Functional dissociation of saccade and hand reaching control with bilateral lesions of the medial wall of the intraparietal sulcus: implications for optic ataxia.

The posterior parietal cortex (PPC) is essential for the integration of visuomotor information during visually guided reaching. Studies in macaque monkeys have demonstrated a functional specialisation around the intraparietal sulcus (IPS) with a more medial representation of hand movements ("parietal reach region") and a more lateral representation of saccadic eye movements (lateral intraparietal area, LIP). Here we present evidence for the validity of this concept with respect to the human parietal cortex. We recorded isolated and combined goal-directed eye-hand movements in normal control subjects and in a patient with bilateral parieto-occipital lesions and incomplete Balint's syndrome including severe optic ataxia (misreaching to visual targets). Brain lesions in the patient were caused by acute posterior leucoencephalopathy in association with aortic surgery because of Takayasu's arteritis. MRI scans showed bilateral line-shaped hemorrhagic lesions, restricted to the cortex at the medial banks of the intraparietal sulcus, but leaving its lateral banks largely intact. In the patient visually guided reaching was significantly dysmetric, whereas the metrics of visually guided saccades were within normal limits. Dysmetria was more pronounced for the right visual field, with a gross hypermetria. Variability of the movement improved when a delay of 5 or 10 s was introduced between target presentation and movement execution. Lesion data support the concept of a functional specialisation around the human IPS: The cortex medial to the IPS predominantly controls rapid goal-directed reaching movements, comparable to the parietal reach region in monkeys, whereas saccadic eye movements appear to be controlled rather by the cortex lateral to the IPS.