The influence of coil–skull distance on transcranial magnetic stimulation motor-evoked responses
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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.
KeywordsTranscranial magnetic stimulation Latency MEP Silent period Magnetic coils Coil-to-skull distance
This work was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia projects 145083 and 143027.
- 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 49:454–459PubMedCrossRefGoogle Scholar
- 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 13:459–470PubMedGoogle Scholar
- 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 91:79–92PubMedCrossRefGoogle Scholar
- 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 18–30Google Scholar