Summary

Abstract

In the first part of this study, we studied functional anatomy of human somatosensory cortex using cortical stimulations and somatosensory evoked responses (SERs). SERS following median nerve stimulation were recorded on ECoG (6 patients), scalp-EEG (5 subjects), and MEG (9 subjects). We applied source localization techniques to study the neuronal sources underlying SERS and thus tried to achieve a better understanding of spatiotemporal information processing in human somatosensory cortex. Finally, we tried to provide exact and objective criteria for the localization of central fissure. We used a combined approach of multivariate statistical methods, i.e. principal component analysis, and of biophysical modeling involving multiple dipoles. The dipoles were fixed in space and had time varying activities and thus allowed modeling of multiple simultaneously active brain regions overlapping both in space and time. The basic principles of the modeling procedure were outlined in a separate section where also simple simulation experiments and typical applications were demonstrated. This approach allowed us to study the spatiotemporal structure of the evoked response over its entire time domain, and to infer the number, three-dimensional intracerebral locations, and time activities of its underlying neuronal sources. We found that two sources were sufficient to explain the large majority of the evoked response on all three recording techniques accounting for 84.6% of the data variance on ECoG, for 87.7% on scalp-EEG, and for 86.4% on MEG. One source had peak latencies at about 20 and 30 msec, and generated the electric N20–P30 components and the magnetic SEF₂₀- SEF₃₀ components (N20-P30 and SEF₂₀- SEF₃₀ source, respectively). The other source had peak latencies at about 25 and 35 msec, and thus generated the electric P25–N35 components and the magnetic SEF₂₅-SEF₃₅ components (P25-N35 and SEF₂₅-SEF₃₅ source, respectively). The spatiotemporal patterns of the two sources showed considerable overlap and could not be readily identified by visual inspection of the raw data. This was especially true for the magnetic recordings, where the SEF₂₅-SEF₃₅ source showed only small contributions to the evoked response. Our results support the hypothesis that the N20-P30 source consists of a horizontal dipole in the posterior wall of central sulcus, and that the P25-N35 source is represented by a radial dipole in the anterior crown of postcentral gyrus. On ECoG, both sources were located in postcentral gyrus at an average distance of 6 mm from central sulcus close to electrodes which elicited sensory experiences in the hand. The distance between the two sources and the C3/C4 position of the International 10–20 System averaged 10 mm on scalp-EEG and 11 mm on MEG.