Fovea and foveation in Parkinson's disease.
Nondemented Parkinson's disease (PD) patients report problems on nonmotor tasks that depend on visual or visuospatial abilities. In PD, foveal vision is impaired. Experimental studies in humans and monkeys established that foveal processing and visuospatial attention may be linked through saccadic eye movements. Saccadic eye movements "bring" eccentric targets to the direct sight line for closer scrutiny by foveal processing. This is called foveation. This review musters the arguments for impaired foveal vision and impaired cortical control of voluntary saccades in PD. Retinal impairment of spatial contrast vision is selective in PD. Thus, the reviewed literature includes a brief survey of the physiology of foveal pathways. This is necessary to understand the specificity of the retinopathy of PD, documented by clinical evidence, relying on psychophysical, electrophysiological, and imaging techniques. These have recently been supplanted by the results of retinal imaging using Optical Coherence Tomography (OCT). Studies of cortical mechanisms in PD reviewed here relied on neuropsychological, electrophysiological (EEG), and imaging techniques. Impaired functional anatomy and electrophysiology in PD are reviewed. The exact relationship of retinal foveal deficits and visuospatial attention and postural control impairment in PD remain challenging research questions. This review will hopefully will provide useful material for future studies.