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• W2041949619 abstract "Transcranial magnetic stimulation (TMS) is a technique for noninvasive stimulation of the human brain. Stimulation is produced by generating a brief, high-intensity magnetic field by passing a brief electric current through a magnetic coil. The field can excite or inhibit a small area of brain below the coil. All parts of the brain just beneath the skull can be influenced, but most studies have been of the motor cortex where a focal muscle twitch can be produced, called the motor-evoked potential. The technique can be used to map brain function and explore the excitability of different regions. Brief interference has allowed mapping of many sensory, motor, and cognitive functions. TMS has some clinical utility, and, because it can influence brain function if delivered repetitively, it is being developed for various therapeutic purposes. Transcranial magnetic stimulation (TMS) is a technique for noninvasive stimulation of the human brain. Stimulation is produced by generating a brief, high-intensity magnetic field by passing a brief electric current through a magnetic coil. The field can excite or inhibit a small area of brain below the coil. All parts of the brain just beneath the skull can be influenced, but most studies have been of the motor cortex where a focal muscle twitch can be produced, called the motor-evoked potential. The technique can be used to map brain function and explore the excitability of different regions. Brief interference has allowed mapping of many sensory, motor, and cognitive functions. TMS has some clinical utility, and, because it can influence brain function if delivered repetitively, it is being developed for various therapeutic purposes. Almost 30 years ago, Merton asked Morton to build a high-voltage electrical stimulator able to activate muscle directly rather than through the small nerve branches in the muscle. When built, he had the idea that this device could also stimulate the motor areas of the human brain through the intact scalp (transcranial electrical stimulation [TES]), and it worked (Merton and Morton, 1980Merton P.A. Morton H.B. Stimulation of the cerebral cortex in the intact human subject.Nature. 1980; 285: 227Crossref PubMed Google Scholar). A brief, high-voltage electric shock over the primary motor cortex (M1) produced a brief, relatively synchronous muscle response, the motor-evoked potential (MEP). It was immediately clear that this would be useful for many different purposes, but a problem with TES is that it is painful. Five years later, Barker et al. (Barker et al., 1985Barker A.T. Jalinous R. Freeston I.L. Noninvasive magnetic stimulation of human motor cortex.Lancet. 1985; 2: 1106-1107Abstract PubMed Google Scholar) solved a number of technical problems and showed that it was possible to stimulate brain (as well as peripheral nerve) with magnetic stimulation (transcranial magnetic stimulation [TMS]), and this could be accomplished with little or no pain. TMS has now come into wide use, and TES is still used for selective purposes. TMS is most frequently used as a research tool to study brain physiology, but it has some clinical utility and is also being developed as a therapeutic tool. For electrical stimulation between two electrodes placed on the scalp, current flows from anode to cathode. Near the scalp, the predominant direction of current flow is radial, but there are return loops that are tangential to the scalp. For magnetic stimulation, a brief, high-current pulse is produced in a coil of wire, called the magnetic coil (Figure 1). A magnetic field is produced with lines of flux passing perpendicularly to the plane of the coil, which ordinarily is placed tangential to the scalp. The magnetic field can reach up to about 2 Tesla and typically lasts for about 100 μs. An electric field is induced perpendicularly to the magnetic field. The voltage of the field itself may excite neurons, but likely more important are the induced currents. In a homogeneous medium, spatial change of the electric field will cause current to flow in loops parallel to the plane of the coil, which will be predominantly tangential in the brain. The loops with the strongest current will be near the circumference of the coil