A unique area of the homonculus: the topography of the primary somatosensory cortex in humans following posterior scalp and shoulder stimulation.

We recorded somatosensory evoked magnetic field (SEF) to investigate the differentiation in the receptive area for the face, lower part of the posterior scalp (mastoid) and shoulder, which occupy an unique area in the homunculus. We analyzed the location of the equivalent current dipole (ECD) of SEF following electrical stimulation of the skin at the face, mastoid and shoulder in 20 normal subjects. Three deflections (1M, 2M and 3M) were obtained within 50 ms of the stimulation in 16 of 20 subjects. The peak latency of the 1M and 2M was not significantly different at any stimulus sites. The amplitude of the 1M was significantly larger following the face than mastoid stimulation (p<0.05). The 16 subjects were classified according to the locations of the ECD on stimulation of the mastoid: close to that for shoulder stimulation, but significantly (p<0.05) more superior and medial to that following the face stimulation (Type 1, eleven subjects); close to that for face stimulation, but significantly (P<0.05) more inferior and lateral to that following the shoulder stimulation (Type 2, five subjects). The site of the receptive area for the posterior scalp shows interindividual variation, possibly due to anatomical differences.