Parafoveal function is important for daily visual tasks such as reading. Here the variability in cone density along the four cardinal meridians in parafoveal regions of the retina was investigated in vivo using an adaptive optics fundus camera. Ten healthy normal trichromatic individuals were included in the study. There were significant differences in cone density between individuals at all four tested eccentricities (0.5, 1, 2 and 3°) and meridians. Cone density ranged from 34,900 to 63,000 cones/mm2 at 1° horizontally, and from 31,600 to 60,700 at 1° vertically. The results were consistent with those of Curcio et al. (1990), although between-individual variability is greater than previously reported in the parafovea from 1 to 3.2°.

Findings are consistent with the visual deficits being caused by a reduced number of healthy cones in the two brothers and the adult female. In the unrelated adult subject, no structural basis for the disorder was found. These data suggest two distinct groups on the basis of structural imaging. It is proposed that the former group with evidence of a reduction in cone numbers is more in keeping with typical OT, with the latter group representing an OT-like phenotype. These two groups may be difficult to readily discern on the basis of phenotypic features alone, and high-resolution imaging may be an effective way to distinguish between these phenotypes.

Five mutations in the S-cone-opsin gene (OPN1SW) that give rise to different single amino-acid substitutions (L56P, G79R, S214P, P264S, R283Q) are known to be associated with tritan color-vision deficiency. Here we report a sixth OPN1SW mutation (T190I) and the associated color vision phenotype. S-opsin genotyping and clinical evaluation of color vision were performed on affected and unaffected family members and normal controls. Chromatic contrast was tested at different levels of retinal illuminance. Affected family members were heterozygous for a nucleotide change that substituted the amino acid isoleucine (I) in place of threonine (T) that is normally present at position 190 of the S-opsin. The mutation is in extracellular loop II (EII). The association between making tritan errors and having the T190I mutant S opsin was strong (p > 0.0001: Fisher's exact test). The performance of subjects with the T190I mutation was significantly different from that of normal trichromats along the tritan vector under all conditions tested (Mann-Whitney U: p < 0.05), but not along the protan or deutan vectors. Individuals with the T190I S-opsin mutation behaved as mild tritans at 12.3–92.3 Td, but as tritanopes at 1.2–9.2 Td, for both light-adapted and dark-adapted conditions. The results are consistent with the mutant opsin causing abnormal S-cone function.

Chromatic contrast sensitivity may be a more sensitive measure of an individual's visual function than achromatic contrast sensitivity. Here, the first aim was to quantify individual- and age-related variations in chromatic contrast sensitivity to a range of spatial frequencies for stimuli along two complementary directions in color space. The second aim was to examine whether polymorphisms at specific amino acid residues of the L- and M-opsin genes (OPN1LW and OPN1MW) known to affect spectral tuning of the photoreceptors could influence spatio-chromatic contrast sensitivity. Chromatic contrast sensitivity functions were measured in 50 healthy individuals (20-71 years) employing a novel pseudo-isochromatic grating stimulus. The spatio-chromatic contrast sensitivity functions were found to be low pass for all subjects, independent of age and color vision. The results revealed a senescent decline in spatio-chromatic contrast sensitivity. There were considerable between-individual differences in sensitivity within each age decade for individuals 49 years old or younger, and age did not predict sensitivity for these age decades alone. Forty-six subjects (including a color deficient male and eight female carriers) were genotyped for L- and M-opsin genes. The Ser180Ala polymorphisms on the L-opsin gene were found to influence the subject's color discrimination and their sensitivity to spatio-chromatic patterns. The results expose the significant role of neural and genetic factors in the deterioration of visual function with increasing age.