Absence of a common functional denominator of visual disturbances in cerebellar disease

Several studies have demonstrated disturbances of visual perception in pati ents suffering from cerebellar disease. In an attempt to determine the caus e of these visual disturbances and thereby the cerebellar contribution to v ision, we designed two sets of experiments in which we tested (i) the possi bility of a general magnocellular deficit in cerebellar disease and (ii) th e alternative possibility of impaired spatial attention underlying visual d isturbances in cerebellar patients. The first set of experiments consisted of a test of position discrimination, a parvocellular function and tests ta pping different aspects of motion perception including speed discrimination , direction discrimination and the ability to extract a coherent motion sig nal embedded in noise. The second set of experiments compared the performan ce on two different classes of texture discrimination. The first one requir ed fast and precise shifts of focal spatial attention ('serial search'), th e second one, testing preattentive texture discrimination ('pop-out') did n ot. In the first set of experiments cerebellar patients were impaired on th e position discrimination task as well as several, albeit not all, tests of motion perception. The pattern of disturbances obtained was neither compat ible with the notion of a selective magnocellular deficit nor the idea, ori ginally put forward by Ivry and Diener (J Cogn Neurosci 1991; 3: 355-56) th at visual deficits are secondary to an impaired measurement of time. In the second set of experiments, cerebellar patients showed normal performance o n pop-out tasks and normal performance on all variants of the serial search task except for the one requiring comparison of a single element presented with a sample of the target in short-term memory. In summary, our results support the existence of visual disturbances in cerebellar disease, but pro vide evidence against a common, simple denominator such as a timing deficit , deficient cerebellar modulation of magnocellular circuitry, deficits of s patial attention or visual working memory.