Abstract
Neurones can be excited by an externally applied time-varying electromagnetic field. Focused magnetic brain stimulation is attained using multiple small coils instead of one large coil, the resultant induced electric field being a superposition of the fields from each coil. In multichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusing of the stimulating field, and independent coil channels allow targeting of the stimuli on a given spot without moving the coils. The problem of shaping the stimulating field in multichannel stimulation is analysed, and a method is derived that yields the driving currents required to induce a field with a user-defined shape. The formulation makes use of lead fields and minimumnorm estimation from magneto-encephalography. Using these methods, some properties of multichannel coil arrays are examined. Computer-assisted multichannel stimulation of the cortex will enable several new studies, including quick determination of the cortical regions, the stimulation of which disrupts cortical processing required by a task.
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References
Barker, A. T., Jalinous, R. andFreeston, I. L. (1985): ‘Noninvasive magnetic stimulation of human motor cortex’,Lancet,1, pp. 1106–1107
Beckers, G. andZeki, S. (1995): ‘The consequences of inactivating areas V1 and V5 on visual motion perception’,Brain,118, pp. 49–60
Chokroverty, S. (Ed.) (1990): ‘Magnetic stimulation in clinical neurophysiology’, (Butterworth Publications, Boston)
Cohen, D. andCuffin, B. N. (1991): ‘Developing a more focal magnetic stimulator. Part I: some basic principles’,J. Clin. Neurophysiol.,8, pp. 102–111
Eaton, H. (1992): ‘Electric field induced in a spherical volume conductor from arbitrary coils: application to magnetic stimulation and MEG’,Med. Biol. Eng. Comput.,30, pp. 433–440
Hämäläinen, M. S. andIlmoniemi, R. J. (1984): ‘Interpreting measured magnetic fields of the brain: estimates of current distributions,’ Technical Report TKK-F-A559, Helsinki University of Technology
Hämäläinen, M. S. andIlmoniemi, R. J. (1994): ‘Interpreting magnetic fields of the brain: minimum-norm estimates’,Med. Biol. Eng. Comput.,32, pp. 35–42
Heller, L. andvan Hulsteyn, D. B. (1992): ‘Brain stimulation using electromagnetic sources: theoretical aspects’,Biophys. J.,63, pp. 129–138
Himmelblau, D. M. (1972): ‘Applied nonlinear programming’ (McGraw-Hill Book Company, New York)
Morioka, T., Yamamotot, T., Mizushima, A., Tombimatsu, S., Shigeto, H., Hasuo, K., Fuji, K. andFukui, M. (1995): ‘Comparison of magnetoencephalography, functional MRI, and motor evoked potentials in the localization of the sensory-motor cortex’,Neurol. Res.,17, pp. 361–367
Nilsson, J., Panizza, M. andGrandori, F. (Eds.), (1996): Advances in magnetic stimulation: mathematical modeling and clinical applications (Salvatore Maugeri Foundation, Pavia, Italy)
Pascual-Leone, A., Rubio, B., Pallardo, F. andCatala, M. D. (1996): ‘Rapid-rate transcranial magnetic stimulation of left dorsolateral prefrontal cortex in drug-resistance depression’,Lancet,348, pp. 233–237
Ravazzani, P., Ruohonen, J., Grandori, F. andTognola, G. (1996): ‘Magnetic stimulation of the nervous system: induced electric field in unbounded, semi-infinite, spherical, and cylindrical media’,Ann. Biomed. Eng.,24, pp. 606–616
Roth, B. J. (1994): ‘Mechanisms for electrical stimulation of excitable tissue’,Crit. Rev. Biomed. Eng.,22, pp. 253–305
Ruohonen, J. andIlmoniemi, R. J. (1996): ‘Multichannel magnetic stimulation: improved stimulus targeting’,inNilsson, J., Panizza, M. andGrandori, F. (Eds.): ‘Advances in magnetic stimulation: mathematical modeling and clinical applications’ (Salvatore Maugeri Foundation, Pavia, Italy) pp. 55–64
Ruohonen, J., Ravazzani, P., Ilmoniemi, R. J., Galardi, G., Nilsson, J., Panizza, M., Amadio, S., Grandori, F. andComi, G. (1996): ‘Motor cortex mapping with combined MEG and magnetic stimulation’,inArber, C., Celesia, G., Comi, G. andMauguière, F. (Eds.): ‘Functional neuroscience’ (Elsevier Science, Amsterdam) pp. 317–322
Ruohonen, J., Virtanen, J. andIlmoniemi, R. J. (1997): ‘Coil optimization for magnetic brain stimulation’,Ann. Biomed. Engl.,25, pp. 840–849
Ueno, S., Tashiro, T. andHarada, K. (1988): ‘Localized stimulation of neural tissues in the brain by means of a paired configuration of time-varying magnetic fields’,J. Appl. Phys.,64, pp. 5862–5864
Ueno, S., Matsuda, T. andFujiki, M. (1990): ‘Functional mapping of the human motor cortex obtained by focal and vectorial magnetic stimulation of the brain’,IEEE Trans. Magn.,26, pp. 1539–1544
Wassermann, E. M., McShane, L. M., Hallett, M. andCohen, L. G. (1992): ‘Noninvasive mapping of muscle representations in human motor cortex’,Electroenceph. Clin. Neurophysiol.,85, pp. 1–8
Yonokuchi, K. andCohen, D. (1991): ‘Developing a more focal magnetic stimulator. Part II: fabricating coils and measuring the induced current distributions’,J. Clin. Neurophysiol.,8, pp. 112–120
Ziemann, U., Lönnecker, S., Steinhoff, B. J. andPaulis, W. (1996): ‘Effects of antiepileptic drugs on motor cortex excitability in humans: a transcranial magnetic stimulation study’,Ann. Neurol.,40, pp. 367–378
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Ruohonen, J., Ilmoniemi, R.J. Focusing and targeting of magnetic brain stimulation using multiple coils. Med. Biol. Eng. Comput. 36, 297–301 (1998). https://doi.org/10.1007/BF02522474
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DOI: https://doi.org/10.1007/BF02522474