Brain electrical source analysis of primary cortical components of the tibial nerve somatosensory evoked potential using regional sources

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings reveale d a source in the foot area of the contralateral hemisphere. We now investi gated this paradoxical lateralization by performing a brain electrical sour ce analysis in the P40 time window (34-46 ms). The tibial nerve was stimula ted behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verifie d electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitud e was higher ipsilaterally (0.45 +/- 0.14 mu V) than contralaterally (-0.49 +/- 0.16 mu V). The best fitting regional source, however, was always loca ted in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a fron tal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation c ompared to the tangential P37 dipole, it is systematically underestimated i n MEG. Dipole orientations explained interindividual variability of scalp p otential distribution. SEP amplitudes were smaller when generated in the do minant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (Delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.