Figure 1.

A: Calculation of averaged virtual sensors using the SAM minimum‐variance beamformer algorithm for a single dipole location in the head as indicated by the solid arrow. The beamformer weights are derived from the covariance matrix based on the unaveraged single trial data and normalized by the estimated uncorrelated noise n_w (left). Dipole orientation u is then iteratively adjusted to maximize total source power integrated over all trials (center). The virtual sensors for the optimal source orientation are then averaged across trials with respect to a stimulus event shown by the vertical line (right). The plus‐minus average (± Average) is also computed by multiplying odd numbered trials by –1.0. B: Virtual sensors are computed for each node of a 2D or 3D lattice of n voxels covering a region of interest (left). Virtual sensor amplitude at selected latencies is rectified and mapped onto the subject's MRI scan. The amplitude map of the 20‐ms response to electrical median nerve stimulation of the right wrist is shown superimposed on the axial slice through the primary somatosensory cortex in one subject.