Again, a Request That a Measurement be Made

by Gerald Huth on April 3, 2007


I repeat my proposal that measurement of the wavelength sensitivity of ‘retinal mosaics’, made by Rooda & Williams at a retinal eccentricity of one degree, be made at larger eccentricities. I predict that such measurements will show an increasing density of green ‘cones’ with increasing retinal angle reaching a maximum density (i.e., total green response) at retinal angles of 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 in this region.



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 eccentricity 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 eccentricity


I now believe that I misspoke and that their measurements did represent a distribution of “color detection centers” at this point but that these are not due to the response of cones but are the distribution of these centers is fully consistent with my explanation. This rather heroic imaging methodology developed by the group used probably portrays an accurate picture of the distribution of such centers and, as such,it may be used to validate my model.

The authors state that their measurements were made at a retinal eccentricity of one degree and, further, that all of the color centers are cones. Further, that no rod receptors appear in the views. George Wald, however, in Figure 9 of his paper (“Blue-Blindness in the Normal Fovea”, Jour. Opt. Soc. Am, Vol. 57, No.11, November 1967) recounting Osterberg’s retinal morphology 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 changing precipitously and that this introduces 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 eccentricity, 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 nearly equal, the cones are almost twice as large as rods. A statistical distribution of rod-cone appositions therefore begins to appear reaching a point at 7-8 degrees where sufficient rods are present to completely surround each cone and form an octagonal motif. These are the green detection centers that I would believe are the green centers observed by Roorda and Williams.

To say again, if this group would extend their measurements to retinal eccentricities beyond one degree I believe that 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.



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