Color Vision: A New Understanding
John A. Medeiros
Summary, Final Note
I have covered a wide range and large number of issues related to human color vision. I hope that in the process I have shed some light on the topic, Color Vision: A New Understanding. While I have made what I believe is a very strong case for the inadequacy (at least) of the three-cone model of color vision as based on "red", "green" and "blue" photopigments uniquely sequestered within each cone type, there is still, of course, a very important role for photopigments in any model of color vision. The photopigments provide the first step in the transduction of light into a visual sensation. However, I am suggesting that the role for photopigments is ancillary to the basic spectral dispersion mechanism of waveguide mode cutoff. Photopigments do the work of absorbing and converting the light into an electrical signal and a differential distribution of photopigments with different absorption curves could well enhance and improve the operation of the basic mechanism proposed here.
By way of a summary, some of what has been covered in this document includes:
- A description of the approximate three dimensional nature of human color vision: how that dimensionality plays in metameric matches, how the ratio of cone bipolar cells to cones varies from 3:1 in the central retina where color vision is trichromatic, to 2:1 in the intermediate retina where color vision is dichromatic and to 1: 1 in the peripheral retina where color vision is essentially monochromatic. The bottom line is that the dimensionality of color perception is evidently more closely tied to the processing circuitry of the retina rather than to separate cone classes.
- A description of the Ives' experiment that invalidates the basic premise of the standard three-cone model through the demonstration of the breakdown of statically established metameric matches under dynamic presentation and the validation of that result in our own repetition of the experiment.
- The explanation of the Ives' result in terms of the chromatic latency of color perception as measured in our experiments where we made use of the separate and simultaneous observation of rod and cone responses using moving slits of light.
- A cursory review of the evidence from molecular genetics and microspectrophotometry that shows that the evidence does indeed suggest the existence of multiple pigments but that these are not necessarily the same thing as multiple cones.
- Reviewed the anatomical evidence that shows that instead of separate classes of cones, all the cones in a given area of the retina are essentially identical and that there is a systematic change in the shape of the cone photosensitive outer segments from being long and gently tapering in the central retina where color vision is best to being short, squat and more evidently tapered in the periphery where color vision is diminished.
- Described a spectroscopic property of small tapered fibers where low-order waveguide mode cutoff disperses light in a systematic spectral order along the length of a cone.
- Pointed out that the cones of the human retina have just the right parameters (size, shape and refractive index) to optimally exhibit the spectral dispersion effect.
- Described an experimental demonstration of this spectral dispersion in tapered fibers and showed photographs of the spectral dispersion of light emerging from a cone as a result of mode cutoff.
- Described the induction of subjective colors with purely black and white illumination presented in the appropriate temporal order, and how the temporally ordered colors match the measured chromatic latency of color perception.
- Described how the photoabsorption event within a cone is local, so that information on absorption as a function of position along the length of the cone is available for some read-out mechanism. The absorption event is, in fact, localizable to less than 1.0 µm so that for the the 40 µm long foveal cones, the potential wavelength discrimination (for a single cone) is 1/40th of the 650 to 450 nm range of vision or 5 nm.
- Described how microscaddic eye movements can provide the synchronization signal necessary to convert the spectral dispersion of color information along the length of the cones into a temporal code with the characteristics matching the induction of subjective colors.
- Examined a list of 42 items, including anatomical features of the cones and retina and of color vision functions and effects, that any model of color vision must explain or at least with which it must be consistent. The proposed cone spectrometer model accounts well for virtually the entire list, while the standard three-cone model predominantly fails.
- Described a photpigment-independent calculation that accounts for the hue discrimination function on the basis of the cone spectrometer action alone.
- Described how the model accounts for the similarity of the perception of violet and purple as a result of second order mode propagation for light of sufficiently short wavelength.
- Described how mistuning of the cone parameters can lead to the common forms of color deficit vision and discussed how the understanding of this process might lead to new ways to clinically address color blindness.
The bottom line to all this is that the eye is indeed a marvelous instrument of seeing, perhaps even more cleverly constructed than had been previously imagined. There is reason, after all, that we can each detect seven million different colors or so and do it over a dynamic range of light intensity that spans some ten orders of magnitude, a feat no invention of technology has yet achieved. Each human eye is composed of an array of millions of sublimely constructed spectroscopic detectors that can each resolve the world of colors in a way no mere gross partitioning into three buckets of color could ever hope to accomplish.
Given the scope of what I have covered here, I can hardly do justice to the vast body of scientific research, studies, publications, and data that has been generated about human color vision over the last two hundred years. There is clearly very much more to do, but perhaps the framework of the cone spectrometer model described both here and in the book, Cone Shape & Color Vision, can yet help with the process of clarifying the understanding of color vision.
I apologize in advance to any whose favorite project or effect or issue I did not address (although, perhaps I should be apologizing to those whose issues I did address!). The subject is simply much too vast to cover everything in one document or even one book. I do really feel that what is discussed here, as comprehensive as I have tried to make it, is only a beginning. My greatest joy after so many years of working on this issue would be to see a revitalization of the field of color vision with a new understanding that I may have had a small part in generating.
Anyway, thanks to Nancy for discussions and in helping to proof all this and to David for some excellent ideas for making this a better document. Any omissions, errors, or gaffs, unintended as they may be, are entirely my own responsibility.
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