Visual dysfunction in Alzheimer's disease: relation to normal aging.

In patients with Alzheimer's disease (AD), compared with age-matched and young healthy control subjects, visual deficits in the following functions were observed: color, stereoacuity, contrast sensitivity, and backward masking (homogeneous and pattern). Critical flicker fusion thresholds were normal, relative to age-matched healthy subjects. For color, the majority of the errors were tritanomalous (blue axis). Color and stereoacuity deficits were unrelated to severity of dementia, in accordance with models of vision that describe these functions as modular rather than diffuse for cortical localization. Although contrast sensitivity was depressed throughout the frequency range in AD, more patients were impaired at low than at high spatial frequencies, contrasting with the observed normal aging pattern of high-frequency loss. Healthy elderly subjects showed depressed critical flicker fusion thresholds and reduced contrast sensitivity at high frequencies, relative to the young group; differences between these groups were not found for the other vision tests. A subset of the AD group received detailed neuro-ophthalmological examination, and no abnormalities were found. This finding, taken together with normal thresholds for critical flicker fusion, suggests that the widespread visual dysfunction reported here is more likely to be related to known pathological changes in primary visual and association cortex in AD than to changes in the retina or optic nerve.