A model of the Parkinsonian visual system: support for the dark adaptation hypothesis.

Considerable evidence suggests that some visual abnormalities in Parkinson's disease are mediated by disruption of dopaminergic processes in the retina. Since dopamine is thought to be involved in the process of dark adaptation, and some of these abnormalities are similar to the changes which accompany dark adaptation in normal subjects, it has been proposed that the parkinsonian retina behaves as though inappropriately dark-adapted. In Parkinson's disease, the apparent contrast of peripherally viewed medium and high spatial frequency gratings is reduced. In our first experiment, normal subjects were dark-adapted, and were required to match the apparent contrast of a peripherally viewed grating to that of a foveally viewed grating. The results showed an interaction between spatial frequency and dark adaptation, reflecting a greater reduction in the apparent contrast of peripheral high spatial frequency gratings. In a second experiment, no effect of dark adaptation was found on the apparent spatial frequency of a peripherally viewed grating required to match that of a foveally viewed grating. The first experiment supports the dark adaptation hypothesis of parkinsonian vision, and the second suggests that the changes in apparent contrast are mediated by different amounts of change in contrast gain in central and peripheral vision, rather than by differential changes in receptive field size.