The influence of spatial coordinates in a case of an optic ataxia like syndrome following cerebellar and thalamic lesion

We report the case of a patient (S. C.) who, after a lesion involving the midbrain, thalamus, and cerebellum on the left side, was unable to reach a target at which he was not directly gazing. When attempting to do so with either hand, he showed an optic ataxia-like behaviour: A rightward deviation was manifest with respect to the real position of the target object, accuracy being worse for the right than for the left hand and for the right than for the left hemifield. To assess whether S.C.'s reaching accuracy was affected by the position of the target with respect to the retina, eyes, and/or head, he was asked to perform a pointing task in different conditions. By manipulating eyes and head position, the relative location of the target with respect to these body parts was altered. Further, to verify the relevance of the visual feedback for the accuracy of the reaching responses, the task was also performed either with or without vision of the performing hand (closed- and open-loop conditions). The results showed that the patient's reaching impairment reflects a deficit in spatio-motor coding mainly within head-centred coordinates. The lack of visual feedback about the hand spatial location worsened the reaching performance. This new pattern of reaching deficits is discussed in relation to the theoretical framework of both parietal optic ataxia and cerebellar ataxia.