On Color Variant Vision (Usually Termed Color “Blindness”)

The retina of the eye is shown in this simple geometric explanation to detect three – and only three –  wavelengths of light  and these are detected on the retinal surface in circular bands that surround the central fovea. Further, it is the intensity of these  wavelengths that varies across each band. Traditionally, we have termed these three wavelengths “primary” and then, inappropriately, proceed to call them “colors”. At this point in the vision process these wavelengths represent only the basis for the subsequent synthesis by the eye of the endless hues of color that we see.

This explanation for light interaction with the retina is consistent with, and for the first time makes sense of, the 1935 work of Osterberg who recorded in detail the asymmetric morphology of cone and rod receptors on the retinal surface.

As described in the body of this work, the assumption that light is absorbed via spatially-dimensioned “antenna” structures, together with a simple geometric insight, shows that the retina actually functions as the Fourier plane of the optics of the eye in contrast to the historic and incorrect assumption that it is an image plane (as in a camera). Vision is shown to be a diffractive Fourier-transforming process.

With this view of the retina in mind it becomes apparent that Edwin Land’s work on color vision, without any knowledge of the retinal structure that I propose, was brilliant indeed! Land posited from measurements made external to the eye that a fixed reference point (that he termed a “fulcrum”) must exist somewhere on the retinal surface. It becomes clear in this work that this is the geometrically determined mid-band point at 7-8 degrees of retinal eccentricity where the density of rods is sufficient to completely surround each cone. This is a fixed reference point where the detection and the refractive properties of the lens and body of the eye coincide! All subsequent color processing is made relative to this fixed reference wavelength.

Land further deduced that the hues that we term color were determined by a ratio of the light intensities incident on either side of this fixed point – a deduction that is also consistent with this explanation. Land further found in simulations based on his model that any, even slight, shift in the position of this point on the retinal surface would drastically alter the perception of color. (please review Land’s work for further understanding here). To say again, refraction in the lens and body of the eye is directs the mid band wavelength (~550 nanometer) to this specific angle on the retina that has evolutionarily been defined to be geometrically tuned to this wavelength.  This is the point on the retina  at 7-8 degrees of retinal eccentricity where the “pure” eight-around-one motif of rods around cones exists (see Osterberg). It should be obvious that any alteration in the ratio of the diameter of cones to rods, occasioned by a change in the diameter of either one, would cause this crucial reference to be moved to a different retinal eccentricity thus affecting perception of color. The same result might be a result of a change in the shape of the eye that would cause the mid- band wavelength to be refracted to a different point on the retina. I believe, however, that the former is more logical.

I would propose therefore that a change in the ratio of the diameter of cones to rods is a logical cause of what is termed color variant vision. In modern times it has become fashionable to invoke a genetic explanation to explain this visual condition (and most everything!). Genes express proteins and one should look to the specific genes that express the proteins that determine the diameter of the outer light interactive segments of the retinal receptors to understand the cause of this visual abnormality. So, at the basis, it a genetic effect but this explains nothing about the mechanism involved.

I am indebted to Peter Kaiser (“Human Color Vision” Second edition, Kaiser and Boynton) for introducing me to the term “color variant vision”. It really is not a “blindness” at all but just another unfortunate term that has been introduced into the science of vision.

GCH

3/02/08