itself. The current loops become weak near the center of the coil, and there is no current at the center itself. Neuronal elements are activated by the induced electric field by two mechanisms. If the field is parallel to the neuronal element, then the field will be most effective where the intensity changes as a function of distance. If the field is not completely parallel, activation will occur at bends in the neural element. Magnetic coils may have different shapes (Figure 2). Round coils are relatively powerful. Figure-of-eight-shaped coils are more focal, producing maximal current at the intersection of the two round components. A figure-of-eight-shaped coil with the two components at an angle, the cone-shaped coil, increases the power at the intersection. Another configuration is called the H-coil, with complex windings that permit a slower fall-off of the intensity of the magnetic field with depth (Zangen et al., 2005Zangen A. Roth Y. Voller B. Hallett M. Transcranial magnetic stimulation of deep brain regions: evidence for efficacy of the H-coil.Clin. Neurophysiol. 2005; 116: 775-779Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar). In another design, the windings of a coil are around an iron core rather than air; this focuses the field and allows greater strength and depth of penetration (Epstein and Davey, 2002Epstein C.M. Davey K.R. Iron-core coils for transcranial magnetic stimulation.J. Clin. Neurophysiol. 2002; 19: 376-381Crossref PubMed Google Scholar). The results of TMS over M1 appear similar to those of TES. One difference, however, is that the latency of response is slightly shorter with TES, and explaining this difference opens the door to understanding the excitation mechanism of the two types of stimulation. It is likely that the mechanism of stimulation is similar in many parts of the brain, but we have detailed information only from M1, since the results can be measured in such detail. The difference in latency appears to be related to the nature of the descending volley in the corticospinal tract produced by the two types of stimulation (Figure 3) (Di Lazzaro et al., 1998Di Lazzaro V. Oliviero A. Profice P. Saturno E. Pilato F. Insola A. Mazzone P. Tonali P. Rothwell J.C. Comparison of descending volleys evoked by transcranial magnetic and electric stimulation in conscious humans.Electroencephalogr. Clin. Neurophysiol. 1998; 109: 397-401Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar). With TES, but typically not with TMS, there is an early D wave (direct wave) that reflects direct activation of descending axons. With both types of stimulation, there is a series of later I waves (indirect waves) that reflect synaptic activation of the corticospinal neurons. The mechanism of I wave production is not completely clear. I waves come at intervals of about 1.5 ms and are either generated by increasingly long polysynaptic networks or recurrent synaptic networks. Comparing the responses from rotating the magnetic coil in different angles, the largest MEPs are produced when the current in the brain is directed in the posterior-anterior direction (optimally at an angle perpendicular to the central sulcus), and the first wave produced is typically the I1 wave (at about a 1.5 ms interval from the D wave). When brain current is lateral-medial, there can be a D wave produced first. When the brain current is anterior-posterior, the I3 wave (at about a 4.5 ms interval from the D wave) can be produced first. MEPs are also larger and earlier when the muscle is contracting at baseline as opposed to when it is at rest. This is largely due to the fact that the motor neuron pool is at a higher level of activity and it is easier to provoke an increase of activation. Delivering a single pulse of TMS to the brain is very safe. Devices are now available that are capable of delivering high-frequency (1–50 Hz), repetitive TMS (rTMS). This can produce powerful effects that outlast the period of stimulation, inhibition with stimulation at about 1 Hz, and excitation with stimulation at 5 Hz and higher. rTMS, however, has the potential to cause seizures even in normal individuals. Safety guidelines describing limits for combinations of frequency, intensity, and train length have been developed, which should prevent most problems (Wassermann, 1998Wassermann E.M. Risk and safety of repetitive transcranial magnetic stimulation: report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, June 5–7, 1996.Electroencephalogr. Clin. Neurophysiol. 