Primary Visual and Visuocognitive Deficits (2012)

Visual abnormalities in individuals with Parkinson’s disease (PD) are unlikely to be uncovered during routine neurological examination. However, more sophisticated electrophysiological testing using techniques, such as the visual-evoked potential (VEP) and pattern electroretinogram (PERG), confirm the presence of visual system involvement in PD. Clinical observations, electrophysiological, and anatomical studies provide functional evidence that foveal vision is predominantly affected in PD. In vivo retinal imaging, using optical coherence tomography, reveals that the foveal retina is thinned in PD patients. In this chapter, we summarize the evidence of retinal impairment and its potential relevance to higher cognitive visual and visuomotor impairment. Although the retina is the earliest, it is not the only site of visual pathology in PD. Foveal visual processing has a preferential role beyond the retina, beyond simple visual detection. Foveal signals are of crucial importance in visual categorization and, importantly, in visuospatial attention. More complex visuocognitive difficulties, e.g., impairment of consciously controlled visual information processing, have also been identified in PD. Some of these higher visual functions are affected in this disorder and one may ask if retinal foveal dysfunction contributes to visuospatial ­attention and other higher order visual impairment in PD. The exact relationship of foveal dopaminergic retinal deficits and visuocognitive impairment is not known in any detail; thus, the effect of foveal visual impairment on higher visual functions poses an exciting and challenging research question. Visual event-related potentials (ERPs) in PD patients, obtained using foveal visual stimuli, demonstrate a delay in the P300 component, beyond a delay in the primary visual responses, and suggest slowness of visual information processing. Whereas some investigators have noted this abnormality only in demented patients with PD, others have indicated its presence in both demented and nondemented individuals. The visuospatial sketchpad, a component of the working memory system, shows a specific selective impairment in PD, and visual categorization deficits (suggesting involvement of the posterior part of the cortex) have also been found. Concurrent electrophysiological recordings of primary and visuocognitive responses reveal that the impairment of higher order visual processing in PD is not simply a consequence of retinal dopaminergic deficiency. Electrophysiological, neuropsychological, and functional neuroimaging data imply that both frontal and posterior cortico-subcortical circuits may be involved. Foveal processing and visuospatial attention possibly may be linked through processes that involve saccadic eye movements. Saccadic eye movements are freely executed many thousand times a day by healthy observers. Part of their role is to “bring” eccentric targets to the direct sight line for closer scrutiny by foveal processing. This is called foveation: with functional imaging (fMRI), it was shown that the distributed centers of cortical control of saccades are disrupted in PD. It is even suggested that in the early stages of PD, patients have changes in visual input that impair their postural control.