Dermatome versus homunculus; detailed topography of the primary somatosensory cortex following trunk stimulation

Objective: Identification of a detailed topography of the receptive area fo r each of the thoracic dermatomes in humans using somatosensory evoked magn etic fields (SEF). Methods: We analyzed the location of the equivalent current dipole (ECD) of SEF following electrical stimulation of the skin at Th4, Th6, Th8, Th10 an d Th12 dermatomes in 14 normal subjects. Results: Three deflections, M18, M25 and M40, were obtained within 60 ms of stimulation of Th6, Th8 and Th10 dermatomes. No consistent deflection coul d be identified following Th4 and Th12 dermatomal stimulation, probably due to a poor signal-to-noise ratio and difficulty in fixing the stimulation e lectrodes. Mis was absent or small in amplitude. The latency of M25 ranged from short to long in the order Th6, Th8 and Th10 (P < 0.05). ECDs of all c omponents for each site stimulation were located in the truncal area of the primary somatosensory cortex. Although the locations of the ECDs tend to b e arranged from lateral to medial in the sequence Th6, Th8 and Th10, the di fference was not significant. Conclusion: The representation area of the trunk is small, and the receptiv e areas for the stimulation of Th6, Th8 and Th10 dermatomes are considered to be very close or to overlap. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.