ACTIVITY IN POSTERIOR PARIETAL CORTEX FOLLOWING SOMATOSENSORY STIMULATION IN MAN - MAGNETOENCEPHALOGRAPHIC STUDY USING SPATIOTEMPORAL SOURCE ANALYSIS

We investigated the activation of posterior parietal cortex (PPC) to s omatosensory stimulation in humans to determine its fundamental role a s a somatosensory associated area using magnetoencephalography (MEG). We studied somatosensory evoked magnetic fields (SEF) after stimulatio n of median nerve, posterior tibial nerve and lip, and analyzed them b y the single dipole model and also by the multidipole model using brai n electric source analysis (BESA) system. In single source model analy sis, the dipole at the peak latency of short-latency components follow ing each site stimulation were located in the corresponding receptive fields in the primary somatosensory cortex (SI) contralateral to the s timulation. The dipole at the peak latency of the middle latency compo nents were located in bilateral upper bank of Sylvian fissure (SII). B y contrast, in the five-dipole model of BESA, the equivalent current d ipoles (ECDs) of the middle-latency SEF after stimulation of median ne rve and posterior tibial nerve were identified in the contralateral SI and in the bilateral SII and PPC, while all activities of middle-late ncy SEF after lip stimulation appeared to be restricted in the contral ateral SI and bilateral SII. Around 80 msec in latency, the ECD locati on in PPC after median nerve stimulation was, on the average, 2.4 cm p osterior, 2.9 cm medial and 2.6 cm superior to the hand area in SI. Th e ECD in PPC after posterior tibial nerve stimulation was also located posterior to the foot area in SI, but it was close to the SI area of foot, their distance being approximately 1.3 cm. ECD in PPC was almost equally demonstrated in each hemisphere. These findings suggested tha t the somatosensory associated cortex in PPC represented somatotopic o rganization in parallel with 'homunculus' in SI, but the hand area was much wider than the foot area. It was not clear whether the lip area in PPC was absent or was too close to be separated from the SI.