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.
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.
- Hallett, M. (2007). Transcranial magnetic stimulation: A primer. Neuron, 55, 187–199.
- 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.
- 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.