A Reprise on the Subject of “Classes of Cones”
Friday, February 29th, 2008Regarding a Letter to Nature (Nature, Vol. 397, 11 February 199, pp520-522)
This letter titled “The Arrangement of the Three Cone Classes in the Living Retina” by Roorda and Williams is interesting. This (and remember that it is published in 1999) represents the traditionally held view that “Human colour vision depends on three classes of receptor, the short (S), medium (M), and long (L) wavelength-sensitive cones”. The paper then goes on to state that ” these cones are interleaved in a single mosaic” so that “at each point on the retina only a single class of cones samples the retinal image”. I readily admit that I do not know what this means! They seem to assume (as has the history of vision) that the retina acts as the intensity-only sensitive “image plane” of the optics of the eye. This is the plane where photographic film is placed in our camera technology. If this were so, however, there would have to be some logical spatial order to the three light detection centers - as the RGB triads or stripes used in televison imaging tubes or camera CCD surfaces. But…there is none of this on the retina - only Osterberg’s asymmetric array of cones and rods. What on earth is assumed here? How is an image (and ,even further , a color image) formed? Is there some logic here that is eluding me?
Incidentally, the asymmetric distribution of cones and rods as measured by Osterberg in 1935 forms the exact basis for my explanation of light interaction with the retina and the genesis of color vision. The meaning of this distribution seems to be explained for the first time
A further sentence in the Roorda Letter states that “although the topography of human S cones is known 2, 3…...” (p.520). If this statement were accurate, namely that the so-called S cones have been shown to exist and that they are arranged in some known topography, I would would have to concede that my explanation of the vision process was incorrect.
I humbly submit, however, that a review of the references cited ( #2 an #3 as noted above), purportedly that support this contention, do not seem at all to do this!!!! I stand to be enlightened but this is my reading of the references.
The Williams reference 2 is simply inconsistent with their statement. In the reference, and after considerable effort, a single cone receptor is finally located which is defined as an S cone. One might question how this finding can be reconciled with the assertion that a known topography exists. How is one to ascribe a topography to a single cone?
The Curcio reference 3 is even more curious, again, not at all supporting the statement. This paper relates a much more intensive effort than pursued by Williams to seek the elusive S cone using a number of diverse approaches. But the concluding comment of the paper is most telling stating that “all of the evidence (for the existence of the S cone) must be viewed as inferential only” . The italics are mine but this is the exact term used in the paper. I really do not understand what this means but it is certainly not very positive about the existence of the S cone (or a topography)!
One more note about this piece – as the authors state, their measurements were consistently made “at a retinal eccentricity of one degree nasal from the foveal center”. This region, i.e., at the “edge” of the fovea, is the retinal angle at which there is a greatest interplay of cones and rods – cone density is rapidly diminishing and rods are being introduced into the admixture (again, please take note of the historic data of Osterberg). I would therefore expect to see what R&W imaged at one degree! The pseudocolored images that they present as Figure 3 (p.522) represent exactly what I would expect composed of predominantly red centers representing the long wavelength sensitive cone/cone appositions of the fovea, some number of green centers entering the picture representing the gathering statistical number of cone/rod appositions (and that geometrically define mid band wavelength), and a far fewer number of blue centers characteristic of the, still random at this angle, pairing of rod receptors.
I therefore would agree with the validity of their experimental method recognizing that their measurements were made at one degree of retinal angle. And….these findings at one degree of angle….are in complete consonance with my explanation for light interaction with the retina - and with the long accepted morphology of receptors on the retinal surface.
THERE IS, HOWEVER, ONLY ONE PROBLEM - THE WAVELENGTH DETECTING CENTERS THAT THEY IDENTIFY DO NOT REPRESENT CONES BUT RATHER RECEPTOR APPOSITIONS.
I would propose that if they made a similar type of measurement at 7-8 degrees of retinal eccentricity they would find, accepting that their experimental methodology was correct, that green centers would predominate.
GCH/Ojai,CA
2/29/08
1A. Roorda, and D.R.Williams, Nature, 397, No.11, February 1999
2 Williams, D.R. et al “Punctate Sensitivity of the Blue Sensitive Mechanism”, Vision Res, 21, 1357-1375, (1981)
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