1998; 108: 1-16Abstract Full Text Full Text PDF PubMed Scopus (1287) Google Scholar). One of the obvious measurements that can be made with TMS is central motor conduction time. This is the time from motor cortex to the motor neuron pool in the spinal cord or brainstem. It is calculated by taking the latency of the MEP and subtracting the peripheral conduction time. Peripheral conduction time may be obtained in two ways. The first is to stimulate over the spine that activates the nerve roots in the intravertebral foramina. This is slightly in error since it misses the segment from the spinal cord to the foraminal region. The second method is to use the F wave, using the formula (F wave latency + M wave latency − 1)/2. Upon stimulating a motor nerve, the M wave is the direct muscle response, and the F wave is the muscle response produced by activation of the alpha motoneuron by the antidromic volley. This is more accurate, but a bit more time consuming (and painful). Using TMS, the brain can be briefly activated or briefly inhibited; in fact, likely both occur with each stimulus in differing amounts and with different time courses. This effect can be used to localize brain functions in both space and time. Applications were first in the motor system but have now been used to map sensory processes and cognitive function. Mapping the motor cortex by moving the coil over the surface of the scalp and recording MEPs from different muscles has been fairly straightforward. MEP mapping is an example of mapping in space with activation. Different body parts, such as arm and leg, are completely separate, but there is overlapping of muscles in the same body part (Wassermann et al., 1992Wassermann E.M. McShane L.M. Hallett M. Cohen L.G. Noninvasive mapping of muscle representations in human motor cortex.Electroencephalogr. Clin. Neurophysiol. 1992; 85: 1-8Abstract Full Text PDF PubMed Google Scholar) (Figure 4). Such studies have also allowed the demonstration of weak ipsilateral pathways to upper extremity muscles as well as the more powerful contralateral ones. Mapping of cranial nerve muscles has also been done, revealing innervations that are bilateral, bilaterally asymmetric, and unilateral, and also allowed confirmation of the innervation of orbicularis oculi by the cingulate cortex (Sohn et al., 2004Sohn Y.H. Voller B. Dimyan M. St Clair Gibson A. Hanakawa T. Leon-Sarmiento F.E. Jung H.Y. Hallett M. Cortical control of voluntary blinking: a transcranial magnetic stimulation study.Clin. Neurophysiol. 2004; 115: 341-347Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar). The patterns of muscle activity provoked by TMS have some physiological relevance, as these can be recognized as principal components of natural movement (Gentner and Classen, 2006Gentner R. Classen J. Modular organization of finger movements by the human central nervous system.Neuron. 2006; 52: 731-742Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar). While TMS of occipital cortex can produce phosphenes, it can also produce a transient scotoma. Scotoma mapping is an example of mapping in time with inhibition. In the first demonstration of this, subjects were shown briefly presented, randomly generated letters on a visual monitor, and TMS was delivered after the visual stimulus (Amassian et al., 1989Amassian V.E. Cracco R.Q. Maccabee P.J. Cracco J.B. Rudell A. Eberle L. Suppression of visual perception by magnetic coil stimulation of human occipital cortex.Electroencephalogr. Clin. Neurophysiol. 1989; 74: 458-462Abstract Full Text PDF PubMed Google Scholar). When delivered at an interval less than 40–60 ms or more than 120–140 ms, letters were correctly reported; but at intervals of 80–100 ms, a blur or nothing was seen. Presumably this indicates important visual processing during that time interval. Subsequent studies with more sensitive techniques indicate also an earlier period of suppression at about 30 ms, likely indicating the initial arrival of visual information to occipital cortex (Figure 5) (Corthout et al., 1999bCorthout E. Uttl B. Ziemann U. Cowey A. Hallett M. Two periods of processing in the (circum)striate visual cortex as revealed by transcranial magnetic stimulation.Neuropsychologia. 