Somatosensory evoked magnetic fields following passive movement compared with tactile stimulation of the index finger.

Cortical processing of passive finger movement was assessed magnetoencephalographically in 12 healthy volunteers and compared with somatosensory evoked magnetic fields (SEF) following tactile stimulation. A new device comprising a clamp-like digit holder facilitated bilateral guidance of the briskly elevated index finger. Both passive movement and tactile stimulation induced activation of the contralateral primary somatosensory (SI) cortex, indicated by six SEF deflections with inter-individually rather consistent peak latencies of 20-230 ms following proprioceptive and 20-300 ms following tactile stimulation. SEF responses to the two stimulus modalities clearly differed with regard to peak latencies, amplitudes and orientations of equivalent current dipoles (ECDs). The strength and orientation of proprioception-related ECDs suggested sequential activation of SI generators, with possible involvement of areas 3a and/or 2 at around 20 ms, area 4 at approximate peak latencies of 65 and 100 ms and area 3b between 150 to 230 ms. Passive movement elicited additional activation of cortical regions outside SI, including the bilateral perisylvian regions and the contralateral cingulate gyrus at latencies of 40-470 and 150-500 ms respectively. The study provides new results with respect to the spatiotemporal analysis of proprioception-related cortical processing and may contribute to a better understanding of the modality-specific organization of the human somatosensory cortex.