Optic ataxia affects the lower limbs: evidence from a single case study.
Evans C., Milner AD., Humphreys GW., Cavina-Pratesi C.
Optic ataxia represents a spatial impairment of visually guided reaching following bilateral or unilateral damage to the posterior parietal cortex that is independent of purely motor or visual deficits. Research to date has focused on reaching actions performed with the upper limbs but has neglected to explore whether or not optic ataxia affects the lower limbs, that is, whether it is effector-specific. We asked patient M.H., who suffers from unilateral optic ataxia from left hemispheric damage, and eight age-matched controls, to perform leg movements by stepping down from a wooden block towards a visually presented target. Steps were performed using the left or the right leg, in conditions of central fixation or free viewing. Patient M.H. performed significantly worse than controls. His errors in step accuracy were most pronounced when stepping into the visual periphery (during central fixation), particularly while using the contralesional right foot towards the contralesional right hemispace. This behaviour is consistent with M.H.'s impairments in optic ataxia previously recorded for reaching and grasping actions with the upper limbs. The lesion affecting M.H.'s brain is quite large, encompassing functional areas associated with visuomotor transformations performed with different effectors such as arm and eye (superior parietal-occipital cortex and medial intraparietal sulcus). Our data suggest that optic ataxia is not completely effector-specific, and that neurons encoding visuomotor transformations for both arm and leg are probably both affected by the damage. Our results support the notion that lesions affecting the medial portion of the left posterior parietal cortex similarly affect different effectors (arm and leg) when visually guided actions are directed towards the same contralesional hemispace. In addition they may help explain why patients with optic ataxia have been reported to have difficulties in certain aspects of visually guided locomotion.