1999; 37: 137-145Crossref PubMed Scopus (67) Google Scholar). Additionally, TMS of V5 can selectively interfere with the perception of motion of a stimulus without impairing its recognition (Beckers and Zeki, 1995Beckers G. Zeki S. The consequences of inactivating areas V1 and V5 on visual motion perception.Brain. 1995; 118: 49-60Crossref PubMed Google Scholar, Walsh et al., 1998Walsh V. Ellison A. Battelli L. Cowey A. Task-specific impairments and enhancements induced by magnetic stimulation of human visual area V5.Proc. Biol. Sci. 1998; 265: 537-543Crossref PubMed Scopus (127) Google Scholar). Such data provide support to the concept arising from imaging studies that V5 is the motion perception region of the brain. Studies of vision have also revealed the importance of backprojections for perception. For example, there appears to be an important projection from V5 to V1. TMS over V5 can produce a moving phosphene, but when the V5 stimulus is followed by a TMS over V1 at an interval of 5–45 ms, the phosphene is degraded (Pascual-Leone and Walsh, 2001Pascual-Leone A. Walsh V. Fast backprojections from the motion to the primary visual area necessary for visual awareness.Science. 2001; 292: 510-512Crossref PubMed Google Scholar). Moreover, a similar backprojection exists from the frontal eye field (FEF) to V5. TMS over FEF impairs visual target discrimination (independent of its role in eye movements) (O'Shea et al., 2004O'Shea J. Muggleton N.G. Cowey A. Walsh V. Timing of target discrimination in human frontal eye fields.J. Cogn. Neurosci. 2004; 16: 1060-1067Crossref PubMed Scopus (93) Google Scholar) and, at an interval of 20–40 ms, can modify the phosphene threshold of TMS over V5 (Silvanto et al., 2006Silvanto J. Lavie N. Walsh V. Stimulation of the human frontal eye fields modulates sensitivity of extrastriate visual cortex.J. Neurophysiol. 2006; 96: 941-945Crossref PubMed Scopus (81) Google Scholar). High-frequency rTMS, at about 5–10 Hz, has been used as a more powerful stimulus to produce a brief period of inhibition in space and time. One example in the motor system is the study of the role of the supplementary motor cortex (SMA; more exactly, the mesial frontocentral cortex) in the production of sequential finger movements (Gerloff et al., 1997Gerloff C. Corwell B. Chen R. Hallett M. Cohen L.G. Stimulation over the human supplementary motor area interferes with the organization of future elements in complex motor sequences.Brain. 1997; 120: 1587-1602Crossref PubMed Scopus (158) Google Scholar). Stimulation over the SMA induced accuracy errors in complex, but not simple, sequences. Additionally, the errors occurred in subsequent elements of the sequence rather than those occurring at the time of the stimulation itself. The data support a critical role of the SMA in the organization of forthcoming movements in complex motor sequences. When patients who are blind from early life read Braille, they activate their occipital cortex, as demonstrated by functional neuroimaging (Sadato et al., 1996Sadato N. Pascual-Leone A. Grafman J. Ibanez V. Deiber M.P. Dold G. Hallett M. Activation of the primary visual cortex by Braille reading in blind subjects.Nature. 1996; 380: 526-528Crossref PubMed Scopus (550) Google Scholar). This is a striking example of transmodal plasticity, where somatosensory information gets routed to the visual cortex. The observation from neuroimaging alone, however, did not prove that the activity in visual cortex was being used for actual useful analysis of the information. Using rTMS during the reading showed that function was impaired when the visual cortex was disrupted (Cohen et al., 1997Cohen L.G. Celnik P. Pascual-Leone A. Corwell B. Falz L. Dambrosia J. Honda M. Sadato N. Gerloff C. Catala M.D. Hallett M. Functional relevance of cross-modal plasticity in blind humans.Nature. 