NEUROMAGNETIC STUDIES OF THE LIP AREA OF PRIMARY SOMATOSENSORY CORTEXIN HUMANS - EVIDENCE FOR AN OSCILLOTOPIC ORGANIZATION

Magnetic trigeminal somatosensory responses from human subjects were r ecorded using a 14-channel magnetoencephalographic system. Sensory sti muli comprising a 15-ms vibration at frequencies of 50 Hz, 150 Hz and 250 Hz were given at randomized interstimulus intervals. Using a singl e dipole model, the neuronal sources of the evoked responses were dete rmined, and mapped onto magnetic resonance images of each subject. Sou rce localization analysis was based on the main peak of the averaged s ignal (M55). All of the sources were located deep in the anterior bank of the postcentral gyrus, corresponding to area 3b of somatosensory c ortex SI. In all cases, the source for the upper lip was significantly higher in the vertical axis (0.6-1.1 cm) than for the lower lip, whil e the lower lip stimulation produced a larger response than the upper lip. Furthermore, statistically significant differences were found bet ween the locations of the dipoles evoked by different frequency stimul ation. The location of the response shifted with change in stimulation frequency, showing a trend among all subjects with a medial shift bet ween 150 and 250 Hz for both upper and lower lip. The accuracy of sour ce localization calculated from magnetic fields ranged between +/-0.9 and +/-3.0 mm (SEM). These results demonstrate (1) that a large area o f the somatosensory cortex is utilized for lip representation and (2) that the spatial displacement of the trigeminal somatosensory response may be related to the discrimination of frequency.