Transcranial magnetic stimulation (TMS) is a neuromodulation technique that has been used for 30 years. This method uses an alternating magnetic field to induce electrical excitation in a specific area of the cortex, via electromagnetic induction. A stimulator generates a pulsed current, connected to a magnetic coil placed against the skull. The stimulator contains a capacitor that produces a voltage current of up to 3000 V and an intensity reaching 20,000 A. This leads to the depolarization of the neuronal cell membrane and the activation of the cortex directly beneath the stimulated area. This excitation can propagate through synapses and neighboring neurons to the brain structures to which this cortical area is connected. TMS thus makes it possible to stimulate the brain electrically, without the use of electrodes.
Stimulation of the motor cortex in the region responsible for the movement of the fingers or toes is performed in order to verify that, in an unconscious patient, there is no rupture of the spinal cord (to see if the impulse originating from the motor cortex can travel through the intact corticospinal tract). A single stimulus applied to an appropriate area of the cortex can also disrupt ongoing neuronal interactions in that area. Thus, for example, it is possible to demonstrate the involvement of a given cortical region in certain perceptual processes.
TMS also makes it possible to map connections within the brain: effects can be observed not only in the stimulated structure, but also in the regions to which that structure is connected. It also makes it possible to stimulate or inhibit a specific region of the cortex, through a series of pulses – called rTMS (repetitive TMS). Pulses applied at a frequency of about 4 Hz have a stimulating effect, which can last from a few minutes to several tens of minutes after the end of the stimulation. In the case of inhibitory stimulation, what is called a “virtual lesion” is created, i.e., a temporary blockage of the activity of a chosen region of the cortex. The first demonstration of this method was the temporary inhibition of language by creating a “virtual lesion” of Broca’s area in healthy volunteers. In a famous study on brain plasticity, subjects were blindfolded for a week and learned to read braille. After a week, it turned out that tactile stimuli (raised braille letters) were not processed by the somatosensory cortex, but by the visual cortex – a plastic reorganization of the brain had taken place. This was proven by interrupting the activity of the visual cortex with TMS: subjects were then unable to distinguish the braille letters.
What Is TMS Used For in Therapy?
For years, specialists have been seeking therapeutic applications for TMS. Great hopes have been placed on adding TMS to neurorehabilitation after a stroke. The most frequently studied version is the suppression, via TMS, of interhemispheric inhibition. In the motor and sensory areas of the cerebral cortex, the effect of one hemisphere on the other is inhibitory. It was thought that, for example, in the case of a unilateral lesion of the motor cortex following a stroke, the inhibitory effects of the intact hemisphere could hinder the plastic changes that rehabilitation aimed to trigger in the intact areas around the stroke site. Some attempts improved the rehabilitation process, but many failures were also observed. Trials have also been conducted to treat aphasia (with little effect) and spatial neglect (with more promising results).
The reduction of chronic pain has been the subject of several experiments in which TMS was used to decrease neuropathic pain. Despite positive results in small experimental groups, meta-analyses have not shown clinical efficacy.