1997; 389: 180-183Crossref PubMed Scopus (458) Google Scholar). Hence, TMS showed that the occipital activity was a necessary component of the processing. In a similar situation, studies with fMRI showed that the ventral premotor cortex was activated with counting of large numbers, but not small ones (up to 4) (Kansaku et al., 2007Kansaku K. Carver B. Johnson A. Matsuda K. Sadato N. Hallett M. The role of the human ventral premotor cortex in counting successive stimuli.Exp. Brain Res. 2007; 178: 339-350Crossref PubMed Scopus (11) Google Scholar). Correlative studies with rTMS showed that disruption of the ventral premotor cortex interfered with this counting behavior, showing that this region appears to be necessary for it. TMS has helped localize memory processes. For example, several studies give evidence for a role of the left dorsolateral prefrontal cortex (DLPFC) in working memory. Single-pulse TMS between presentations of letters impaired ability to match letters on a three-back task (Mull and Seyal, 2001Mull B.R. Seyal M. Transcranial magnetic stimulation of left prefrontal cortex impairs working memory.Clin. Neurophysiol. 2001; 112: 1672-1675Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). Low-frequency rTMS over the left DLPFC interfered with short-term memory for words, but not for faces (Skrdlantova et al., 2005Skrdlantova L. Horacek J. Dockery C. Lukavsky J. Kopecek M. Preiss M. Novak T. Hoschl C. The influence of low-frequency left prefrontal repetitive transcranial magnetic stimulation on memory for words but not for faces.Physiol. Res. 2005; 54: 123-128PubMed Google Scholar). Double-pulse TMS over DLPFC at 100 ms interval interfered with working memory for words after a reading task (Osaka et al., 2007Osaka N. Otsuka Y. Hirose N. Ikeda T. Mima T. Fukuyama H. Osaka M. Transcranial magnetic stimulation (TMS) applied to left dorsolateral prefrontal cortex disrupts verbal working memory performance in humans.Neurosci. Lett. 2007; 418: 232-235Crossref PubMed Scopus (14) Google Scholar). Consolidation of a simple motor skill, phasic pinch force, was disrupted by stimulation selectively over M1, without disruption of other aspects of motor function (Muellbacher et al., 2002Muellbacher W. Ziemann U. Wissel J. Dang N. Kofler M. Facchini S. Boroojerdi B. Poewe W. Hallett M. Early consolidation in human primary motor cortex.Nature. 2002; 415: 640-644Crossref PubMed Scopus (321) Google Scholar). Another study confirmed this finding, but failed to find a similar disruption of learning of movement dynamics in a force field, suggesting that only some types of motor consolidation occur in M1 (Baraduc et al., 2004Baraduc P. Lang N. Rothwell J.C. Wolpert D.M. Consolidation of dynamic motor learning is not disrupted by rTMS of primary motor cortex.Curr. Biol. 2004; 14: 252-256Abstract Full Text Full Text PDF PubMed Google Scholar). On the other hand, rTMS of M1 prior to learning of movement dynamics did interfere with consolidation without interfering with the learning itself (Richardson et al., 2006Richardson A.G. Overduin S.A. Valero-Cabre A. Padoa-Schioppa C. Pascual-Leone A. Bizzi E. Press D.Z. Disruption of primary motor cortex before learning impairs memory of movement dynamics.J. Neurosci. 2006; 26: 12466-12470Crossref PubMed Scopus (61) Google Scholar). There are numerous examples of how this technique has helped localize a wide variety of other cognitive functions; a few other findings are noted here. Low-frequency rTMS over either the right or left prefrontal cortex (but not the parieto-occipital cortex) impaired behavior on a task involving visuo-spatial planning (Basso et al., 2006Basso D. Lotze M. Vitale L. Ferreri F. Bisiacchi P. Olivetti Belardinelli M. Rossini P.M. Birbaumer N. The role of prefrontal cortex in visuo-spatial planning: A repetitive TMS study.Exp. Brain Res. 2006; 171: 411-415Crossref PubMed Scopus (15) Google Scholar). Disruption of the right (but not left) dorsolateral prefrontal cortex reduced a subject's willingness to reject an unfair offer, even though they still could appreciate the offer as unfair (Knoch et al., 2006Knoch D. Pascual-Leone A. Meyer K. Treyer V. Fehr E. Diminishing reciprocal fairness by disrupting the right prefrontal cortex.Science. 2006; 314: 829-832Crossref PubMed Scopus (280) Google Scholar). Selective stimulation over Wernicke's area improves cognitive function by shortening the latency for picture naming (Mottaghy et al., 2006Mottaghy F.M. Sparing R. Topper R. Enhancing picture naming with transcranial magnetic stimulation.Behav. Neurol. 