A visuomotor disorder in the absence of movement: does optic ataxia generalize to learned isometric hand action?

Visuomotor deficits in parietal patients suffering from Optic Ataxia (OA) have been so far studied during natural reaching movements. We aimed at understanding if these disorders are also present when more abstract visuomotor transformations are involved. A patient with unilateral OA was tested during both standard reaches and isometric actions, therefore in the absence of hand displacement. Isometric action was affected similarly to standard reaches, with endpoint errors to visual targets that were found in both central and peripheral vision. The dissociation of perceptual and motor components of errors highlighted the existence of field, hand and hemispace effects, which depended on the type of error investigated. A generalization of the reaching disorder to learned isometric conditions would suggest that lesions of posterior parietal cortex (PPC) affect sensory-motor transformations not only for standard reaches, but also when visual signals need to be aligned with information from hand force receptors, therefore regardless of the specific remapping required to generate the directional motor output. The isometric impairment emerged with high and similar severity regardless of whether targets were in central or peripheral vision. Since under all isometric conditions gaze and hand position were decoupled, the spatial correspondence between the hand and the gaze seems to play a critical role in this syndrome. This indicates that regardless of the action to be performed and the specific remapping required, there exists in PPC an abstract representation of the directional motor output, where the computation of eye-hand alignment by parietal neurons plays a crucial role.