Neuromagnetic gamma-band activity in the primary and secondary somatosensory areas.

To evaluate the gamma-band activity related to somatosensory processing, we recorded neuromagnetic signals from seven healthy subjects. The source power changes evoked by electrical stimulation of the median nerve were estimated with synthetic aperture magnetometry (SAM). Source power in the low gamma band (40 Hz) decreased in the contralateral primary somatosensory cortex (SI) for a few hundred milliseconds (i.e. middle and long latency) and then increased inversely. Source power in the high gamma band (70-90 Hz) increased simultaneously both in the contralateral SI and contra/ipsilateral secondary somatosensory cortex (SII) in 80-180 ms. These results suggest that low and high gamma oscillations work under independent mechanisms during somatosensory processing. In particular, high gamma oscillations may play an essential role in making a functional connection between SI and SII.