There may be more to reaching than meets the eye: Re-thinking optic ataxia

Optic ataxia (OA) is generally thought of as a disorder of visually guided reaching movements that cannot be explained by any simple deficit in visual or motor processing. In this paper we offer a new perspective on optic ataxia; we argue that the popular characterisation of this disorder is misleading and is unrepresentative of the pattern of reaching errors typically observed in OA patients. We begin our paper by reviewing recent neurophysiological, neuropsychological, and functional brain imaging studies that have led to the proposal that the medial parietal cortex in the vicinity of the parietal-occipital junction (POJ) - the key anatomical site associated with OA - represents reaching movements in eye-centred coordinates, and that this ability is impaired in optic ataxia. Our perspective stresses the importance of the POJ and superior parietal regions of the human PPC for representing reaching movements in both extrinsic (eye-centred) and intrinsic (postural) coordinates, and proposes that it is the ability to simultaneously represent multiple spatial locations that must be directly compared with one another that is impaired in non-foveal OA patients. In support of this idea we review recent fMRI and behavioural studies conducted by our group that have investigated the anatomical correlates of posturally guided movements, and the movements guided by postural cues in patients presenting with optic ataxia.