Can spatial resolution reveal individual differences in the L:M cone ratio?

We measured spatial resolution in the parafovea for targets designed to isolate either the long-wavelength (L) or the middle-wavelength (M) cones. Landolt C optotypes were presented for 100 ms on a calibrated monitor at an eccentricity of 5° to the left or right of fixation. There were large individual differences in the ratio of the resolution obtained with L targets to that obtained with M targets, and we suggest that these differences reflect variations in the relative sampling densities of L and M cones in the parafovea. In Experiment 1, we measured contrast thresholds for targets of varying size. Among 10 unselected observers, there was a threefold variation in the ratio of the contrast thresholds for the smallest targets. In Experiments 2 and 3, we held contrast constant and we varied size, in order to establish the minimal target that could be discriminated for each of the two classes of cone. In Experiment 2, two groups of observers, selected on the basis of their settings on a flicker-photometric test, showed a highly significant difference in the ratio of the M and L acuities on the spatial task. In Experiment 3, female carriers of protan or deutan deficiencies, classified only on the basis of their sons' phenotypes, also showed a large difference in the ratio of their acuities for M and L targets. In all three experiments, there was a strong correlation between the ratio of M and L spatial acuities and a flicker-photometric measure of relative sensitivity to long- and middle-wavelength light.