The separation of overlapping neuromagnetic sources in first and second somatosensory cortices.

In response to a somatosensory stimulus, two cortical centers in each hemisphere produce neural mass activity large enough to be detected with electric (EEG) or magnetic (MEG) measurements. Both the primary somatosensory cortex (S-I), located in the postcentral sulcus and in the depths of the central sulcus, as well as the secondary somatic sensory cortex (S-II), lying in the upper bank of the Sylvian fissure, respond within the first 100 ms such that the two activities overlap in time. We demonstrate that this overlap can be disentangled using a MUSIC-type approach, as suggested by Oppelt and Scholz. It needs no a priori information about the sources. As the results show, there are several instances in time in which only one of the two centers (SI, SII) is active. It is only for these time segments that a single moving dipole yields meaningful results. Such time intervals occur during the upstroke of the late component around 60 ms (only SI activity) and during the down-stroke around 120 ms (only SII activity). In these time intervals the activity of one of the somatosensory areas is still large enough, while the other center is not yet or is no longer active.