Perceptual abnormalities in Parkinson's disease: top-down or bottom-up processes?

A wide variety of perceptual impairments have been reported in patients with Parkinson's disease (PD) in recent years; the underlying causes of these impairments have been variously attributed to different levels of the visual-cognitive system, from the retina to frontal cortex. Parkinsonian perceptual abnormalities could thus be interpreted as indirectly caused either by 'bottom-up' effects, stemming from dopaminergic dysfunction in the retina, or by 'top-down' effects, stemming from deficits in attention due to disturbances in the striatal-frontal system. Alternatively, a direct visuospatial impairment, perhaps related to the motor symptoms, has been considered. Data on three basic aspects of visual perception (3-D stereo vision, figure-ground discrimination, and pattern perception) which might be expected to give difficulty to PD subjects suffering changes in early processing mechanisms are reported. Visual complexity and the degree of mental manipulation of the material required both varied in different parts of the tests. PD patients were on stable drug regimens. It was found that disease severity interacted with performance: patients with mild PD showed little perceptual abnormality, patients with moderate PD showed only top-down effects, and patients with severe PD showed evidence both of top-down and of bottom-up deficits. Thus it appears that any retinal effects on perception in PD occur only in the advanced stages of the disease; in earlier stages any visual dysfunction probably reflects top-down disturbances from higher levels of the cognitive-behavioural system.