A request that retinal response be measured at larger retinal angles

Added March 19, 2006

I reiterate my earlier Comment proposing that measurement of the wavelength sensitivity of ‘retinal mosaics’, made by Rooda & Williams at a retinal angle of one degree, be made at larger retinal angles. I predict that such measurements will show an increasing density of green ‘cones’ with increasing retinal angle reaching a maximum density (i.e., all green response) at 7-8 degrees. These green sensitive centers will not (could not because there are very few cones here) correspond to cone response but rather to cone-rod appositions which reach a maximum at that angle.
GCH

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I had proposed in my original paper that the figures in the paper by Roorda / Williams (Nature, Vol.397, pp 520-522, 11 February 1999) portrayed regions somewhere beyond the one degree retinal angle claimed by the authors. This was based on the presence of regions of green and blue sensitivity (their Figure 3). The authors claimed that these were due to the presence of green and blue sensitive cone receptors and that the views represented the configuration of these different species of cones at one degree of retinal angle. .
I now believe that I misspoke and that the views did represent the distribution of color detection centers at this angle but that these are not due to the response of cones but rather seem consistent with my hypothesis. I also believe that the rather heroic imaging methodology used in these measurements probably portrays an accurate picture of the distribution of such centers and that it may be used to validate my model.
The authors state that their measurements were made at a retinal angle of one degree and that all of the color centers are cones and that therefore no rod receptors appear in the views. However, Wald in Figure 9 below taken from his paper (“Blue-Blindness in the Normal Fovea”, Jour. Opt. Soc. Am, Vol. 57, No.11, November 1967) recounting Osterberg’s data notes that at one degree the density of cones and rods is approximately equal at 40,000 per sq. mm. Now, I will grant that at this angle cone and rod receptor densities are falling and rising precipitously introducing the possibility of error in measurements made in this region. However, I believe a more logical explanation is that the measurements are accurate and are explained by my model.

At one degree of retinal angle, and as I have said previously, the smaller rod receptors are beginning to “intrude” in a statistical manner into the hexagonally ordered (and tight packed) array of larger cones (remember that although the densities of the two receptors are about equal the cones are almost twice as large as rods). Therefore, a statistical distribution of rod-cone appositions is observed (Osterberg) that I would claim form the green centers observed by R/W. As rod density rapidly increases a statistically smaller number of rod-rod appositions is present (again, see Osterberg’s figures) that I would claim correspond to the fewer blue sensitive centers in this region.
I have proposed that if R/W would extend their measurements to retinal angles beyond one degree they would observe an increasing number of green sensitive centers until at 7-8 degrees the field would be totally green, i.e., rod density is sufficient to completely surround each cone at this point presenting a field of green detection centers.

GCH