Somatotopy of human hand somatosensory cortex revealed by dipole source analysis of early somatosensory evoked potentials and 3D-NMR tomography

Effect of movement on dipolar source activities of somatosensory evoked potentials

“…The topographic differences of the dipole 2 location between median and tibial nerve models match well the somatotopy of the foot and hand in the primary somatosensory cortex, as demonstrated in humans by Penfield and Rasmussen. 21 The somatotopic alignment of dipole 2 in our models, confirming what had already been shown by Buchner et al, 2 and its biphasic activity, very similar to the morphology of the so-called primary response, 38 suggest that this dipolar source may be located in area 3b. 1,26 Even if dipoles 3 and 4 showed significant location differences between the median and tibial nerve models, we failed to demonstrate any shift of dipoles toward the mesial hemispheric surface after tibial nerve stimulation, as expected from Penfield and Rasmussen's homunculus representation.…”

“…Because this dipole could not be fitted because of the lack of recording electrodes at the base of the skull, we chose a low location for it, according to the position of the lemniscal activity source in previous BESA models. 2,4 Interestingly, in the median nerve model, dipole 1 also showed a later long-lasting activity, which could partially account for the N18 far field. However, we decided not to analyze the modifications of this activity in the gating conditions, because our reference electrode at the ipsilateral ear does not permit the activity of the N18 generator to be picked up entirely.…”

High-Frequency Somatosensory Thalamocortical Oscillations and Psychopathology in Schizophrenia

“…In order to reduce the data to a single waveform weighted on the activity of the neuronal generators of the SEPs, single dipole sources were fitted for each subject and each side for a time period between 14 and 25 ms. To adjust the individual data for average head geometry and conductivities, a spherical three-shell (scalp, skull, brain) head model was used with the assumptions of a scalp thickness of 6 mm (0. A single dipole is not an appropriate model for the early SEPs, and to model all the different early SEPs, a more complex source configuration with at least three dipoles is needed (for an explanation, see Buchner et al [31]). The approach, chosen with one dipole based on the limitations of the investigated patient group, produced a unique solution and was considered to be sufficient to demonstrate differences in signal composition between the subgroups.…”

“…Human median nerve SEPs provide the possibility of investigating thalamocortical and early cortical processing more intensely by means of advanced analysis filtering tools [31]: SEPs of the median nerve when digitally high-pass filtered show a brief oscillatory burst with low amplitudes (!500 nV) and high frequency (600 Hz). These high-frequency oscillations (HFOs) are superimposed on the primary cortical low-frequency response represented by a parietal negativity peak component approximately 20 ms after stimulation, i.e., the N20 [32].…”

Central scalp projection of the N30 SEP source activity after median nerve stimulation

“…Moreover, the SEPs recorded in our subjects underwent dipolar source modeling. This technique has proved helpful to localize the generators of evoked potentials (EPs) reflecting responses from well-identified primary sensory areas 3,7,21,22,26 and to separate clearly the activities of neighboring cerebral structures. We used source modeling to separate the different SEP generators and then projected the field distribution corresponding to their activities on the scalp.…”