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At the current state of technology, multichannel simultaneous recording of combined electric potentials and magnetic fields should constitute the most powerful tool for separation and localization of focal brain activity. We performed an explorative study of multichannel simultaneous electric SEPs and magnetically recorded SEFs. MEG only sees tangentially oriented sources, while EEG signals include the entire activity of the brain. These characteristics were found to be very useful in separating multiple sources with overlap of activity in time. The electrically recorded SEPs were adequately modelled by three equivalent dipoles located: (1) in the region of the brainstem, modelling the P14 peak at the scalp, (2) a tangentially oriented dipole, modelling the N20-P20 and N30-P30 peaks, and part of the P45, and (3) a radially oriented dipole, modelling the P22 peak and part of the P45, both located in the region of the somatosensory cortex. Magnetically recorded SEFs were adequately modelled by a single equivalent dipole, modelling the N20-P20 and N30-P30 peaks, located close to the posterior bank of the central sulcus, in area 3b (mean deviation: 3 mm). The tangential sources in the electrical data were located 6 mm on average from the area 3b. MEG and EEG was able to locate the sources of finger stimulated SEFs in accordance with the somatotopic arrangement along the central fissure. A combined analysis demonstrated that MEG can provide constraints to the orientation and location of sources and helps to stabilize the inverse solution in a multiple-source model of the EEG.

Surgery of lesions within or close to the central area of the brain always carries the risk of iatrogenic motor or sensory deficits. Functional localization by means of intraoperative direct stimulation of the motor area or by recording somatosensory evoked potentials (SSEP's) from the surface of the somatosensory cortex is believed to reduce the operative risk. The authors introduce the combination of dipole source analysis of scalp-recorded SSEP's with three-dimensional (3-D) magnetic resonance (MR) imaging as a tool for preoperative localization of the central sulcus. This provides information on both functional and structural localization for preoperative planning. Four repeated measurements of right and left median nerve SSEP's were obtained from 20 subjects. Dipole source analysis showed a retest reliability of the 3-D localization error of 2.9 +/- 2.0 mm. Compared to the MR evaluation, dipole source analysis was found to mark the central sulcus within 3 mm for 15 conditions (subjects x side of stimulation), while the 3-D MR measurement was accurate to within 6 mm for 10 conditions and 9 mm for 14 conditions. Dipole locations were confirmed in six patients who underwent surgery of the central region. With respect to this application, dipole source analysis combined with 3-D MR imaging appears to be a valuable tool for preoperative functional localization. The accuracy in localization will be further improved when realistic head models become available that can take into account individual head geometry. Further development of the proposed new method holds promise that evoked potentials and electroencephalography will gain greater use in presurgical functional localization.

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