Activity in posterior parietal cortex following somatosensory stimulation in man: Magnetoencephalographic study using spatio-temporal source analysis

We investigated the activation of posterior parietal cortex (PPC) to somatosensory stimulation in humans to determine its fundamental role as a somatosensory associated area using magnetoencephalography (MEG). We studied somatosensory evoked magnetic fields (SEF) after stimulation of median nerve, posterior tibial nerve and lip, and analyzed them by the single dipole model and also by the multidipole model using brain electric source analysis (BESA) system. In single source model analysis, the dipole at the peak latency of short-latency components following each site stimulation were located in the corresponding receptive fields in the primary somatosensory cortex (SI) contralateral to the stimulation. The dipole at the peak latency of the middle latency components were located in bilateral upper bank of Sylvian fissure (SII). By contrast, in the five-dipole model of BESA, the equivalent current dipoles (ECDs) of the middle-latency SEF after stimulation of median nerve and posterior tibial nerve were identified in the contralateral SI and in the bilateral SII and PPC, while all activities of middle-latency SEF after lip stimulation appeared to be restricted in the contralateral SI and bilateral SII. Around 80 msec in latency, the ECD location in PPC after median nerve stimulation was, on the average, 2.4 cm posterior, 2.9 cm medial and 2.6 cm superior to the hand area in SI. The 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 that the somatosensory associated cortex in PPC represented somatotopic organization 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.