B. Brain imaging methods: Functional imaging of electric brain activity requires specific models to
transform the signals recorded at the surface of the human head into an image [8]. Two
categories of model are available: 1) single-time-point and 2) spatio-temporal methods. The
instantaneous methods rely only on few voltage differences measured at one sampling point. To
create a spatial image from this limited information, they require strict assumptions that rarely
conform to the underlying physiology. Spatio-temporal models create two kinds of images: first, a
spatial image of discrete equivalent multiple dipoles or regional sources, and second, an image of
source current waveforms that reflect the temporal dynamics of the brain activity in circumscribed
areas. The accuracy of the spatial image is model dependent and limited, but it can be validated
from the spatio-temporal data by the "regional source imaging" technique, introduced here. The
source waveforms are linear combinations of the scalp waveforms, and thus, specific derivations
which image local brain activities at a macroscopic level.
Brain source imaging of somatosensory evoked potentials revealed temporally overlapping
activities from the brainstem, thalamus, and from multiple sources in the region of the
contralateral somatosensory projection areas.