Spatio-motor representations in reaching: Evidence for subtypes of optic ataxia

Optic ataxia, a disorder of visually guided reaching, has been attributed to a failure in the transmission of visuospatial information to motor systems. This account fails to explain why many patients with optic ataxia misreach only when targets are presented in non-foveal vision, but not when targets are viewed directly. We present data from two patients with unusually severe optic ataxia that suggest an alternative account of this relatively common pattern of misreaching. DP is utterly unable to reach to targets unless he gazes directly at them. We provide evidence that he is unable to program reaches in appropriate shoulder and hand-centred spatio-motor coordinates, but instead reaches using oculocentric coding. The gaze (and visual attention) of AM is frequently "captured" by his own hand, and his attentional allocation is influenced by the location at which he performs a hand movement. This suggests that he programs eye movements within hand-centred spatial coordinate frames. Taken together, these data suggest that residual reaching in optic ataxia may be enabled by linkage of the systems coding the spatio-motor actions of the eye and hand.