Topographic distributions of L-and M-cones


The relative numbers of L- and M-cones in the inner fovea have been estimated by various psychophysical means (see Psychophysical estimates of the relative numbers of L- and M-cones). Taken together, the mean ratios yielded by these methods (with the exception of spectral acuity) tend to suggest that there are twice as many L- as M-cones. However, it should be pointed out that each method has serious problems of interpretation and each ultimately depends on the reliability of cone isolation procedures. Moreover, although the mean estimates of L- to M-cone ratios suggest that there are twice as many L- as M- cones in the central fovea, the individual ratios are highly variable between observers, ranging from 0.33:1 to 10:1. The largest variations occur in individual photometric functions and in individual red-green flicker ratios -- which are the ratio of flicker sensitivity for red and green lights.

The similar amino acid sequences of the L- and M- cones, so far have prevented the development of an anatomical staining method for differentiating between them in the human retina. However, a histochemical assay has been used in the baboon retina to estimate that there are fewer L- than M-cones, in a ratio of 1:2 (Marc & Sperling, 1977). And direct MSP measurements of intact patches of foveal retina in Old World (talapoin) monkeys have been used to demonstrate that the distribution of L- and M-cones is locally random and that the two cone types are present in almost equal numbers (Mollon & Bowmaker, 1992). In humans, MSP measurements of patches of foveal and parafoveal retina so far have not permitted any direct statements about the dependence of relative numbers on retinal location. However, the findings of 22 (59%) L- and 12 (32%) M- cones in 37 cone records (including 3 S-cones) from one eye (Bowmaker & Dartnall, 1980) and of 69 (53%) L- and 49 (38%) M-cones in 129 records from seven eyes (Dartnall et al., 1983) suggest a greater preponderance of L-cones. In comparison, in talapoin monkeys, 256 (46%) L- and 289 (52%) M-cones have been measured in 557 cone records from intact patches and from isolated receptors (Mollon & Bowmaker, 1992).


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