Summary
Theoretically, the information we can obtain about the functional localization of a source of brain activity from the scalp, for instance evoked by a sensory stimulus, is the same whether one uses EEG or MEG recordings. However, the nature of the sources and, especially of the volume conductor, poses constraints such that appreciable differences between both types of data may exist. We present here empirical and theoretical data that illustrate which are the main constraints and to what extent they may affect electric potential and magnetic field maps. The empirical data consists of visual evoked potential and magnetic fields to the appearance of a checkerboard pattern (half-visual field stimulation). The concept of equivalent dipole is presented and its limitations are discussed. It is considered that the concept of equivalent dipole (ED) yields only an approximate description of the activity of a patch of cortex. A main difference between EEG and MEG recordings is the fact that radially oriented dipoles can hardly be seen in the MEG in contrast with the EEG. Accordingly, a weak tangential dipole component is difficult to distinguish in the EEG if a strong radial component is also present. However, a combination of both methods can give useful complementary information in such cases. A factor that influences largely such differences is the model of volume conductor used. A four concentric spheres model, as commonly used for solving the inverse problem of source localization, causes appreciable errors when EEG data are used but much less in case of the MEG. The use of a model consisting of eccentric spheres fitting the four compartments, brain, CSF, skull and scalp, provides a better approximation of the real geometry of the head and allows to obtain comparable results for visual evoked potentials and magnetic fields. It is emphasized that for precise localization of EDs, especially based on EEG recordings, a realistic model of the different compartments of the head is necessary. The latter must be tailor made to a given subject using MRI-scans, in view of the large variability in head geometry between subjects.
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References
Akima, H. A method of bivariate interpolation and smooth surface fitting for irregularly distributed data points (E1). ACM Trans. Math. Software, 1978, 4: 160–164.
Brindley, G.S. The variability of the human striate cortex. J. Physiol. (London), 1972, 25: 1–3.
Cohen, D. and Cuffin, B.N. Demonstration of useful differences between magnetoencephalogram and electroencephalogram. Electroenceph. clin. Neurophysiol., 1983, 56: 38–51.
Cohen, D., Cuffin, B.N., Yunokuchi, K., Maniewski, R., Purcell, C., Cosgrove, G.R., Ives, J., Kennedy, J.G. and Schomer, D.L. MEG versus EEG localization test using implanted sources in the human brain. Ann. Neurol., 1990, 28: 811–817.
Dagnelie, G. Pattern and motion processing in primate visual cortex: a study in visually evoked potentials. Ph.D. Thesis, University of Amsterdam, 1986: 326pp.
De Munck, J.C., Van Dijk, B.W. and Spekreijse, H. An analytic method to determine the effect of source modelling errors on the apparent location and direction of biological sources. J. Appl. Physics, 1988a, 63: 944–956.
De Munck, J.C., VanDijk, B.W. and Spekreijse, H. Mathematical dipoles are adequate to described realistic generators of human brain activity. IEEE Trans. Biomed. Engn., 1988B, BME-35: 980–986.
De Munck, J.C. A mathematical and physical interpretation of the electromagnetic field of the brain. Ph.D. Thesis, University of Amsterdam, 1989.
De Waal, B.J., Reits, D., Spekreijse, H. and Grinsbergen, C.A. Implementation of a portable pattern stimulator and VEP/ERG recording system based on an Apple micro-computer. Doc. Ophthal., 1983: 209–216.
Drasdo, N. Cortical potentials evoked by pattern presentation in the foveal region. In: C. Barber (Ed.), Evoked potentials. University Park Press, Baltimore, MD, 1980: 167–174.
Freeman, W.J. Mass action in the nervous system, Academic Press, New York, 1975: 1–489.
Gevins, A.S. and Bressler, S.L. Functional topography of the human brain. In: G. Pfurtscheller/F.H. Lopes da Silva (Eds.) Functional brain imaging, Hans Huber Publishers, Stuttgart, 1988: 99–116.
Lopes da Silva, F.H. and Spekreijse, H. Localization of brain sources of visually evoked responses: using single and multiple dipoles. An overview of different approaches. In: Event-related Brain Research, C.H.M. Brunia, G. Mulder and M.N. Verbaten (Eds.). Elsevier: Amsterdam, Suppl. 42, EEG Journal, 1991: 38–46.
Maier, J., Dagnelie, G., Spekreijse, H. and Van Dijk, B.W. Principal components analysis for source localization of VEPs in man. Vision Res., 1987, 27: 165–177.
Meijs, J.W.H. and Peters, M.J. The EEG and MEG, using a model of eccentric spheres to describe the head. IEEE Trans. Biomed Eng., 1987, 34: 913–920.
Meijs, J.W.H., Bosch, F.G.C., Peters, M.J. and Lopes da Silva, F.H. On the magnetic field distribution generated by a dipolar current source situated in a realistically shaped compartment model of the head. Electroenceph. clin. Neurophysiol., 1987, 66: 286–298.
Scherg, M. and Von Cramon, D. Two bilateral sources of the late AEP as identified by a spatio-temporal dipole model. Electroenceph. clin. Neurophysiol., 1985, 62: 32–34.
Schimmel, H. The ( ± ) reference: accuracy of estimated mean components in average response studies. Science, 1967, 157: 92–93.
Snijder, W.V. Contour plotting (J6). ACM Trans. Math. Software, 1978, 4: 290–294.
Spekreijse, H. and Estevez, D.R. Visual evoked potentials and a physical analysis of visual processes in man. In: J.E. Desmedt (Ed.), Visual evoked potentials in man. Clarendon Press, Oxford, 1977: 16–89.
Stok, C.J. The inverse problem in EEG and MEG with application to visual evoked responses. Ph.D. Thesis, University of Twente, Enschede, 1986.
Stok, C.J., Spekreijse, M.J., Peters, M.J., Boom, H.B.K. and Lopes da Silva, F.H. A comparative EEG/MEG equivalent dipole study of the pattern onset visual response. New Trends and Advanced Techniques in Clin. Neurophysiol., EEG Suppl., 1990, 41: 34–50.
Stok, C.J. The influence of mode parameters on EEG/MEG single dipole source estimation. IEEE Trans. Biomed. Engn., BME. 1987, 34: 289–296.
Van Dijk, V.W. and Spekreijse, H. Localization of electric and magnetic sources of brain activity. In: J.E. Desmedt (Eds.), Visual evoked potentials, 1990, 5: 58–75.
Wieringa, H.H. and Peters, M.J. MRI and MEG, 3-D models and display. In: Biomagetic localization and 3D imaging, H.M. Rajala and T. Katila (Eds.). J. Nenonen, Sjökulla Finland, 1991: 10–22.
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Lopes da Silva, F.H., Wieringa, H.J. & Peters, M.J. Source localization of EEG versus MEG: Empirical comparison using visually evoked responses and theoretical considerations. Brain Topogr 4, 133–142 (1991). https://doi.org/10.1007/BF01132770
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DOI: https://doi.org/10.1007/BF01132770