Anatomical physiology of spatial extinction.

Neurologically intact volunteers participated in a functional magnetic resonance imaging experiment that simulated the unilateral (focal) and bilateral (global) stimulations used to elicit extinction in patients with hemispatial neglect. In peristriate areas, attentional modulations were selectively sensitive to contralaterally directed attention. A higher level of mapping was observed in the intraparietal sulcus (IPS), inferior parietal lobule (IPL), and inferior frontal gyrus (IFG). In these areas, there was no distinction between contralateral and ipsilateral focal attention, and the need to distribute attention globally led to greater activity than either focal condition. These physiological characteristics were symmetrically distributed in the IPS and IFG, suggesting that the effects of unilateral lesions in these 2 areas can be compensated by the contralateral hemisphere. In the IPL, the greater activation by the bilateral attentional mode was seen only in the right hemisphere. Its contralateral counterpart displayed equivalent activations when attention was distributed to the right, to the left, or bilaterally. Within the context of this experiment, the IPL of the right hemisphere emerged as the one area where unilateral lesions can cause the most uncompensated and selective impairment of global attention (without interfering with unilateral attention to either side), giving rise to the phenomenon of extinction.