jbm@eos.UUCP (Jeffrey Mulligan) (03/29/88)
From article <3267@tekgvs.TEK.COM>, by mikec@tekgvs.TEK.COM (Micheal Cranford): > ...[ material deleted ]... > Not every human is a trichromat (three primary color > sensors). In addition to the obvious color blindness (monochromats and > dichromats) a few people are tetrachromats (four primary color sensors). > This is a good example of evolution (i.e. change) in action. > If a person were to have four color "sensors," he might or might not be functionally tetrachromatic (in the sense of requiring four primaries to match an arbitrary color). This is because functional trichromacy could equally well arise from a recoding into three *neural* coding channels. The first microspectrophotometric measurements of cone pigments were only made within the last decade; that was the first direct evidence that there are indeed a small number of photopigments. There was strong indirect evidence that human trichromacy DID arise from three cone pigments: namely, the invariance of metameric matches under chromatic adaptation. This would not hold for an observer having more than 3 distinct photopigments whose signals were transformed into three neural channels. I have heard of anomalous trichromacy, and I may have heard that some anomalous trichromats have both the anomalous and the normal pigment, but I have never heard of an actual tetrachromat; if you could post a reference I would be quite interested. > [5]. Retinal rods are part of the Scotopic visual system and do not take part > in the discrimination of color. Retinal cones, being part of the Photopic > visual system, do discriminate colors. Since rods and cones use the same optic nerve fibers to signal the visual areas of the brain, I'm not sure how useful it is to talk about distinct scotopic and photopic "visual systems." Supposedly, rods do influence color perception at low (mesopic) luminances below the rod saturation point. Neither rods nor cones are capable by themselves of discriminating colors. (A single photoreceptor cannot tell a color change from an intensity change). Color discrimintation can only happen at a later neural stage, where signals from the different cone classes are compared. -- Jeff Mulligan (jbm@ames-aurora.arpa) NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035 (415) 694-5150