Frequency-dependent volumetric distribution of somatosensory evoked magnetic fields

Event-related desynchronization (ERD) is attenuation in the amplitude of oscillations in specific frequency that occurs in relation to a specific neural activity [1]. The opposite phenomenon, event-related synchronization (ERS), is an increase in the amplitude [2]. A previous MEG study demonstrated that ERD occurs in alpha and beta bands following electrical stimulation of the median nerve or self-paced finger movements in human [3]. However, the changes in brain rhythm in higher frequency bands following somatosensory stimulation have been reported only in the animal brain [4]. Furthermore, the volumetric distribution of those changes has not been demonstrated previously. In the present study, we have developed a novel analysis method, synthetic aperture magnetometry (SAM) [5], to reveal the frequency-dependent volumetric distribution of somatosensory evoked magnetic fields (SEFs) on the individual MR images. SAM is one of the statistical spatial filtering methods using an adaptive beamformer, analogous to the technique which obtains the high spatial selectivity from radio antenna arrays [5,6,7].