Transcranial magnetic stimulation during positron emission tomography: a new method for studying connectivity of the human cerebral cortex.

We describe a new technique permitting the mapping of neural connections in the living human brain. The method combines two well established tools of brain research: transcranial magnetic stimulation (TMS) and positron emission tomography (PET). We use TMS to stimulate directly a selected cortical area while simultaneously measuring changes in brain activity, indexed by cerebral blood flow (CBF), with PET. The exact location of the stimulation site is achieved by means of frameless stereotaxy. In the first study using this technique, we found significant positive correlations between CBF and the number of TMS pulse trains at the stimulation site, namely the left frontal eye field (FEF) and, most importantly, in the visual cortex of the superior parietal and medial parieto-occipital regions. The pattern of these distal effects was consistent with the known anatomic connectivity of the monkey FEF. We suggest that the combined TMS/PET technique offers an objective tool for assessing the state of functional connectivity without requiring the subject to engage in any specific behavior.