Historically, research into the human brain often depended on special patients, who had suffered damage in certain areas of their brains due to accidents, wounds, or brain haemorrhages, for example. Indeed, with specific damage in certain specific brain areas, quite specific symptoms may also occur. For example, limitations in certain forms of perception, behavior, or cognition. By mapping out the exact symptoms and identifying the specific brain areas affected, scientists were able to explore, bit by bit, the relationships between brain and mind.
With the advent of fMRI, EEG, and other research methods to map the healthy human brain, much changed. But even today, the brains of patients with brain damage are examined using the same method, because it is still very informative to see exactly what happens to behavior and cognition, when something goes wrong in a specific brain area.
Nowadays, however, there is also a method that applies almost the same logic as using brain damage patients to people with completely healthy brains. Transcranial magnetic stimulation (TMS) is a technique in which magnetic pulses are sent into the brain. This can primarily stimulate brain areas in the outer layer, the neocortex. If the magnetic pulses are strong enough, neurons in the underlying brain area can be made active; the brain is then briefly ‘activated’ there.
The special thing about TMS, however, is that it can have different kinds of effects on a brain region, or even a network of regions. By repeatedly offering the magnetic pulses in a certain rhythm, the stimulated area can be made temporarily more or less receptive to signals. Simplistically, this is often translated as: a brain area can be temporarily activated or inhibited with TMS. It temporarily works better than usual, or a little less well than usual, to put it even more simply. What that means for behavior and cognition is always the question. The answer depends on the type of TMS, the stimulated area, the behavior that the test subjects or patients perform, and often it also depends on the test subject itself. Every brain is just a little bit different.
On average, there are some rules of thumb. You can say that rhythmic TMS (repetitive TMS: rTMS) with 1 pulse per second (1 Hertz) or less usually results in the stimulated area being inhibited for a while. Stimulation with more pulses per second, for example 10 Hertz, usually actually leads to a temporarily more active brain area, which reacts more strongly to incoming signals than it normally would. The latter is for example the inspiration for research into improving human performance on certain tasks using TMS, or applications of TMS in the clinic to boost certain brain areas that are not active enough. There are also clinical applications of TMS that attempt to make areas less active, for example, if they are overactive and therefore cause certain symptoms in patients. There is a great deal of research into the possible clinical applications of TMS. In the treatment of depression, for example, the findings are already so convincing that TMS is now used as a treatment method in several countries around the world, including the Netherlands.
In addition to clinical applications, research is also being conducted with TMS in healthy brains. Sometimes this research takes on a form that is very similar to traditional brain research, whereby a temporary disruption is caused in a specific brain area - a kind of temporary simulated 'damage' - in order to measure and evaluate the consequences of this on certain cognitions or behavior. It should be noted that, as mentioned, this is always temporary; there are no lasting effects from this type of TMS.
Even more temporary are the effects in research with event-related TMS. In this type of research, often only a few pulses, or even a single pulse, are administered to a brain area to disrupt the function of that area at that very moment. Here we are talking about effects that last only milliseconds, too short to notice. However, if that brain area is trying to perform a certain task at exactly that moment, that brief disruption can indeed lead to measurable effects. The most obvious example is the perception of briefly presented pictures. Many experiments have confirmed that if a TMS pulse is administered to visual brain areas about 100 milliseconds after a picture briefly appeared on a screen, subjects sometimes do not perceive that picture at all!
Since TMS affects the brain, it is important to look closely at whether there is any danger involved. In doing so, especially in the early days of the method, it was noted that the most serious risk of TMS was the possible induction of epileptic seizures. However, international guidelines have been established based on research. If an epileptic seizure is induced at all, it is in principle a one-time event with no lasting consequences. But in reality, the risk of this is almost negligible if the TMS applied is within the guidelines. Therefore, TMS is increasingly used in research and the clinic, in the Netherlands and abroad. If you would like to experience TMS yourself in research, or are considering treatment, just be well informed. But basically, this exciting method is especially interesting!
Author: Tom de Graaf (translated by Melanie Smekal)