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Almost Reason Enough for Having Eyes Almost Reason Enough for Having Eyes Jay Neitz, Joseph Carroll and Maureen Neitz t is a commonly held misconception that dogs are color blind. The truth is that a dog’s ability to see colors is similar to that of most other mammals,1 although different from that of humans with nor- I mal vision. Cross-species comparisons of differences in color vision have helped us appreciate our own ability to see colors, as well as to understand the underlying mechanisms of color vision. Among humans with normal color vision, there are differences in color perception that can also help us under- stand the fundamental processes of color vision.2,3 Over the past decade, comparisons and contrasts made at the cellular and molecular level among different color vision systems have brought us to a new under- standing of color vision, one that differs in many unexpected ways from the tenets of conventional wisdom. With this understanding comes a new appreciation of the fact that wavelength sensing, in the form of color discrimination, is a fundamental visual capacity that serves to greatly expand the eye’s information- gathering power. Color vision has played an important role in how the biological machinery for seeing has developed over the course of evolution. 26 Optics & Photonics News n January 2001 1047-6938/01/01/0026/8-$0015.00 ' Optical Society of America COLOR VISION Neural mechanisms for seeing when the white light from the star stimu- zero input to maximum activity, in the color lates the photoreceptors. Activation of this white labeled line; sensations from dark The underlying machinery of a biological labeled line is responsible for the percep- grays to black would correspond to graded system for seeing color is composed of two tion of white. Another line, the labeled line levels of activity in the black labeled line. stages: the first consists of light-sensitive for “blackness,” carries output collected Hue, which is determined by the wave- receptors and the second consists of the from the very same photoreceptors. length content of colors, is the property neural components needed to process, However, this line derives from neural cir- that allows us to perceive colors as ranging partition, and encode information about cuits with physiologically different com- from red through yellow, green and blue. wavelength that the photoreceptors col- ponents that invert the signal relative to In the human visual system, the second lect. Our understanding of how visual the whiteness labeled line. Because the sig- information is encoded in humans has nal carried by the blackness line is sign- reversed, its activity is maximally silenced developed from studies of human percep- A short glossary of terms tion coupled with research on the physiol- when the star illuminates the photorecep- 4-7 tors. Conversely, the neural line labeled Percept: A mental impression of some- ogy and behavior of animals. The cen- thing perceived by the senses, viewed as tral idea is that, from the retinal image, “blackness” is activated when the contrast the basic component in the formation of neural elements extract the relative re- is reversed, for instance when we fixate on concepts; a sense datum. sponses of neighboring receptors and a printed black dot against the background Labeled line code: A coding mecha- process the information through a se- of a white page. Since one of the two out- nism used by the nervous system for stim- quence of stages to ultimately encode the put lines is inverted relative to the other ulus quality. Information in a neural mes- information using a system of “labeled (although both access the same photore- sage depends on the set of nerve fibers (axons) that are active. The identity or lines.” ceptors), the activities of the two lines are label for an active line tells the "mean- The concept can be illustrated in the always opposed. The activity in these two ing" of the activity.Thus, for example, at a case of the percepts of black and white parallel output lines is presumably respon- more basic level, increased discharge in that, at higher levels of visual processing, sible for the percepts not only of black and axons coming from the ear gives rise to are carried by a pair of labeled lines. white, but also of all of the intermediate the sensation of sound only; stimulation of the optic nerve from the eye evokes a Consider the small cluster of photorecep- shades of gray. There is not general agree- visual sensation only. Similarly, within the tors that are illuminated by a bright star as ment about how the activities of black and visual areas of the brain nerve fibers are we fixate on it: multiple neural output white labeled lines are related to percepts tuned, for example, so that red objects lines gather information from the illumi- of gray. However, one theory is that there increase discharge in one set of fibers and green objects in another set of fibers. It is nated receptors, as well as from their is no labeled line for gray. Rather, “gray” is the label on the active fibers that immediate neighbors. In the brain, one set the percept that occurs when neither line informs the brain that the sensation is one of nerve fibers that carry output from this is active. A series of sensations from light of redness or greenness. cluster of receptors constitute the labeled gray to white would correspond to line for “whiteness;” this line is activated increasing levels of activity, ranging from Optics & Photonics News n January 2001 27 COLOR VISION stage for encoding hue has two compo- However, the two systems diverge at the ed to the human S cone, the other to the nents8: each is responsible for a pair of sen- level of the cone photoreceptor output: human M and L cones. The photoreceptor sations, and just as in the case of the black- one system draws output from the S cones, basis of wavelength discrimination medi- white system, the sensations in each pair comparing it to the L plus M cone ated by this system is illustrated in Fig. 1. are opposed to one another. One hue sys- responses; the other extracts wavelength Over a wide band of the spectrum, each tem is responsible for perceiving the hue- information from a comparison of the rel- wavelength of light produces a unique pair blue and yellow; the other is responsi- ative L and M cone responses.9 The fact ratio of activation in the two different ble for the hue-pair red and green. Each of that only one system depends on S cones is photoreceptors. The neural circuits these two systems is believed to share significant because S cones differ from M responsible for color vision compute the properties with the black-white system: and L cones both in physiology and in reti- ratio of absorption of the two types of each presumably draws from a common nal distribution. We have also learned that cone to provide a continuum of wave- set of photoreceptors, but outputs the the underlying neural circuits of the sys- length information through the region information through different where the absorption spectra of the pig- neural components to consti- ments overlap. Figure 1 represents an tute the different labeled lines. attempt to illustrate the color world of The blue-yellow and red- creatures with dichromatic color vision. green systems are remarkably Wavelengths in which the relative absorp- different and separate. Togeth- tion by the S cones is highest are shown as er, the two systems extract in- appearing blue to the animal (by analogy formation used for color vision to the human blue-yellow system) and from the three traditionally rec- wavelengths in which the relative absorp- ognized classes of cone pho- tion by the other cone type (L in the dog) toreceptors: short-, middle-, is highest are shown as appearing yellow. and long-wavelength-sensitive, From the middle of the spectrum to the abbreviated S, M, and L. ends, changes in color are seen solely as changes in saturation (i.e., vividness of hue, or degree of difference from gray). In the middle there is a wavelength for which tems are quite different in design, opera- the outputs of the two cones are balanced tion,7 and vulnerabilities: the blue-yellow and the appearance is colorless (shown as system is more likely to succumb to gray in Fig.1), which corresponds to mini- insults, such as toxic exposure, eye disease mum saturation of both cone types. Thus, or a trauma such as retinal detachment; over an interval of about 100 nm, dichro- the red-green system, less susceptible to mats can discriminate among 20 to 30 gra- acquired disorders, is subject to an aston- dations of monochromatic light.1 Pre- ishing number of congenital defects that sumably, the neural circuit that compares rarely occur in the blue-yellow system. A the responses of the S and L cones to pro- comparative approach has provided clues duce an output with the sign “plus toward understanding these two systems S/minus L,” constitutes the neural sub- from an evolutionary perspective. In fact, strate for a labeled line for “blueness,” one with the exception of a few differences in which short-wavelength (blue) light driven by utility, in each case the differ- produces positive activity. The same ences between the systems can be inputs fed through a sign-reversing circuit explained by a surprisingly unique evolu- would constitute the line for “yellowness,” Figure 1. Absorption spectra of the two different tionary history. which is silenced by blue and activated by cone photopigments responsible for color vision in light at the other end of the spectrum.
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