SOMATOSENSORY HOMUNCULUS AS DRAWN BY MEG

We studied a detailed somatosensory representation map of the human pr imary somatosensory cortex using magnetoencephalography. Somatosensory -evoked magnetic fields following tactile stimulation of multiple poin ts in the right hemibody (including the tongue, lips, fingers, arm, tr unk, leg, and foot) were analyzed in five normal subjects. We were abl e to estimate equivalent current dipoles (ECDs) following stimulation of the tongue, lips, fingers, palm, forearm, elbow, upper arm, and toe s in most subjects and those following the stimulation of the chest, a nkle, and thigh in one subject. The ECDs were located in the postcentr al gyrus and generally arranged in order along the central sulcus, whi ch is compatible with the somatosensory ''homunculus.'' Linear distanc es, averaged in five subjects, from the receptive area of the thumb to that of the tongue, little finger, forearm, upper arm, and toes were estimated to be 2.42 +/- 0.28, 1.25 +/- 0.28, 2.21 +/- 0.72, 2.75 +/- 0.63, and 5.29 +/- 0.48 cm, respectively. The moment of each ECD, whic h suggested the size of the cortical areas responsive to the stimulati on, was also compatible with the bizarre proportion of the homunculus with a large tongue, lips, and fingers. According to these results, we were able to reproduce a large part of the somatosensory homunculus q uantitatively on an individual brain MRI. (C) 1998 Academic Press.