More on Retinal Pigments
January 18th, 2006 | No Comments »I have proposed that it is the nanostructure of the cone and rod organization of the retina that controls light interaction with this surface. In this new view the eye evolved to detect incoming light as the wave of classical physics in inter-receptor spaces while the cones and rods themselves perform the function of “quantum confined electron sites” or, as generic “nanowires”. This is at odds with the long held “purely quantum” view that “light photons interact with pigment molecules contained within receptors”. I must note that my view in no way conflicts with quantum reality….one must simply replace the idea that “a photon interacts” with the more general “a quantized interaction occurs”. I have been informed by elements of the vision science community (in no uncertain terms!) that my view cannot be correct in light of the historically held pigment mental construction.
In my initial paper I had proposed thinking of the receptors themselves as entities emphasizing the nanostructural overview (the simple geometric logic of that overview is still the overriding factor in light detection!). I viewed the retinal/opsin pigment complexes contained within the thylakoid disks of receptors as energy-accepting sites with the chromophoric retinal molecule performing that (among other) function with the various opsin configurations providing the structural framework for spatially defining (or orienting) the position of the pigment complex within the bilipid membrane of the thylakoid disk.
Upon reflection, and jogged by criticism that I was disregarding the pigment model, it became clear that the structure of the pigments should differ to accommodate the three different receptor dimensional arrangements (i.e., the C-C, C-R, and R-R appositions). These three appositional dimensionalities define the three light absorption peaks of my proposal and thus three quantitatively different energy situations (I suppose in “optical antenna” terms a “resonance” might be assumed). This would explain the differing energy of pigments that has been historically found.
To the subject of pigments themselves……I began by reviewing a paper by Mollon (Proc. Natl. Acad. Sci. USA, Vol. 96, pp4743-4745, April 1999). The author presents (his Fig.1, p 4743) the same photopigment response curves published over and over elsewhere (and which I have critiqued in my section “I Do Not Believe That Classes of Cones Exist”), I have noted that the curves, based on microspectrophotometric (MSP) type of measurement (see below), have strange characteristics. First, they are normalized to the same sensitivity levels when I claim that there is not evidence for doing this ..or positioning the blue (or “S”) response at this level (or indeed, at any level -but that is another story). The green and red photopigment response curves labeled here as “M” (or midband) and “L” (or long wave) overlap and are exceedingly closely spaced with the red curve showing vanishing sensitivity at the red wavelengths (650-700 nm) to which it is supposed to be responsive. Mollon does include a color bar in the figure that is helpful..showing that the L curve peaks in the yellow/green region of the spectrum. Interestingly, the M and L peaks sit almost exactly astride the 550 nm midband point of the visible spectrum ..more about this below.
Nathans (Neuron, Vol.24, 299-312, October 1999) quantitiates the above M and L curves to ~530 and ~560 nm. He also makes a statement (p. 302) that I will quote directly “Why do humans and other trichromatic primates have cone pigments with absorption maxima at ~425, ~530, and ~560 nm rather than at some other set of wavelengths? In particular, why are the absorption spectra of the green and red (I note – not “M” and “L”) pigments so close together relative to the blue pigment spectrum?” I must note that he goes on to provide answers to these questions that I will discuss below.
Now…further on in Nathans, in his explanation of the above, on p 303, he presents psychophysically derived(important!) pigment absorption spectra. These absorption curves - as opposed to the MSP derived curves noted above - seem to solve the red sensitivity issue that I raised with both the M and L curves displaying finite sensitivity in the 650-700 nm region (they are still, however, normalized to the same sensitivity levels that I do not understand)…but they do, importantly, display red response!!!!
