A visuomotor disorder in absence of movement: Optic ataxia generalizes to learnt isometric hand action

It is well known that parietal lobe lesions often result in Optic Ataxia (OA), a disorder in which patients make inaccurate reaches to visual targets. However, it is not clear if lesions of the cortical areas involved in sensorimotor transformation, such as PPC, result in similar reach defects regardless of the type of the sensory signals to be aligned, therefore of the specific remapping required. We asked a patient with OA from a unilateral tumor lesion of the right superior parietal lobule (SPL) and a group of healthy age-matched control subjects to perform a conventional center-out task involving either natural reaching movements toward targets over a touchscreen, or a learnt isometric hand action as to move a visual cursor toward the same target by controlling the force applied on a isometric joystick. Both tasks were performed under two conditions, in the first subjects were allowed to move the eye to the visual targets, therefore hand movement was performed when targets were in central vision; in the second, subjects were requested to maintain fixation on a central target while reaching to eccentric ones. Learnt isometric action was affected similarly to natural reaches in both central and peripheral vision, with abnormal endpoint errors and spatial dispersion of hand-guided cursor trajectories toward visual targets. Perceptual and motor components of hand errors were dissociated showing that OA consists of both spatial and motor components, since a field effect emerged in the process of target localization, in addition to a hand effect observed only when considering the motor components of OA. This suggests that lesion of posterior parietal cortex affects sensory-motor transformations not only when they require a natural displacement of the hand, but also after learning a task in which visual signals about target location need to be aligned with information from force receptors, therefore regardless of the specific remapping required.