Transcranial Magnetic Simulation

Transcranial magnetic stimulation (TMS) is a non-invasive method of activating regions of the brain by small electric currents generated by a magnetic pulse.  Over  the  last  25  years,  TMS  has  become an  integral  technique  for  studying  brain  function in  humans  in  both  healthy  and  diseased  states and  is  a  valid  and  reliable  technique  for  investigating brain mechanisms of popular interventions in  sport  and  exercise  psychology,  such  as  action observation and imagery.

Principles of TMS

Based  on  the  principle  of  electromagnetic  induction,  a  brief,  high-intensity  current  is  passed through  a  coil  composed  of  copper  windings encased  in  plastic,  creating  a  transient  magnetic field. The magnetic field has the capacity to induce a  small  electric  current  in  nearby  excitable  tissue,  including  brain  cells,  peripheral  nerves,  and muscle. When placed on the surface of the scalp, the induced current can cause neurons in the outer part of the cortex to fire. Because magnetic fields pass through the body without activating sensory afferents like cutaneous and pain receptors, TMS has been referred to as ouchless stimulation.

Types of TMS

Three  distinct  types  of  TMS  are  routinely  used. The  first,  single  pulse  TMS,  when  applied  to  the primary  motor  cortex,  produces  muscle  activity known as a motor evoked potential (MEP), which can  be  recorded  in  the  electromyogram.  Single pulse TMS is routinely used in research to monitor  changes  in  motor  pathways,  for  example,  in response to exercise or skill acquisition. Clinically, single pulse TMS is used to assess the integrity of corticospinal pathways for diagnostic purposes in conditions such as stroke, spinal cord injury, multiple sclerosis, and motor neuron disease.

In   the   second   type,   paired   pulse   TMS, two   stimulation   pulses   are   delivered   either through  the  same  stimulating  coil  or  through two  separate  coils.  Delivering  two  stimuli  in  a conditioning-test  format  through  the  same  coil allows  local  inhibitory  or  excitatory  circuits to  be  assessed,  depending  on  the  interstimulus interval. For short intervals (2–4 milliseconds) a subthreshold  conditioning  stimulus  followed  by a suprathreshold test stimulus is thought to activate short-interval intracortical inhibition (SICI) networks  within  the  local  circuits  of  the  motor cortex. In contrast, longer intervals (8–20 milliseconds) are thought to activate local excitatory circuits  and  can  be  used  to  assess  intracortical facilitation (ICF).

Paired  pulses  delivered  by  two  separate  coils can  be  used  to  assess  the  functional  connections between  regions  of  the  cortex.  For  example,  a conditioning pulse delivered to the dorsal premotor area has an effect on a test pulse delivered to the  primary  motor  cortex  8  milliseconds  later.  In this manner, the functional connections of various motor association areas can be examined and their role  in  important  processes  such  as  skill  acquisition can be determined.

The  third  type  of  TMS  involves  repetitive stimulation, known as rTMS. The effects of rTMS outlast  the  period  of  stimulation  by  several  minutes  and,  depending  on  the  specific  stimulation profile  used,  can  induce  long  lasting  potentiation or depression of cortical circuits. As a result, rTMS can  be  used  to  study  such  higher  brain  functions as language, memory, and attention. Furthermore, rTMS  can  result  in  behavioral  changes  and  may be effective for the treatment of a number of neurological  and  psychiatric  disorders.  For  example, in  randomized,  controlled  placebo  trials,  rTMS treatment of depression has been shown to have a comparable effect to pharmacological treatments.

The  rTMS  is  also  being  examined  for  use  in relation to a number of movement-related pathologies, including rehabilitation for aphasia (the loss of the ability to understand or produce speech) and motor function following stroke, improvement of motor  function  in  Parkinson’s  disease,  and  alleviating  the  effects  of  dystonia  (abnormal  muscle tone). Other clinical applications of rTMS include the management of chronic pain, migraines, bipolar disorder, and tinnitus.

TMS in Sport and Exercise Psychology

In  sport  psychology-related  work,  TMS  has  been used  to  examine  the  corticospinal  changes  that occur in response to action observation and motor imagery,  two  fundamental  techniques  used  in skill  acquisition  and  sport  psychology  practice. Observation of an action performed by self or others, in the absence of any recordable overt movement or muscle activity, modulates the excitability of the corticospinal pathway in humans, resulting in  increased  amplitude  of  MEPs  specific  to  the muscles involved in the observed action. Similarly, motor   imagery   has   been   shown   to   result   in increased MEPs in response to TMS. Interestingly, the  increases  in  excitability  of  the  corticospinal pathway  are  greatest  when  the  feel  of  the  movement  is  imagined  (kinesthetic  imagery)  compared with when the look of the movement is imagined (visual imagery). These findings confirm that both action  observation  and  motor  imagery  result  in functional  changes  in  the  central  nervous  system that may explain their effectiveness. It is also likely that  TMS  can  be  used  to  examine  the  quality  of imagery since the degree of corticospinal modulation  is  correlated  to  the  vividness  of  imagery  as assessed by self-report questionnaires.

Safety Considerations

Although  considered  safe,  TMS  is  not  without risk.  Of  primary  concern  is  the  risk,  although small,  of  inducing  seizures.  Of  the  small  number of TMS-induced seizures reported in the literature,  in  many  incidences  there  were  predisposing  factors, including brain lesions and familial history of epilepsy. Other associated risks include discomfort as a result of stimulation of the nerves and muscles of the scalp resulting in localized muscle contraction, headaches, and some mild effects on hearing (as there is a loud click when the coil discharges). TMS should never be applied in conjunction with metal electrodes or in persons with metal implants near  the  site  of  stimulation  as  the  magnetic  discharge  can  cause  heating  in  the  metal.  In  2009, the Safety of TMS Consensus Group issued comprehensive  guidelines  for  the  application  of  TMS in research and clinical applications and considers the overall risk of TMS to be low.

References:

  1. Hallett, M. (2007). Transcranial magnetic stimulation: A primer. Neuron, 55, 187–199.
  2. Loporto, M., McAllister, C., Williams, J., Hardwick, R., & Holmes, P. (2011). Investigating central mechanisms underlying the effects of action observation and imagery through transcranial magnetic stimulation. Journal of Motor Behavior, 43, 361–373.
  3. Rossi, S., Hallett, M., Rossini, P. M., Pascual-Leone, A., & Safety of TMS Consensus Group. (2009). Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research. Clinical Neurophysiology, 120(12), 2008–2039.

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