The attentional role of the left parietal cortex: The distinct lateralization and localization of motor attention in the human brain

It is widely agreed that visuospatial orienting attention depends on a netw ork of frontal and parietal areas in the right hemisphere. It is thought th at the visuospatial orienting role of the right parietal lobe is related to its role in the production of oven eye movements. The experiments reported here test the possibility that other parietal regions may be important for directing attention in relation to response modalities other than eye move ment. Specifically, we used positron emission tomography (PET) to test the hypothesis that a 'left' parietal area. the supramarginal gyrus, is importa nt for attention in relation to limb movements (Rushworth et al. 1997; Rush worth. Ellison. & Walsh. in press). We have referred to this process as 'mo tor attention' to distinguish it from orienting attention. In one condition subjects spent most of the scanning period covertly attending to 'left' ha nd movements that they were about to make. Activity in this first condition was compared with a second condition with identical stimuli and movement r esponses bur lacking motor attention periods. Comparison of the conditions revealed that motor attention-related activity was almost exclusively restr icted to the 'left' hemisphere despite the fact that subjects only ever mad e ipsilateral, left-hand responses. Left parietal activity was prominent in this comparison, within the parietal lobe the critical region for motor at tention was the supramarginal gyrus and the adjacent anterior intraparietal sulcus (AIP), a region anterior to the posterior parietal cortex identifie d with orienting attention. In a second part of the experiment we compared a condition in which subjects covertly rehearsed verbal responses with a co ndition in which they made verbal responses immediately without rehearsal. A comparison of the two conditions revealed verbal rehearsal-related activi ty in several anterior left hemisphere areas including Broca's area. The la ck of verbal rehearsal-related activity in the left supramarginal gyrus con firms that this area plays a direct role in motor attention that cannot be attributed to any strategy of verbal mediation. The results also provide ev idence concerning the importance of ventral premotor (PMv) and Broca's area in motor attention and language processes.