Reaching a moveable visual target: Dissociations in brain tumour patients

Damage to the posterior parietal cortex (PPC) can lead to Optic Ataxia (OA), in which patients misreach to peripheral targets. Recent research suggested that the PPC might be involved not only in simple reaching tasks toward peripheral targets, but also in changing the hand movement trajectory in real time if the target moves. The present study investigated whether patients with a lesion arising from operation for prefrontal, premotor or parietal tumours are selectively impaired in three experimental pointing conditions: (i) pointing to peripheral targets, (ii) pointing to fixatable targets, and (iii) pointing to moved targets (on-line movement corrections). The study confirmed the selective importance of the parietal cortex in all three tasks. Surprisingly, given clinical claims about OA, the degree of peripheral reaching errors correlated strongly in parietal patients with that to fixatable targets. However, there was no relation between peripheral reaching errors and the ‘shift cost’ of making on-line correction to moved targets, and classical double dissociations between the two skills were observed. The findings suggest that deficits in pointing to peripheral and to moved targets reflect two at least partly independent processes.