2006; 17: 177-186Crossref PubMed Google Scholar). Various TMS measures of the motor cortex can evaluate different aspects of cortical excitability. Such measures are useful in understanding changes in brain physiology seen, for example, in the setting of cortical plasticity and brain disorders. Some of the common measures are listed here. The threshold for producing an MEP reflects the excitability of a central core of neurons that arises from the excitability of individual neurons and their local density. Since it can be influenced by drugs that affect Na and Ca channels, it must indicate membrane excitability (Ziemann, 2004Ziemann U. TMS and drugs.Clin. Neurophysiol. 2004; 115: 1717-1729Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar). Because the MEP is small, the threshold measure (with posterior-anterior brain current flow) reflects the influence of mainly the I1 wave. The recruitment curve is the growth of MEP size as a function of stimulus intensity and background contraction force. This measurement is less well understood but must involve neurons in addition to the core region activated at threshold. These neurons have higher threshold for activation, either because they are intrinsically less excitable or they are spatially further from the center of activation by the magnetic stimulus. These neurons would be part of the “subliminal fringe” and will contribute to I2 and later I waves. Short intracortical inhibition (SICI) and facilitation (ICF) are obtained with paired-pulse studies and reflect interneuron influences in the cortex.(Ziemann et al., 1996Ziemann U. Rothwell J.C. Ridding M.C. Interaction between intracortical inhibition and facilitation in human motor cortex.J. Physiol. 1996; 496: 873-881PubMed Google Scholar) In such studies, an initial conditioning stimulus is given—enough to activate cortical neurons, but small enough so that no descending influence on the spinal cord can be detected and there is no MEP. A second test stimulus, at suprathreshold level, follows at a short interval. Intracortical influences initiated by the conditioning stimulus modulate the amplitude of the MEP produced by the test stimulus. At very short intervals, less than 5 ms, there is inhibition, and at intervals between 8 and 30 ms, there is facilitation (Figure 6). SICI is likely largely a GABAergic effect, specifically GABA-A (Di Lazzaro et al., 2000aDi Lazzaro V. Oliviero A. Meglio M. Cioni B. Tamburrini G. Tonali P. Rothwell J.C. Direct demonstration of the effect of lorazepam on the excitability of the human motor cortex.Clin. Neurophysiol. 2000; 111: 794-799Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar). Paired magnetic pulses are given. In (A), from top down: conditioning pulse alone, conditioning and test pulse at 3 ms interval, conditioning and test pulse at 2 ms interval. The MEP from the test pulse without the conditioning pulse is indicated in the second and third traces with dotted lines. This shows that the conditioning pulse, although not producing an MEP itself, can lead to inhibition of the test pulse. (B) illustrates the average effect on MEPs with paired pulses at different intervals. Error bars denote ± 1 SEM. There is inhibition at 1–5 ms interval and facilitation at 10 and 15 ms interval. From Kujirai et al., 1993Kujirai T. Caramia M.D. Rothwell J.C. Day B.L. Thompson P.D. Ferbert A. Wroe S. Asselman P. Marsden C.D. Corticocortical inhibition in human motor cortex.J. Physiol. 1993; 471: 501-519PubMed Google Scholar, with permission. The silent period (SP) is a pause in ongoing voluntary EMG activity produced by TMS. While the first part of the SP is due in part to spinal cord refractoriness, the latter part is entirely due to cortical inhibition. This type of inhibition seems to be mediated by GABA-B receptors (Werhahn et al., 1999Werhahn K.J. Kunesch E. Noachtar S. Benecke R. Classen J. Differential effects on motorcortical inhibition induced by blockade of GABA uptake in humans.J. Physiol. 1999; 517: 591-597Crossref PubMed Scopus (400) Google Scholar). SICI and the SP clearly reflect different aspects of cortical inhibition. Long intracortical inhibition (LICI) is assessed with paired suprathreshold TMS pulses at intervals from 50 to 200 ms. LICI and SICI differ, as demonstrated by the facts that with increasing test pulse strength, LICI decreases but SICI tends to increase, and that there is no correlation between the degree of SICI and LICI in different individuals (Sanger et al., 2001Sanger T.D. Garg R.R. Chen R. Interactions between two different inhibitory systems in the human motor cortex.J. Physiol. 