, reflectance) as the fundamental stimulus of color, and it emphasizes the importance of boundaries, which allow the eye to estimate lightness by seeking out singularities in the ratio of energy flux from closely spaced points.

References (abridged):

1. E. H. Land, Proc. Natl. Acad. Sci. USA 45, 115 (1959); 45, 636 (1959). E. H. Land, Sci. Am., December 1977, p. 108. A recent account of Land's work and its historical context is given by S. Zeki, A Vision of the Brain, Blackwell Scientific, Boston (1993).

2. J. L. Benton, J. Opt. Soc. Am. 59, 103 (1969). E. H. Land, Sci. Am., May 1959, p. 84.

Physics Today http://www.physicstoday.org

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