The distribution of human rod and cone photoreceptors is known for only a few retinae. This is unfortunate because the large variability between eyes, which is influenced by the age of the donor eye and the size of the counting field, complicates quantitative analysis. In the fovea, there may be as much as a threefold range in maximum cone density of young adult human eyes (Curcio et al., 1987) (see Peak foveal cone densities). Much less variability between observers, however, is found for peripheral cones and rods.
Despite the scant material, what is clear is that cone distribution varies greatly with retinal eccentricity (see Cone numbers and densities). The fovea contains the highest density of cone photoreceptors in the retina (199,000/mm2); the numbers drop to about 50% (100,000/mm2) by 500 μm (c. 1.75 deg) from the fovea centre and to less than 5% (<10,000/mm2) at about 4 mm (c. 20 deg) eccentricity (Curcio et al., 1991). Cone density may increase slightly in the far nasal retina, however (Curcio et al., 1990).
Cone distribution is also radially asymmetrical about the fovea. Higher cone densities have been reported in the nasal and superior retinal quadrants (Østerberg, 1935; Curcio et al., 1987).
The distribution of the S-cones differs from that of the L- and M-cones. There is a central area in which S-cones are missing; the diameter of which is c. 100 μm or 0.35 deg. (Estimates vary slightly, see Cone numbers and densities.) This accords with much prior psychophysical evidence that the centre of the human fovea is tritanopic for very small objects ( Koenig, 1894; Willmer, 1944; Willmer & Wright, 1945; Wald, 1967; Williams et al., 1981a; Castano & Sperling, 1982). A good psychophysical estimate, which takes into account the blur introduced by ocular optics and eye movements, is c. 118 μm or 0.42 deg (Williams et al., 1981a,b).
The highest density for S-cones -- >2000 cones/mm2 (see Curcio et al., 1991) or 10 - 15% of the total (see Ahnelt et al., 1987) -- is found in a ring at about 0.1-0.3 mm eccentricity (see also DeMonasterio et al., 1985). This also accords with psychophysical evidence that the maximum of blue sensitivity is at about 1 deg (280-300 μm) (Brindley, 1954; Stiles, 1949; Wald, 1964; Wooten & Wald, 1973; Castano & Sperling, 1982). In the peripheral retina (10 - 20 deg eccentricity), 7-8% of the cones may be S-cones (Ahnelt et al., 1987).
The S-cone mosaic appears to be formed by a separate process than those forming the M- and L-cone mosaics. The S-cones are nonrandomly, but irregularly distributed (Williams et al., 1981; Ahnelt et al., 1987; Nork et al., 1990; Curcio et al., 1991); whereas the L- and M-cones appear to be randomly distributed with the numbers of each differing. Psychophysical estimates of the relative numbers of L- and M-cones suggest, on average, that there are twice as many L- as M-cones in the central fovea. However, individual ratios are highly variable between observers, ranging from 0.33:1 to 10:1 (see Topographic distributions of L-and M-cones).
Rod density also varies with eccentricity. The central foveola, c. 350 μm or 1.25 deg in diameter (see Summary table of human rod numbers and densities), is free of rod photoreceptors (Curcio et al., 1990). Ahnelt et al. (1987) report that the first rods are seen at 125 μm from the centre. Foveal rod density increases most rapidly superiorly and least rapidly nasally. The highest rod densities are located along an elliptical ring at the eccentricity of the optic disk (c. 4 min or 20 deg from the foveal centre) and extending into nasal retina. The ring has a hot spot of peak rod density which is c. 176,000/mm2 (Curcio et al., 1990) Typically but not always, it lies 3-5 mm from the fovea on or near the vertical meridian in superior retina. As the rod-ring crosses the horizontal meridian, rod density drops 15-25% (Curcio et al., 1990).
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