Specific somatosensory processing in somatosensory area 3b for human thumb: a neuromagnetic study

Objectives: We examined the relation between somatosensory N20m primary responses and high-frequency oscillations (HFOs) after thumb and middle finger stimulation. Methods: Somatosensory evoked fields (SEFs) from 12 subjects were measured following electric stimulation of the thumb and middle finger. SEFs were recorded with a wide bandpass (3–2000 Hz) and then N20m and HFOs were separated by subsequent 3–300 and 300–900 Hz bandpass filtering. Results: The N20m peak-to-peak amplitude did not differ significantly between thumb and middle finger SEFs. In contrast, HFOs had a significantly larger number of peaks and were higher in the maximum amplitude and the total amplitude after thumb stimulation than after middle finger stimulation. Conclusions: Our present data demonstrate a different relation between N20m and HFOs after thumb and middle finger stimulation. In view of the fact that the human thumb has uniquely evolved functionally and morphologically, the somatosensory information from the thumb will be processed differently for a fine motor control. We speculate that HFOs are generated by inhibitory interneurons in layer 4 in area 3b. Thus, enhanced activity of interneurons reflected by high amplitude HFOs exerts stronger inhibition on downstream pyramidal cells in area 3b for thumb stimulation.