Errors in the visual perception of size in left hemi‐Parkinson's disease

Abstract

Purpose: Although Parkinson's disease (PD) has been considered to be primarily a motor disorder, there is increasing evidence for perceptual abnormality in the illness, which might impair the visual control of movements. We investigated how the perceived size of visual stimuli in PD depends on their position in external space, as well as on the involvement of the right hemisphere.Methods: Patients with mild/moderate idiopathic PD were allocated to a primarily left‐ or a primarily right‐sided group (LPD, RPD; n = 7 or 8 in different experiments) from the severity of tremor, rigidity, and akinesia in the left and right limbs. The ratio of severity of symptoms between the worse and least affected side averaged more than 3 : 1. In a method of constant stimuli, observers judged whether the width or height of a (variable) rectangle in one region of space was greater than that of a (standard) rectangle in another region. Psychometric functions were fitted to each observer's data by probit analysis, from which the mean (point of subjective equality) and slope were obtained.Results: In LPD (but not RPD or controls) rectangles in left space had to be wider than identical rectangles in right space, and rectangles in right space narrower than identical rectangles in left space, to be perceived as of equal width. Similarly, a rectangle in lower space had to be shorter than an identical rectangle in upper space to be perceived as of equal height, whether the rectangles were positioned in right or in left space.Conclusions: The results are consistent with a perceptual compression of objects in left and upper space in LPD (but not in RPD), implicating right‐hemisphere dopaminergic mechanisms in the visual perception of size. The effects are small (about 4%) but may be important in the visual control of precise movements in LPD.