THE LEFT PARIETAL CORTEX AND MOTOR ATTENTION

The posterior parietal cortex, particularly in the right hemisphere, i s crucially important for covert orienting; lesions impair the ability to disengage the focus of covert orienting attention from one potenti al saccade target to another (Posner, M. I. el al., Journal of Neurosc ience, 1984, 4, 1863-1874). We have developed a task where precues all ow subjects to covertly prepare subsequent cued hand movements, as opp osed to an orienting or eye movement. We refer to this process as moto r attention to distinguish it from orienting attention. Nine subjects with lesions that included the left parietal cortex and nine subjects with lesions including the right parietal cortex were compared with co ntrol subjects on the task. The left hemisphere subjects showed the sa me ability as controls to engage attention to a movement when they wer e forewarned by a valid precue. The left hemisphere subjects, however, were impaired in their ability to disengage the focus of motor attent ion from one movement to another when the precue was incorrect. The re sults support the existence of two distinct attentional systems allied to the orienting and limb motor systems. Damage to either system caus es analogous problems in disengaging from one orienting/movement targe t to another. The left parietal cortex, particularly the supramarginal gyrus, is associated with motor attention. All the left hemisphere su bjects had ideomotor apraxia and had particular problems performing se quences of movements. We suggest that the well documented left hemisph ere and apraxic impairment in movement sequencing is the consequence o f a difficulty in shifting the focus of motor attention from one movem ent in a sequence to the next. (C) 1997 Elsevier Science Ltd.