Spatial filters for the same experimental effect are different for individual subjects. The topography of the spatial filters for each of the 10 subjects used to produce Figure 1B (a stationary template was used for each subject). Only the latitudinal gradiometers are shown for clarity. The average topography over the 10 subjects is shown in the center, exposing the cluster that was identified using FieldTrip’s cluster-based permutation test , and used to define the region of interest (ROI; black dots). Each of the 10 subject-specific topographies (including the magnetometers and latitudinal gradiometers, not shown) yields a time course that maximally separates the means of the lag-9 and control conditions for that subject. It is evident that, while the ROI captures a region that tends to have a higher amplitude signal on average across subjects, it is not an especially prominent region of activity for any individual subject. EMS filtering thus factors out anatomical variability across subjects in order to focus specifically on the time course of the experimental effect.
Schurger et al. BMC Neuroscience 2013 14:122 doi:10.1186/1471-2202-14-122