Enrichment of MP can enhance visual performance
The field of ophthalmology is concerned with the detection and treatment of eye disease, and alleviation or amelioration of the pathology's impact on vision. However, there is a fundamental and unmet need to recognize the considerable and underappreciated variability in visual performance and experience in healthy patients with no evidence of eye disease. In this context, visual performance, reflected in the subject's visual experience, can be assessed with a variety of techniques, and should not be restricted to standard and typical visual acuity (VA) testing.
Measures of visual performance
VA is a useful tool for testing the resolving power of the eye. It involves the presentation of a target that has a substantially different luminance to its background. The subject's task is to read the line of the smallest letters visible to him/her. However, VA's limitations rest primarily on the fact that testing is performed under conditions of 100% contrast, and therefore does not reflect the subject's function in relation to vision because so many other parameters of the visual experience are not being tested.
Contrast sensitivity (CS) refers to the ability of the visual system to distinguish an object from its background.1 High contrast involves the presentation of two visual stimuli of substantially different luminance, whereas low contrast involves the presentation of two visual stimuli of comparable luminance. CS testing determines the lowest level of contrast required to detect the target against its background.
CS is adversely affected by increasing age, cataracts, diabetic maculopathy and age-related macular degeneration (AMD), even in cases where measures of VA are unaffected. Measures of CS have been shown to better represent the impact of eye disease on a subject's visual function than do measures of VA.2 This is unsurprising, given that the real world visual experience is not confined to a high contrast environment.
Importantly, and especially in the context of this review, CS is adversely affected by chromatic aberration (CA) to a greater extent than is VA.3
Glare refers to a reduction in visual performance or a sense of discomfort because of a relatively bright light source within the field of view.4 Clinically, there are two types of glare, and these can be classed as glare discomfort and glare disability. Glare discomfort refers to an unpleasant sensation one experiences when subjected to illumination that is too bright, for example, full beam headlights from an approaching vehicle when driving. Glare disability, however, refers to a reduction in one's ability to perceive visual information (without necessarily causing discomfort) because of a relatively bright light source in the field of vision.5 CS is adversely affected by glare disability, and this deleterious impact on CS is attributable to veiling luminance (which in turn is caused by light scattering), ultimately adversely impacting the visibility of objects in one's field of view.