MERGING OF OCULOMOTOR AND SOMATOMOTOR SPACE CODING IN THE HUMAN RIGHTPRECENTRAL GYRUS

Using PET and (H2O)-O-15, we investigated the cortical areas that merg e two different ways of coding space in the cerebral cortex, those con cerned with the oculomotor and the somatomotor space. Normal subjects performed a visuomotor task that required the spatial coding of visual stimuli in oculomotor space and of motor responses in somatomotor spa ce. We manipulated the mapping of oculomotor and somatomotor space by instructing subjects to respond in half of the PET scans with uncrosse d hands, i.e. each hand was in the homonymous hemispace (standard ocul omotor-somatomotor mapping), and in the other half with crossed hands, i.e. with the left hand in the right hemispace and the right hand in the left hemispace (nonstandard oculomotor-somatomotor mapping). React ion times were slower for crossed hands than uncrossed hands. Crossed hands produced increases in blood flow in the precentral and postcentr al gyri of the right hemisphere. Increases in blood flow in the precen tral gyrus were correlated with increases in reaction time comparing t he crossed-hand task with the uncrossed one, whereas rite increases in blood flow in the postcentral gyrus were not. These findings suggest that the right precentral gyrus merges oculomotor and somatomotor spac e coding in the human brain.