Source analysis of median nerve and finger stimulated somatosensory evoked potentials: Multichannel simultaneous recording of electric and magnetic fields combined with 3d-MR tomography

Abstract

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.

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

“…In most of the somatosensory studies which we have performed (e.g., Elbert et al 1994Elbert et al ,1995, we do not obtain satisfying single ECD-fits at or after the peak of· the first major component (35-90 msec), but achieve excellent models during the upstroke of this component. This too would be in agreement with the assumption that S-II activation comes in only at stages beyond 50 msec, an assumption which is also consistent with the recent report by Buchner et al (1994) showing that early SEFs (N20-P20) and N30-P30 peaks) can be adequately modeled by a single EeD located close to the posterior bank of the central sulcus, in area 3b.…”

Dipole Source Analyses of Early Median Nerve SEP Components Obtained From Subdural Grid Recordings

“…Because there is no unique solution, it is necessary to apply constraints to the model, which may prove as powerful tool to localize sources without invasive measures. With respect to localization of median nerve SEP sources, dipole localization has been performed so far by using surface recordings only (Buchner et al 1994;Jung et al 2003Jung et al , 2008. We have successfully used this technique in an anatomically difficult region, the parasylvian cortex, to estimate the source of pain evoked potentials from subdural grid recordings (Vogel et al 2003), and the results were independently confirmed by another laboratory using depth recordings (Frot and Mauguière 2003).…”

Magnetoencephalography

“…4). Many investigations have been dedicated to the localizatioa of somatosensory-evoked fields (SEF) (28)(29)(30)(31)(32) (Fig. 4) reflecting the homuncular organization of the somatosensory cortex.…”