Stockman et al (J. Opt. Soc. Am. A, Vol.10, No.12, Dec. 1993) discusses the differences between psychophysically-derived and microspectrophotometrically-derived (MSP) types of photopigment measurement. The authors take issue with the MSP measurement for some of the same reasons that I had noted…that they are made transversely whereas the traditional photon/pigment interaction model supposes as axial incidence (even Stockman, however, does not mention the transverse dichroic orientation of the photopigment chromophore that supports my view..and has never seemingly been explained). I would propose, among other issues, that the MSP measurement that is made with the receptor removed from its natural (nanostructural!) milieu within the retina,i.e., “laying it on it’s side in a bath of fluid whose index of refraction differs….etc.” (such a measurement can be considered as “interrogating a gradient of refractive indices across the receptor”). The psychophysical measurement, on the other hand, interrogates the photopigment in its natural, nanostructurally controlled environment…and thus, elicits a truer result.
HEREIN I BELIEVE LIES THE ANSWER WITH THE DIFFERENCE IN THE TWO TYPES OF MEASUREMENT SUPPORTING MY ASSERTION THAT THE IN-VIVO (ACTUALLY “LIVING”) NANOSTRUCTURE OF THE RETINA CONTROLS LIGHT ABSORPTION. THE MSP TYPE OF MEASUREMENT MERELY QUANTIFIES DIFFERENCES OF PHOTOPIGMENT RESPONSE .. REFLECTING ENERGY LOSSES THAT WILL PROBABLY BE FOUND TO CORRELLATE WITH THE STRUCTRURE OF THE RETINAL/OPSIN COMPLEXES.
Back to the Nathans paper(p.303) and his method for calculating spectral discrimination curves from visual pigment absorption spectra. I point specifically to his Figure 5 A and B. He uses a rather complicated “amplitude of differencing” method that he further refers to as “line element analysis”…”first introduced by Helmholtz with subsequent refinements by Schrodinger…”.and who am I to quarrel with those distinguished gentlemen! (I am reminded here of an anti-Land letter that I received pointing to a reference “Land! Land! where the author began by excoriating Land “who was he to dispute Helmholtz! ..which Land did not do!).
The method described by Nathans (and repeated in Mollon) posits the photon-interacting-with-pigment hypothesis and uses the “quantum catch” notion that it is the number of photons “caught” by a receptor that leads ultimately to the logic of spectral discrimination. A statement is made that I believe should give anyone pause..that the photon “loses its identity” (i.e., any information about any wavelength association) upon the process of absorption . This would make me nervous in placing belief in any hypothesis using this assumption..somewhat like the conundrum in quantum physics of the “collapse of the wave function”.
In any event, Nathans in Figure 5B, using the above method, derives three retinal response peaks that are reminiscent of the three peaks that I derive using (I believe!) exquisite geometrical simplicity (Figure 3 of my original paper). A distinct midband peak at 550 nm is calculated by Nathans which is where it should be. I locate this peak with simple geometric logic aand, moreover, can define it as a precise “fixed reference point on the retinal surface.
In this rudimentary exercise of reviewing pigment literature one is left with a few distinct impressions.
First, that the M and L responses (Nathans Figure 5 A-H) control spectral discrimination with much less concern with, the blue or “S” cone specie. In the analysis following from this figure the blue peak is “just there” normalized to it’s improper level (my view) while the positions of the M and L peaks control things.
Which brings me to the point, it seems to me that this entire “quantum catch”- based method of calculating spectral discrimination is based on normalized peak levels…which seem arbitrary to me……how can this be?
On blue cones from Nathans (p.304), “”The primate retina has solved both of these problems (discussed above) by sprinkling the retina only sparingly with blue cones, and, in some species, decreasing their representation in the central fovea (references)”.
The point that the M and L curves (and noting that they are labeled as such) straddling the 550 nm mid-band point seems to echo Land’s theory that “color” is determined by the eye obtaining a ratio of signals (“brightnesses”) on either side of mid-band. The above goes into how such a ratio of signals might be affected by retinal circuitry. I have not concerned myself with this limiting my attention to the physics of light interaction with the retina itself..
Although this may be somewhat disjointed I am going to post it anyway as this is all the time I have
GCH
