There is a regional variation in the length of the outer segments of the cone photoreceptors in going from the central fovea (average length = 35μm) to the parafovea (average length = 22μm), which will affect the pigment optical density. As the length of the cone outer segment decreases with eccentricity, so does the effective optical density of the cone photopigments. With decreasing pigment concentration, the spectral sensitivity curve narrows, though the λmax stays the same. Thus, foveal cones will have a broader spectral sensitivity than parafoveal ones.
Good estimates of foveal pigment optical density are hard to obtain and depend greatly on the method of measurement. They can be collected: (i) by a comparison of data obtained under bleached versus unbleached conditions; (ii) by comparison of data obtained for obliquely versus axially presented lights (i.e. by the Stiles-Crawford effect); (iii) by microspectrophotometry (MSP); or (iv) by retinal densitometry Bleaching experiments tend to yield mean density values for the central fovea in the range 0.3 - 0.6 , Stiles-Crawford analyses in the range 0.7 - 1.0 and MSP and densitometric analyses in the range 0.35 - 0.50. See Summary table of optical density estimates and references.
The bleaching data suggest a lower density for M- than for L-cones. But the other evidence contradicts this (see Stockman et al., 1993). There are no good estimates of pigment optical densities for the S-cones.
Kilbride et al. (1983), on the basis of retinal densitometry, provide rough evidence supporting a decrease in the pigment optical densities of the L- and M-cones with eccentricity. Their results suggest a change in density from 0.35 (foveola) to 0.15 (1 deg eccentricity) to 0.10 (2 deg eccentricity).
Likewise, Elsner et al. (1993), on the basis of retinal densitometry, find a decrease in cone photopigment optical density (presumably for combined L- and M-cone pigments) with eccentricity: on average a reduction of 0.33 (0.24 - 0.41) to 0.24 (0.12 - 0.35) going from a retinal location of 0.5 deg to 4 deg.
MSP does not provide direct measurements of optical pigment measurements. This is because it employs light stimuli that pass transversely through the outer segment rather than axially as in normal vision. This introduces two difficulties. First, it is not clear how the interaction between light and the photoreceptor modifies the axial action spectrum. Second, the axial visual pigment density is much higher than the transverse density. Thus, relative to the transverse spectral-sensitivity measurements, the axial spectral sensitivity is broadened by self-screening. MSP suggests a specific density in the macaque of 0.015 +/- 0.004 μm-1 for the M-cones and 0.013 +/- 0.002 μm-1 for the L-cones (Bowmaker et al., 1978). If one assumes a foveal cone outer segment length of 35 μm (see Outer segments of receptors), these values give axial photopigment densities of approximately 0.5 (Bowmaker & Dartnall, 1980).
Bowmaker, J.K., Dartnall, H.J.A., Lythgoe, J.N. & Mollon, J.D. (1978). The visual pigments of rods and cones in the rhesus monkey Macaca mulatta. Journal of Physiology, London, 274, 329-348.
Bowmaker, J.K. & Dartnall, H.J.A. (1980). Visual pigments of rods and cones in a human retina. Journal of Physiology, London, 298, 501-511.
Burns, S.A., Elsner, A.E., Lobes, L.A. Jr. & Doft, B.H. (1987). A psychophysical technique for measuring cone photopigment bleaching. Investigative Ophthalmology & Visual Science 28, 711-717.
Elsner, A.E., Burns, S.A. & Webb, R.H. (1993). Mapping cone photopigment optical density. Journal of the Optical Society of America A 10, 52-58.
Kilbride, Read, Fishman & Fishman (1983). Determination of human cone pigment density difference spectra in spatially resolved regions of the fovea. Vision Research 23, 1341-1350.
Stockman, A., MacLeod, D.IA. & Johnson, N.E. (1993). Spectral sensitivities of the human cones. Journal of the Optical Society of America A, 10, 2491-2521.