Oscillatory gamma synchronization binds the primary and secondary somatosensory areas in humans

Induced gamma activity has a key role in the temporal binding of distributed cortico-cortical processing. To elucidate the neural synchronization in the early-stage somatosensory processing, we studied the functional connectivity between the primary and secondary somatosensory cortices (SI and SII) in healthy subjects using magnetoencephalography (MEG) with excellent spatiotemporal resolution. First, somatosensory-evoked magnetic fields were recorded to determine the locations of each cortical activity. Then we analyzed the phase-locking values (PLVs) of the induced gamma activity to assess neural synchrony within the somatosensory cortical network. We also assessed PLVs in patients with multiple sclerosis (MS) to validate our PLV analysis in evaluating the inter-areal functional connectivity, which can often be impaired in MS. The PLVs of the induced gamma activity were calculated for each pair of unaveraged MEG signals that represented the activities of the contralateral SI and bilateral SII areas. Analysis of PLVs between the SI and SII areas showed significantly increased PLVs for gamma-band activities, starting at an early post-stimulus stage in normal controls, whereas this increase in PLVs was apparently diminished in MS. The PLV analysis provided evidence for early-latency, gamma-band neuronal synchronization between the SI and SII areas in normal controls. Our study first demonstrates the gamma-band synchrony in the early-stage human somatosensory processing.