title: The influence of coil-skull distance on transcranial magnetic stimulation motor-evoked responses.
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.
authors.affiliation: Department of Neurophysiology, Institute for Medical Research, Belgrade, PO Box 11121, Serbia.
authors.forename: M
authors.initials: M
authors.lastname: Cukic
authors.forename: A
authors.initials: A
authors.lastname: Kalauzi
authors.forename: T
authors.initials: T
authors.lastname: Ilic
authors.forename: M
authors.initials: M
authors.lastname: Miskovic
authors.forename: M
authors.initials: M
authors.lastname: Ljubisavljevic
mesh_terms: D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006225:Hand / Q000294:innervation / Q000502:physiology; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000033:anatomy & histology / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009431:Neural Conduction / Q000502:physiology; D011712:Pyramidal Tracts / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D012886:Skull / Q000033:anatomy & histology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation* / Q000379:methods*
references.citation: J Neuropsychiatry Clin Neurosci. 2000 Summer;12(3):376-84
references.pmid: 10956572
references.citation: Electroencephalogr Clin Neurophysiol. 1994 Aug;91(2):79-92
references.pmid: 7519144
references.citation: J Physiol. 1994 Jun 1;477(Pt 2):223-35
references.pmid: 7932215
references.citation: Neurosurgery. 1987 Jan;20(1):74-93
references.pmid: 3543727
references.citation: J Physiol. 1993 Oct;470:383-93
references.pmid: 8068071
references.citation: J Clin Neurophysiol. 1992 Jan;9(1):132-6
references.pmid: 1552001
references.citation: Arch Gen Psychiatry. 2002 Jun;59(6):560-1
references.pmid: 12044199
references.citation: Clin Neurophysiol. 2007 Jul;118(7):1617-25
references.pmid: 17524764
references.citation: Electroencephalogr Clin Neurophysiol Suppl. 1999;51:120-6
references.pmid: 10590942
references.citation: Electroencephalogr Clin Neurophysiol. 1998 Oct;109(5):397-401
references.pmid: 9851296
references.citation: J Neuropsychiatry Clin Neurosci. 2001 Fall;13(4):459-70
references.pmid: 11748315
references.citation: Neurosci Lett. 2004 Jan 9;354(2):91-4
references.pmid: 14698446
references.citation: Electroencephalogr Clin Neurophysiol. 1990 Apr;75(4):350-7
references.pmid: 1691084
references.citation: Electroencephalogr Clin Neurophysiol. 1994 Feb;93(1):42-8
references.pmid: 7511521
references.citation: J Clin Neurophysiol. 1991 Jan;8(1):10-25
references.pmid: 2019644
references.citation: J Physiol. 1998 Apr 15;508 ( Pt 2):625-33
references.pmid: 9508823
references.citation: Biol Psychiatry. 2001 Mar 1;49(5):454-9
references.pmid: 11274657
references.citation: Exp Physiol. 1991 Mar;76(2):159-200
references.pmid: 2059424
references.citation: Phys Med. 2008 Jun;24(2):80-6
references.pmid: 18296093