John A. Medeiros
By all accounts, the mystery of how the eye sees color is solved.
Yet, contradictions, puzzles, and enigmas about human color vision persist.
And how can there be no connection between form and function, cone structure and color perception?
Perhaps it is time to consider another approach to understanding color vision.
With the press of everyday affairs, it's quite natural to take our sense of color vision for granted. And yet, when we do stop and take the time to think about it, like we might when we see a particularly awesome sunset, it’s hard not to marvel at the richness and depth our color sense adds to the visual experience.
Philosophers, scientists, and laymen alike have all long wondered how color vision works. Many of the giants of scientific thinking, including Newton, Goethe, Young, Maxwell, Helmholtz, and many others have explored various aspects of vision and pondered the question of just how the eye sees colors and all have contributed in important ways to our understanding of the process.
Well, here we are in the Twenty-First Century - the age of supercomputers, molecular genetics, the Internet and space travel so by now we surely know the answer to just exactly how we see colors. Do we not?
What About Our Understanding of Color Vision?
The standard, near universally accepted, Young-Helmholtz trichromatic model explains color vision through the identification of three cone types which explains at a stroke the fundamental fact of trichromacy. Supporting this model, opsins (the protein portion of the photosensitive pigments) that are sensitive to different parts of the visible spectrum have been genetically isolated.
But there are a number of aspects of color vision about which the standard model says nothing, or indeed, says the wrong thing. Consider just ten such items:
All these facts are mysteries -- or even paradoxes -- under the Young-Helmholtz model. However, they are unproblematic (in some cases, required) for an alternative model of how color vision works, what is here called the Cone Spectrometer Model (CSM).
Consider the fact that the cones are just the right size to serve as effective waveguides (optical fibers) to carry light from any part of the visible spectrum. Optical waveguides transmit light along their length in discrete waveguide modes which essentially correspond to light propagating along the fiber by bouncing at specific angles of reflection with the fiber wall and surround interface. Now, for appropriately "sized" cones, due to the tapering which gives cones their name, successively deeper (narrower) parts of the cone can't carry light of longer wavelengths. Red light only fits into the wide end of the cones, and travels but a short distance before being "squeezed" out by mode cut off; green light gets deeper into the cone, and blue light can shine all the way to the bottom.
The accompanying photograph shows a highly-magnified view of this effect occurring in a small tapered glass fiber immersed in a liquid with a refractive index only slightly smaller than that of the fiber. Long wavelength light is seen to leak out first and progressively shorter wavelengths are excluded at successively smaller portions of the cone. Actually, it is evident that this happens more than once in the fiber. Near the top where the initially white light is incident, the colors leaking out and visible along both "edges" of the first third of the glass fiber are first white, then rose-colored, then greenish to blue where the second-order mode is cutting off first. There is then a well-order red through blue dispersion of the spectrum along the last two-thirds of the photograph as the lowest-order fundamental waveguide mode cuts off along the smallest part of the taper.
Now light, of course, shines into the cones at the speed of light, but once a detection event happens, the resulting information travels much more slowly as a nerve impulse. The cones are, somewhat perversely, "wired backwards", so that the red-detecting, wide ends are effectively closer to the cone output at their synapse with the bipolar cells. Detection events at the deeper, blue end must propagate back up the length of the cone at this slow speed; as a result, detection of red light is signaled earlier than blue light (and in general, detection time will be proportional to how deep the detection happened). In other words, the taper cutting off different wavelengths at different depths corresponds to differences in timing (of light detection information reaching the brain).
Effectively, the shape of cones sorts wavelength information into a difference in timing; I propose that this is the essential mechanism of color vision.
In this context, what does the CSM model say about the ten problematical items mentioned above?
There is actually a good deal more the proposed model says about these and many other aspects of color vision. The following sections explore many of the related issues more systematically to build what is arguably a compelling case for the CSM explanation of color vision. The goal is nothing less than the dispelling of what has been many of the long persistent contradictions, puzzles and enigmas about human color vision.
1. The Standard Three-Cone Model of Human Color Vision
2. The Three-Cone Model is Experimentally Falsifiable
3. Multiple Pigments
4. What Does a Color Vision Model Have to Explain?
5. What About the Cone Shape?
6. The Cone Spectrometer Model (CSM)
7. Hue Discrimination and the Similarity of Violet and Purple
8. Color Deficit Vision
9. Summary, Final Note
About the Author
There exists a vast body of scientific research and published literature on human color vision. In part, this is a reflection of the natural interest in the functioning of one of our most profound and treasured human senses. It is also a reflection of the confounding complexity of the perception (involving psychology, physics, neurophysiology, biochemistry, and psychophysics) and the lack to date of a truly comprehensive model to explain the array and diversity of the myriad aspects of color vision. Given the vast scope of phenomenology involved, it is not possible to address all of it here. I do cover more details in the book, Cone Shape and Color Vision: The Unification of Structure and Perception (click for table of contents) although there is still much more to explore. The book is available as a black and white (!) paperback or as a downloadable, color PDF file from all the major online booksellers. A link to the book at Amazon.com is here.
Incidently, I have all the content of the document comprising this websie available as a single, downloadable PDF file here (about 3.4 MB).
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