Reachability judgement in optic ataxia: Effect of peripheral vision on hand and target perception in depth

The concept of peripersonal space was first proposed by Rizzolatti, Scandolara, Matelli, and Gentilucci (1981), who introduced the term to highlight the close links between somatosensory and visual processing for stimuli close to the body and suggested that this near-body space could in fact be characterized as an action space (Rizzolatti, Fadiga, Fogassi, & Gallese, 1997). Supporting this idea, patients with right hemisphere lesions have been described as impaired in performing actions towards objects and in perceiving their location - but only when the objects were presented within arm's reach (Bartolo, Carlier, Hassaini, Martin, & Coello, 2014; Brain, 1941). Whether the deficit of optic ataxia patients in processing target locations for action has an effect on the representation of peripersonal space has never been explored. The present study highlights optic ataxia patients' specific difficulties in processing hand-to-target distances in a motor task and in a perceptual task requiring identification of what is reachable in the visual environment. The difficulties are especially evident when both the target and the hand are perceived in the visual periphery. Indeed, when patient I.G. was able to fixate the target, her reaching accuracy and her perception of reachable space both largely improved. Furthermore, the difficulties were enhanced when the hand and the target were both in the lower visual field (in a fixed-far condition vs a fixed-near condition). This novel up-down dimension of optic ataxia fits with the larger representation of the lower visual field in the posterior parietal cortex (Pitzalis et al., 2013; Previc, 1990).