Localization of brain activity associated with non-time-locked tactile stimulation using synthetic aperture magnetometry (SAM)

It is well know that changes in spontaneous brain rhythms accompany both sensory stimulation and motor activity in humans and that such changes arise from sensorimotor areas of the cortex[1-3]. In particular, sensory input has been observed to suppress activity in both centrally distributed alpha or mu (8-15 Hz) and beta (15-25 Hz) frequency bands and there is evidence that there is a rebound of this activity following electrical stimulation of the digits [1]. Furthermore, such rebound phenomena appear to arise from the hand area of the postcentral gyrus and can be influenced by ongoing motor activity or even motor imagery [1,4]. In the current study, we apply a new method, synthetic aperture magnetometry (SAM) [5,6] to localize frequency specific changes in sensorimotor cortex during passive tactile stimulation. SAM is a minimum variance beamformer which allows three-dimensional localization of source activity at an arbitrary spatial resolution without the need for signal averaging, or a priori specification of the number of active sources. In the current study we compute SAM difference images of source power integrated over 4 s time windows between finger brushing (active) versus rest (control) periods, and superimposed these images on structural MRI scans for each subject.