Loss of visual function is both a prognostic and the worst outcome of visual disease. This is particularly true for conditions that affect the crystalline lens and retina (like age-related cataracts [ARC] and macular degeneration [AMD]). Because treating the underlying disease is often so difficult (especially for conditions like AMD), the approach is often palliative (e.g., correcting refractive errors or magnification). The use of prescription filters, for instance, is becoming increasingly common. These filters are largely aimed at reducing glare by absorbing short-wave light.
1 This is needed because disability due to glare is a common problem for patients with even very early signs of cataract (due to increases in media scattering) and AMD. Patients with early or more severe stages of AMD, for instance, tend to recover from a photostressor 6 to 16 times more slowly, respectively, than age-matched controls.
2 For example, Sandberg and Gaudio
3 showed that when subjects with maculopathy are exposed to bright bleaching lights, visual recovery is significantly slowed despite having normal visual acuity. Disability due to glare results from increased forward light scatter originating in the cornea and lens.
4 Lens irregularities increase with age and incipient cataract.
5
High intraocular scatter may create additional problems. For example, chromatic discrimination can be reduced due to bright light desaturating colors.
6 Color enhances the coding of images at the input stage by facilitating the detection of borders.
7 Isoluminant edges (i.e., edges defined only by chromatic differences) are common in natural scenes
8 and when viewing objects at a distance, because the distance itself tends to equalize differences in luminance that would otherwise have demarcated an edge if the object was closer. Hence, glare disability (GD), photostress recovery (PR), and chromatic contrast (CC) are all measures of visual performance that worsen with increased intraocular scatter, age, and ocular disease.
These visual variables are also united in their tendency to be strongly influenced by short-wave light, as originally noted by Walls and Judd in 1933.
9 It is likely, for instance, that many sources of glare in the environment are broad band (e.g., sunlight, as shown in
Fig. 1) and contain a preponderance of Rayleigh-scattered (blue) light. Such light can degrade vision at a distance (visibility) due to the veiling effects of “blue haze.”
10,11 The pernicious aspects of blue light on vision formed the basis of Walls and Judd's
9 original speculation that the ubiquity of blue-absorbing intraocular filters across species was a response to this natural evolutionary pressure. These authors noted that, based purely on filtering short-wave light, yellow intraocular filters would reduce glare discomfort and “dazzle,” enhance CC (see also Mollon and Regan
12 ), and extend visible range by absorbing blue haze. Like many other species, humans also possess an intraocular blue-absorbing filter in the form of macular pigment (MP). These pigments, a mixture of yellow carotenoids (lutein, zeaxanthin, and meso-zeaxanthin) are concentrated in the inner layers of the retina in and around the foveal depression. Because they are derived from the diet and influenced by numerous personal variables (e.g., sex, iris color, adiposity, tobacco usage
13), their concentrations across individuals vary widely (greater than a factor of 10).
Past data have suggested that there are visual consequences for having reduced MP levels, even for young individuals. For example, empirical data have shown that increased MP density reduces glare discomfort,
14–16 GD, speeds PR,
17–19 and enhances CC.
20 Such relations make sense; the pigments are concentrated in the inner layers of the central retina where they strongly screen foveal cones but only minimally screen the more light-sensitive rods. Unlike external yellow filters, the visual system corrects for MP filtering of blue light by adjusting the gain of the S-cone system
21,22 ; hence, visual performance is less likely to be reduced due to reductions in luminance as they would with external yellow filters. In this study, we further explore the link between MP density and visual performance. This was done by measuring MP and GD, as well as PR and CC in a relatively large sample of young healthy subjects.