Transcranial magnetic stimulation: a primer - Hallet - 2007 - Article

Transcranial magnetic stimulation (TMS) is a noninvasive technique to stimulate the brain. Short, magnetic pulses of high intensity are directed to a specific location on the head. The magnetic field activates or inhibites the electrical activity in the brain, dependent on the stimulation frequency.

TMS can be applied on all parts of the brain directly beneath the skull. However, most studies are directed around the motor cortex.

TMS can be used to get more information about different sensory, motor, and cognitive functions. Besides, also therapeutic treatment is possible, because brain activity can be long term influenced with repeated stimulation.

What is meant by magnetic stimulation?

About thirty years ago, transcranial electrical stimulation (TES) was applied to the motor cortex. It was possible to observe a motor-evoked potential (MEP). Unfortunately, TES is painful. The resolution turned out to be magnetic stimulation, which works with pulses that are not or hardly painful.

When stimulating electrically, an electric current between to electrodes (anode and cathode) will be placed on the skull. When stimulation magnetically, a spindle will be placed on the head. The direction of the electric field is perpendicular on the magnetic field (also, see figure 1 in the article). There are different kinds of spindles: round ones, more powerful ones, ones formed as an 8, ones which are fit for focal stimulation, and ones which are formed as an H (for deeper stimulation). There's also a spindle which is draped around an iron core, to better direct the field, which can deepen and strenghten the stimulation.

Both TES and TMS effects are comparable. However, the latency time with TMS is somewhat shorter. This is problably caused by the type of brainwaves that are produced. With TES, directly after the stimulation direct waves (D-waves) emerge. With both techniques, somewhat later indirect waves (I-waves) emerge. Besides this, the angle at wich the spindle is plaed on the head, influences the stimulation. Also, the brain activates spontaneously. For example, it's easier to stimulate a reaction when the neurons are already active before the stimulation starts.

When considering the safety of TMS, hardly any negative effect is found. There is, however, a slight chance of an epileptic episode. Some protocols limit the frequency, the intensity, and the time, to ensure safety.

What is corticomotor conduction?

The motor conduction time can be measured with TMS. The centrol motor conduction time is the time a signal needs to get from the motor cortex to the motor neurons in the spine. The periferal conduction time is the time neurons in the spine need to signal to the periferal neurons.

How can activation, inhibition, and mapping be made visible?

The planning of the motor cortex can be made visible by studying the reactions on stimulation (for example with MEPs).

Also, sensoric reactions can be activated. Stimulation of the visual cortex can cause phosphenes and tempory facial problems. The perception of movement can alos be disturbed by V5 stimulation. When studying this, the organization of the visual functions can be made visible.

TMS can also localize memory processes. The left dorsolateral prefrontal cortex (DLPFC) seems to be a structure of importance for the memory processes. Stimulation of the motor cortex can disturb the consolidation of motoric learning.

Wat are the aspects of cortical excitation?

TMS can provide insights into the different aspects of the cortical excitation in the motor cortex:

• The threshold to produce a MEP.

• The recruitment curve.

• The short intracortical inhibition (SICI) and facilitation (ICF).

• The silent period (SF).

• The long intracortical inibition (LICI).

• Short and long afferent inhibition (SAI and LAI).

• Transcallosal inhibition (TI).

• Inhibition of the premotor cortex.

How can TMS be used to cause changes in the brain?

TMS can be used to cause changes in the brain. This happened through processes like long term potentiation (LTP) and long term depression (LTD).

Low frequencies between .2 and 1 Hz cause a decrease in brain activity. Higher frequencies like 5 Hz cause an increase in brain activity. Besides, TMS can be used repeatedly. By applying series on a high frequency (like 50 Hz) superimposed on a frecency of 5 Hz, we call this theta burst stimulation (TBS). When doing this every ten seconds, inhibition is created. However, when applying this every fourty seconds, excitation is created.

Another method to facilitate excitation is transcranial direct current stimulation (tDCS). Two electrodes induce an electrical pulse through the brain. Anodal stimulation creates excitation, while cathodal stimultion creates inhibition.

Paired associative stimulation (PAS) enhances hetrosynaptic plasticity. Both periferal stimulation and TMS are provided at the same time.

How is brain activity measured?

Diverse non-invasive techniques can be used to measure brain activity. Every method gives insight in a different aspect of brain activity. To get a direct measure of brain activity, EEG and MEG can be used. However, with these techniques, the spatial resolution is relatively low. With PET and fMRI, a higher spatial resolution is possible, but the temporal resolution is lower.

What is the clinical use of TMS?

TMS can be used in research. The effects of TMS on the motor processes can give insight in, for example, motor deviations, like multiple sclerosis, the motor effects of CVAs, and compressive myelopathy.

Furthermore, studies have been executed in the neuroscientific field which use TMS and study diverse cortical mechanisms like epilepsy, Parkinsons disease, Alzheimers disease, ataxy, and migraine.

Besides research, TMS can also be of use in clinical settings. By repeated stimulation dysfunctional brain processes can be changed long term. TMS can be of therapeutical use to those suffering from neurological diseases like Parkinsons, dystonia, and CVA's, but also from psychiatric diseases like depression.