I will begin by referencing Osterberg’s 1935 diagram of the distribution of cone and rod receptors on the retinal surface. I do not believe that there is any disagreement as to the validity of these data as it is appears in almost every textbook on vision and the eye. This diagram appears as Figure 2 of my original paper where it, in essence, forms the basis for this entire concept and where it is shown that this specific distribution of receptors is in exact consonance with the diffractive properties of the lens and structure of the eye.
But first, I have always been in wonder how anyone could ever reconcile this magnificent asymmetry of cones and rods with the idea that the retina was some sort of analogue of photographic film! Illustrations from vision textbooks repeatedly portray the retina (the “inverted arrow” diagrams) as the “image” plane of the eye. Such a retina would have to be composed of some sort of uniform array of RGB detection centers (or at least three separate color sensitive layers as color photographic film). It is just so obvious that this is is not the case! How might the vision science field have come to the conclusion that it is such a surface?
I will begin with the old saw, again repeated over and over, that “cones detect color” and “rods are the low level black and white receptors of the eye”. (I repeat a comment made to me by a vision science investigator that “no one believes this anymore”…what?) I have provided an explanation for the statement concerning rods elsewhere on the page – to the point that rod receptors that comprise the peripheral retina are linked together (i.e., linked “in parallel” which has been apparently experimentally verified) to form the “light meter” of the eye with this integrated array of receptors controlling pupillary constriction and thus the level of light entering the eye. So, it can be said that rods are involved in low level light detection but not at all as historically assumed. I can see no scientific basis for the assertion that single rod receptors, in themselves, have greater sensitivity to light (than cones).
But, to the historic belief that “cones detect color” and, further, that there are somehow three “classes”of cones (i.e., sensitive to separate red, green, and blue wavelengths). How on earth could this belief have come about?
I will digress here to make the point again that the term “color” itself should not even be associated with light interaction with retinal receptors. Rather this term implying a panoply of hues should properly be applied to a “synthesis of the three signals detected on the retina” appearing in the “color centers” of the brain – as so brilliantly deduced by Edwin Land. The retina itself detects only the “brightness” (or intensity) of three narrow RGB optical wavelengths) as shown in this work.
Further, doesn’t the idea of a single “color detecting cone receptor” imply that such a receptor is acting as the analogue of some sort of “complete laboratory spectrometer”? The answer is really much simpler, i.e., that an apposition of two cones forms a “tuned optical wavelength antenna” that has been located evolutionarily at a position on the retina that is in consonance with the diffractive properties of the lens and structure of the eye. The eye is then a biological structure that evolved using the principles of simple physics and geometry, with the subjective concept of “design” being not at all necessary. Science has done itself a great disservice by not getting at the fundamental principle involved here.
It is axiomatic in this work that it is the function of each cone-cone apposition to detect solely the long (red) wavelength. In fact, it is the dimension of this apposition that defines the long wavelength limit of visual response. (I have proposed that “blue sensitive cones” are actually rod-rod appositions and this is discussed in detail elsewhere on the page). It therefore seems logical how the idea that one of the classes of cones (i.e., the “red” sensitive cone) resides in the all-cone fovea. That much of the “class of cone” rationale is a true statement ..but for the wrong reason.
But where did the idea of the existence of “green sensitive cones“ come from? In this work “green” sensitivity derives from the dimensionality of cone-rod appositions (and, moreover, we show how it geometrically defines the exact mid-band point). Examining Osterberg’s data shown above it can be seen that rod receptors begin to “intrude” into the all-cone foveal region at very small retinal angles. It is not really obvious in the figure how small an angle this is but a statement from Pirenne (Vision and the Eye, The Pilot Press Limited, London, 1948) gives the magnitude…..”The first rod…..is situated at distance of 0.13 mm from the foveal center”. A small density of cone-rod appositions is therefore present immediately adjacent to (or, as I propose, “perceived as within”) the overall array of foveal cones. Dimensions here are extremely small leading to difficulty in differentiating light detection centers …and this has probably led to the misconception that a different type of cone is the source of green wavelength detection.
One can see a closer inspection of the above by studying Fig 28 from Pirenne that presents a diagrammatic representation of cone and rod densities in this crucial sub-millimeter region of the retina.