2001; 530: 307-317Crossref PubMed Scopus (249) Google Scholar). Interestingly, LICI appears to inhibit SICI and shows some interaction of inhibitory mechanisms within the human motor cortex (Sanger et al., 2001Sanger T.D. Garg R.R. Chen R. Interactions between two different inhibitory systems in the human motor cortex.J. Physiol. 2001; 530: 307-317Crossref PubMed Scopus (249) Google Scholar). The mechanisms of LICI and the SP may be similar. Short and long afferent inhibition (SAI and LAI) are produced at latencies of about 20 ms and 200 ms, respectively, after somatosensory stimulation of the hand (Di Lazzaro et al., 2000bDi Lazzaro V. Oliviero A. Profice P. Pennisi M.A. Di Giovanni S. Zito G. Tonali P. Rothwell J.C. Muscarinic receptor blockade has differential effects on the excitability of intracortical circuits in the human motor cortex.Exp. Brain Res. 2000; 135: 455-461Crossref PubMed Scopus (125) Google Scholar). SAI has been demonstrated to be mainly muscarinic by its selective blockage by scopolamine. Transcallosal inhibition (TCI) is the inhibition produced in the primary motor cortex in one hemisphere by stimulation of the opposite primary motor cortex. Inhibition occurs at intervals of 8–50 ms (Ferbert et al., 1992Ferbert A. Priori A. Rothwell J.C. Day B.L. Colebatch J.G. Marsden C.D. Interhemispheric inhibition of the human motor cortex.J. Physiol. 1992; 453: 525-546PubMed Google Scholar). Premotor cortex inhibition is produced by stimulation of the premotor cortex either in the same or opposite hemisphere (Civardi et al., 2001Civardi C. Cantello R. Asselman P. Rothwell J.C. Transcranial magnetic stimulation can be used to test connections to primary motor areas from frontal and medial cortex in humans.Neuroimage. 2001; 14: 1444-1453Crossref PubMed Scopus (123) Google Scholar, Mochizuki et al., 2004Mochizuki H. Huang Y.Z. Rothwell J.C. Interhemispheric interaction between human dorsal premotor and contralateral primary motor cortex.J. Physiol. 2004; 561: 331-338Crossref PubMed Scopus (87) Google Scholar). TMS can be used in a variety of ways to induce plastic changes in the brain, and this can be utilized to assess the capability for plasticity (Table 1). Additionally, induced plastic changes can be exploited therapeutically, and this aspect will be discussed below. An effective way to modulate synaptic efficacy is to activate a cell with two or more inputs at close to the same time. If the stimuli come on the same synaptic pathway, this is called homosynaptic, and, if on different synaptic pathways, this is called heterosynaptic. Increased synaptic strength is called long-term potentiation (LTP); decreased synaptic strength is called long-term depression (LTD).Table 1Summary of Noninvasive Methods for Excitation and InhibitionMethodExcitatory ModeInhibitory ModerTMShigh frequency, ≥5 Hzlow frequency, 0.2–1 HzTBSintermittentcontinuoustDCSanodalcathodalPASsynchronous heterosynaptic stimulationasynchronous heterosynaptic stimulation Open table in a new tab rTMS at slow rates, approximately between 0.2 and 1 Hz, will cause a decrease in brain excitability (Chen et al., 1997Chen R. Classen J. Gerloff C. Celnik P. Wassermann E.M. Hallett M. Cohen L.G. Depression of motor cortex excitability by low-frequency transcranial magnetic stimulation.Neurology. 1997; 48: 1398-1403Crossref PubMed Google Scholar). rTMS at faster rates, approximately 5 Hz or faster, will cause an increase in brain excitability (Pascual-Leone et al., 1994Pascual-Leone A. Valls-Solé J. Wassermann E.M. Hallett M. Responses to rapid-rate transcranial magnetic stimulation of the human motor cortex.Brain. 1994; 117: 847-858Crossref PubMed Google" @default.
• W2041949619 created "2016-06-24" @default.
• W2041949619 creator A5024973951 @default.
• W2041949619 date "2007-07-01" @default.
• W2041949619 modified "2023-10-18" @default.
• W2041949619 title "Transcranial Magnetic Stimulation: A Primer" @default.
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