Abstract
We have investigated the effects of changing the coil-to-skull distance on the motor-evoked responses (MEP) induced with two different magnetic stimulator coils (80 mm round and figure-of-eight coil) at rest and during voluntary muscle contraction. The changes in MEP latency, amplitude and silent period (SP) duration induced by stimulation directly upon the skull, and 1 cm away from the skull were analyzed by computing the probability density distribution (PDD) for the responses obtained from all subjects. This measure corresponds to the finite probability that the event occurs within a given area. Overall, the results were consistent with a distance-induced decrease in magnetic field strength. However, the increase in coil-to-skull distance induced a higher probability of longer latencies in active muscle when stimulating with either coil. Also, stimulating at a distance with the figure-of-eight coil increased the probability of a longer SP duration. The stimulation strength at the two distances was comparable because it was set based on the motor threshold obtained for each distance. Therefore, our results are not entirely compatible with the established exponential drop in magnetic field with increasing distance. Rather, they suggest that a more complex set of interactions occurs in the cortex. The results imply that distinct patterns of cortical network activation may exist related to the distance-induced alterations when the coil is moved away from the skull. Further studies are required to elucidate the precise nature of the distance-related interactions of the magnetic field with the cortex.
References
Amassian VE, Stewart M, Quirk GJ, Rosenthal JL (1987) Physiological basis of motor effects of a transient stimulus to cerebral cortex. Neurosurgery 20:74–93
Brasil-Neto JP, Cohen LG, Panizza M, Nilsson J, Roth BJ, Hallett M (1992) Optimal focal transcranial magnetic activation of the human motor cortex: effects of coil orientation, shape of the induced current pulse, and stimulus intensity. J Clin Neurophysiol 9:132–136
Burke D, Hicks R, Gandevia SC, Stephen J, Woodforth I, Crawford M (1993) Direct comparison of corticospinal volleys in human subjects to transcranial magnetic and electrical stimulation. J Physiol 470:383–393
Cohen LG, Roth BJ, Nilsson J, Dang N, Panizza M, Bandinelli S, Friauf W, Hallett M (1990) Effects of coil design on delivery of focal magnetic stimulation. Technical considerations. Electroencephalogr Clin Neurophysiol 75:350–357
Davey NJ, Romaiguere P, Maskill DW, Ellaway PH (1994) Suppression of voluntary motor activity revealed using transcranial magnetic stimulation of the motor cortex in man. J Physiol 477:223–235
Di Lazzaro V, Oliviero A, Profice P, Saturno E, Pilato F, Insola A, Mazzone P, Tonali P, Rothwell JC (1998a) Comparison of descending volleys evoked by transcranial magnetic and electric stimulation in conscious humans. Electroencephalogr Clin Neurophysiol 109:397–401
Di Lazzaro V, Restuccia D, Oliviero A, Profice P, Ferrara L, Insola A, Mazzone P, Tonali P, Rothwell JC (1998b) Effects of voluntary contraction on descending volleys evoked by transcranial stimulation in conscious humans. J Physiol 508:625–633
Di Lazzaro V, Oliviero A, Profice P, Insola A, Mazzone P, Tonali P, Rothwell JC (1999) Direct recordings of descending volleys after transcranial magnetic and electric motor cortex stimulation in conscious humans. Electroencephalogr Clin Neurophysiol Suppl 51:120–126
Jalinous R (1991) Technical and practical aspects of magnetic nerve stimulation. J Clin Neurophysiol 8:10–25
Kozel FA, Nahas Z, DeBrux C, Molloy M, Lorberbaum JP, Bohning D, Risch SC, George MS (2000) How coil-cortex distance relates to age, motor threshold, and antidepressant response to repetitive transcranial magnetic stimulation. J Neuropsychiatry Clin Neurosci 12:376–384
McConnell KA, Nahas Z, Shastri A, Lorberbaum JP, Kozel FA, Bohning DE, George MS (2001) The transcranial magnetic stimulation motor threshold depends on the distance from coil to underlying cortex: a replication in healthy adults comparing two methods of assessing the distance to cortex. Biol Psychiatry 49:454–459
Mosimann UP, Marre SC, Werlen S, Schmitt W, Hess CW, Fisch HU, Schlaepfer TE (2002) Antidepressant effects of repetitive transcranial magnetic stimulation in the elderly: correlation between effect size and coil-cortex distance. Arch Gen Psychiatry 59:560–561
Nahas Z, Teneback CC, Kozel A, Speer AM, DeBrux C, Molloy M, Stallings L, Spicer KM, Arana G, Bohning DE, Risch SC, George MS (2001) Brain effects of TMS delivered over prefrontal cortex in depressed adults: role of stimulation frequency and coil-cortex distance. J Neuropsychiatry Clin Neurosci 13:459–470
Pascual-Leone A, Cohen LG, Brasil-Neto JP, Hallett M (1994) Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions. Electroencephalogr Clin Neurophysiol 93:42–48
Rossini PM, Barker AT, Berardelli A, Caramia MD, Caruso G, Cracco RQ, Dimitrijevic MR, Hallett M, Katayama Y, Lucking CH (1994) Non-invasive electrical and magnetic stimulation of the brain, spinal cord and roots: basic principles and procedures for routine clinical application. Report of an IFCN committee. Electroencephalogr Clin Neurophysiol 91:79–92
Rothwell JC, Thompson PD, Day BL, Boyd S, Marsden CD (1991) Stimulation of the human motor cortex through the scalp. Exp Physiol 76:159–200
Ruohonen J, Ilmoniemi RJ (2002) Physical principles for transcranial magnetic stimulation. In: Handbook of Transcranial Magnetic Stimulation. In: Pascual-Leone A, Davey NJ, Rothwell JC, Wasserman EM, Puri BK (eds) Transcranial Magneitc Stimlation. New York: Oxford, pp 18–30
Stokes MG, Chambers CD, Gould IC, English T, McNaught E, McDonald O, Mattingley JB (2007) Distance-adjusted motor threshold for transcranial magnetic stimulation. Clin Neurophysiol 118:1617–1625
Toschi N, Welt T, Guerrisi M, Keck ME (2008) A reconstruction of the conductive phenomena elicited by transcranial magnetic stimulation in heterogeneous brain tissue. Phys Med 24:80–86
Wagner T, Gangitano M, Romero R, Theoret H, Kobayashi M, Anschel D, Ives J, Cuffin N, Schomer D, Pascual-Leone A (2004) Intracranial measurement of current densities induced by transcranial magnetic stimulation in the human brain. Neurosci Lett 354:91–94
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This work was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia projects 145083 and 143027.
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Cukic, M., Kalauzi, A., Ilic, T. et al. The influence of coil–skull distance on transcranial magnetic stimulation motor-evoked responses. Exp Brain Res 192, 53–60 (2009). https://doi.org/10.1007/s00221-008-1552-0
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DOI: https://doi.org/10.1007/s00221-008-1552-0