On the Overlap of the Response of M and L Cones
January 20th, 2006 | No Comments »I have proposed that green (midband or M) sensitivity of the retina derives from the optical antenna dimensionalities formed by the appositions of cones to rods. Moreover, such centers actually geometrically define the exact midband sensitive point on the retinal surface.Vision literature portrays midband sensitivity as being derived from the response of “green sensitive cones”..distributed somehow in the foveal area of the retina
The region that I define as M sensitive begins at retinal angles of 1-2 degrees where the first rods appear in the otherwise all-cone fovea (Osterberg states that the first rods appear at 0.13 mm from the foveal center). The M sensitive region then peaks at retinal angles of 7-8 degrees (the fixed midband point where rod density is first sufficient so that eight rods completely surround each cone) yielding to short wavelength (or “S” sensitive) response at angles of ~ 12 degrees (where rod receptors become dominant). This behavior is shown in Figure 3 of my original paper.
Thus I would claim that the M and L (long wavelength) sensitive areas of the retina are distinct and spatially separated with the M region forming a circular ring around the central, all-cone L region. Further, the retina from the all-cone sensitive fovea to the peak of midband M sensitivity lies within angles of less than 8 degrees – BUT THE TWO REGIONS ARE SPATIALLY DISTINCT AND DO NOT AT ALL RESEMBLE UNIFORMLY PIXILATED PHOTOGRAPHIC COLOR FILM.
It may be important that the two regions have different areas with the M region being larger than, and forming a “ring around”, the central L region. These different areas must be calculated andmay bear on “quantal catch” (or, more appropriately, “quantum efficiency”) considerations discussed elsewhere.
Now…it seems that both psychophysical and microspectrophotometric (MSP) measurements agree that response of the M and L regions overlap to a large degree. See the response curves referenced yesterday (but one can find them in any vision text). Specifically, the transient chromatic adaptation (psychophysical) type of measurement of cone sensitivity referenced in Stockman et al (J. Opt. Soc. Am. A, Vol.10, No.12, December 1993) seems to indicate that this type of transient chromatic adaptation measurement is made using lights focused to a central (foveal?) two degrees with a surround encompassing eight degrees of retinal angle. (I am no expert on this type of measurement and stand to be corrected if I am wrong), Assuming this, both the L and M regions that I define would be simultaneously interrogated in this measurement. Two separate regions could possibly be defined (and apparently are) in color comparison measurements as it is generally in this region that wavelength comparison is effected (with the fixed midband point at 8 degrees).
I would propose this as the reason for the overlap in the M and L response curves. It would be natural to assume, as has historically been done, that the M response derives from a “different type of cone” but I do not believe that this is the case.
Which leaves the second (MSP) type of measurement to be dealt with. It is purported that MSP defines separate M and L cone types. Stockman et al, as I noted yesterday, discuss the problems with MSP to which I add physics considerations (diffraction at distances approximating the wavelength of light is a very powerful effect indeed!). In reviewing this lengthy paper one has a difficult time from their Figures 11, 12, and 13 discerning any spectral differences at all in M and L cone response. The only differences seem to be in “quantal sensitivity”. Even after the data referenced are adjusted and corrected in innumerable ways (so stated) the spectral responses appear strikingly the same! Even the measurements made on humans and rhesus monkeys do not appear to differ. I really believe that from my (physics) point of view, this should be so. This, however, is a very difficult point for me with my limited knowledge of this type of measurement to assess…and I stand to be educated.
GCH
