They did not carry information about sensory stimuli, but changed the readiness of neurons to receive and process information about these stimuli. Over time, this term was adopted by a new branch of medicine dealing with directly acting on neurons with the help of a magnetic field and an electric current.
Transcranial magnetic stimultaion (TMS) is a neuromodulation technique that has been used for 30 years. In this technique, an alternating magnetic field induces an electrical excitation in a given area of the cerebral cortex through electromagnetic induction. A stimulator that generates a current pulse is connected to a magnetic coil that adheres to the skull. The stimulator contains a capacitor that generates a current flow with a voltage of up to 3000 V and a current of up to 20,000 A. parallel to the head, which in turn leads to depolarization of the cell membrane of neurons and activation of the cortex located directly under the urethra. This excitation can spread through synapses and subsequent neurons to the structures of the brain with which the given area of the cortex is connected. TMS therefore enables the brain to be electrically stimulated without electrodes.
The response to stimulation of the cerebral cortex with a single TMS pulse depends on the area being stimulated. If it is the motor cortex, the corresponding muscle group will contract. Stimulation of the motor cortex in the place that causes the movement of the fingers or toes is performed in order to check that in the unconscious patient there is no rupture of the spinal cord (whether the impulse from the motor cortex can pass through the intact cortico-spinal tract). If the impulse is applied to the visual cortex, it can trigger phosphenes – flashes of light seen with closed eyes. A single impulse applied to an appropriate area of the cortex may disrupt the interactions of neurons taking place in this area at the moment – in this way, for example, it is possible to prove the involvement of a given area of the cortex in selected perceptual processes.
TMS also makes it possible to map connections with the brain – the effects can be seen not only in the stimulated structure, but also in the areas to which this structure sends connections, as well as silencing or stimulating the examined area of the cerebral cortex. The latter can be achieved using not a single impulse, but a series of impulses – the so-called rTMS (repetitive TMS). Pulses delivered at a frequency of about 4 Hz have a stimulating effect. Such effects last from several to several dozen minutes after the end of stimulation. In the case of inhibitory stimulation, we create what is called a “virtual lesion”, that is, a temporary blockage of the activity of a selected area of the cortex. The first demonstration of the possibility of this method was the temporary inhibition of speech due to “virtual lesion” of the Broc field in healthy volunteers. In a well-known study on brain plasticity, subjects were blindfolded for a week and learned to read Braille. After a week it turned out that tactile stimuli (embossed Braille letters are not reacted by the somatosensory cortex, but the visual cortex – there was a plastic reorganization of the brain. It was proved by disrupting the activity of the visual cortex with TMS – the subjects were then unable to distinguish Braille letters.
What to treat with TMS?
Specialists have been looking for therapeutic applications for TMS for years. Very high hopes were associated with the inclusion of TMS stimulation in neurorehabilitation after a stroke. The most frequently studied variant was TMS silencing of interhemispheric inhibition. In the motor and sensory areas of the cerebral cortex, the influence of one hemisphere of the brain on the other is inhibitory. It was suspected that, for example, in the case of unilateral damage to the motor cortex by stroke, the inhibitory influences from the other, undamaged hemisphere could weaken the plastic changes that rehabilitation wanted to cause in the undamaged areas around the stroke site. Some attempts have improved the rehabilitation process, but there have also been numerous failures. Attempts have also been made in the treatment of aphasia (weak effect) and neglect (more promising results).
Chronic pain reduction has been the subject of a series of experiments in which TMS has been attempted to suppress neuropathic pain. Despite positive results in small experimental groups, the meta-analysis showed no clinical efficacy.