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To execute goal-directed movements, such as reaching to pick up an object, information specified in extrinsic (spatial) coordinates must be transformed into a motor plan that is expressed within intrinsic (motor) coordinates. For reaching movements directed to visually defined targets, this will involve translating visual information that is initially coded in retinotopic coordinates into a motor plan that specifies the sequence of postural changes required to bring the hand to the target. Several lines of evidence point to the important role played by the posterior parietal cortex (PPC) in carrying out the sensorimotor transformations that are associated with goal-directed action. In humans, damage to the PPC, particularly bilateral lesions, leads to impairments in the planning and control of reaching movements directed towards visual targets, known as optic ataxia. Recent theoretical accounts of this disorder propose that it arises as a result of a failure within successive stages of the sensorimotor transformation process. In this chapter, we present evidence from two patients presenting with the most commonly encountered form of misreaching deficit- non-foveal optic ataxia-in support of a novel account. Namely, that a key feature of this form of optic ataxia may be a failure to successfully de-couple the eye and limb visuomotor systems so that each might undertake simultaneously independent actions. © 2005 Psychology Press.

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303 - 324