Photopigment optical density of the human foveola and a paradoxical senescent increase outside the fovea

Abstract

Photopigment optical density (OD) of middle-(M) and long-(L) wavelength-sensitive cones was determined to evaluate the hypothesis that reductions in the amount of photopigment are responsible for age-dependent sensitivity losses of the human cone pathways. Flicker thresholds were measured at the peak and tail of the photoreceptor's absorption spectrum as a function of the intensity of a bleaching background. Photopigment OD was measured at 0 (fovea), 2, 4, and 8 deg in the temporal retina by use of a 0.3-deg-diameter test spot. Seventy-two genetically characterized dichromats were studied so that the L-and M-cones could be analyzed separately. Subjects included 28 protanopes with M-but no L-cones and 44 deuteranopes with L-but no M-cones (all male, age range 12-29 and 55-83 years). Previous methods have not provided estimates of photopigment OD for separate cone classes in the foveola. In this study, it was found that foveolar cones are remarkably efficient, absorbing 78% of the available photons (OD ϭ 0.65). Photopigment OD decreased exponentially with retinal eccentricity independently of age and cone type. Paradoxically, the OD of perifoveal cones increased significantly with age. Over the 70-year age range of our participants, the perifoveal M-and L-cones showed a 14% increase in capacity to absorb photons despite a 30% decrease in visual sensitivity over the same period.

Human trichromacy revisited

“…The vector length of the estimated cone absorptions does not decrease when we reduce the photopigment density, as would be expected in the periphery ( Fig. 5C) (28,29). We performed a systematic search and could find no plausible pigment parameters to align the predicted invisible stimulus from the model with the cone-silent direction.…”

“…Photopigment density variations. Estimates of photopigment optical density in the periphery differ (28,29), but there is agreement that beyond 10°the optical density varies slowly and stays within the range of 0.25-0.30. Optical density differences change the spectral absorption of the photopigments; hence, such variation might provide an additional source of information.…”

Discrimination thresholds of normal and anomalous trichromats: Model of senescent changes in ocular media density on the Cambridge Colour Test

“…Participants in this study were recruited for a number of different studies over the past 10 years in our laboratory. One study explicitly recruited participants with congenital color vision deficiencies [30], hence the high percentage of color vision deficient observers in this paper. A certified ophthalmic technician excluded participants with best-corrected visual acuity (BCVA) worse than 20/25, intraocular pressure >22 mm Hg, abnormal pupil exam, abnormal ocular motility, and abnormal visual fields (assessed with a confrontational visual field test).…”

Age-Related Macular Degeneration: A Complementopathy?

“…The human fovea almost exclusively contains cones, and the lipoprotein content of these photoreceptors is higher than that of the rods, which may also add to the higher lipoprotein deposition under the RPE layer of the fovea. On the other hand, the cone outer segments in the fovea are longer than at the periphery, and the amount of visual pigment in the fovea is higher, offering protection against light [16]. The cellular debris under the RPE cells is thought to activate the complement system, leading to the damaging of the RPE cells and further malfunction of this cell layer, propagating a vicious cycle of local deterioration [17].…”