Transcranial magnetic stimulation (TMS) is the use of powerful rapidly changing magnetic fields to induce electric fields in the brain by electromagnetic induction without the need for surgery or external electrodes. Repetitive transcranial magnetic stimulation is known as rTMS.
The International Federation of Clinical Neurophysiology has developed the following description of TMS and rTMS:
Technical developments in the devices used for TMS made it possible in the late 1980's to apply TMS in trains of multiple stimuli per second. This form of TMS is called repetitive TMS or rTMS. In rTMS, stimuli are applied to the same brain area several times per second during several consecutive seconds. The number of stimuli per second, the strength of the stimuli, the duration of the train of stimulation, the interval between trains, the total number of trains and the total number of stimuli in a given session or to a given brain position can all be varied. All these aspects of rTMS are referred to as stimulation parameters.... Repetitive TMS can be used to study how the brain organizes different functions such as language, memory, vision, or attention. In addition, rTMS seems capable of changing the activity in a brain area even beyond the duration of the rTMS application itself. In other words, it seems possible to make a given brain area work more or less for a period of minutes, hours, days or even weeks when rTMS is applied repeatedly several days in a row. This has opened up the possibility of using rTMS for therapy of some illnesses in neurology and psychiatry. However, this therapeutic potential of rTMS is still being studied and should not be considered proven.
TMS in research
TMS was designed as a brain mapping tool in the 1980s to be used combined with MRI and EEG. By stimulating different points of the cortex and recording responses, e.g., from muscles, one may obtain maps of functional brain areas. By measuring EEG, one may obtain information about the healthiness of the cortex (its reaction to TMS) and about area-to-area connections.
One reason TMS is important in neuroscience is that it can demonstrate causality. A noninvasive mapping technique such as fMRI allows researchers to see what regions of the brain are activated when a subject performs a certain task, but this is not proof that those regions are actually used for the task; it merely shows that the a region is associated with a task. If activity in the associated region is suppressed with TMS stimulation and a subject then performs worse on a task, this is much stronger evidence that the region is used in performing the task.
For instance, subjects asked to memorize and repeat a stream of numbers would likely show, via fMRI, activation in the prefrontal cortex (PFC), which seems to be important in short-term memory. If the researcher then interfered with the PFC via TMS, the subjects' ability to remember numbers would decline, and the researcher would have evidence that the PFC is important for short-term memory, because reducing subjects' PFC capability led to reduced short-term memory.
How TMS works
The exact details of how TMS functions are still being explored, but the MIT Technology Review listed some potential mechanisms:
A doctor typically holds a powerful magnet over the frontal regions of the patient's skull and delivers magnetic pulses for a few minutes a day, over the course of a few weeks. The treatment alters the biochemistry and firing patterns of neurons in the cortex, the part of the brain nearest the surface. Preliminary research indicates that the treatment affects gene activity, levels of neurotransmitters like serotonin and dopamine, and the formation of proteins important for cellular signaling—any of which could play a role in alleviating depression. What's more, magnetic stimulation seems to affect several interconnected brain regions, starting in the cortex and moving to the deep brain, where new cell growth may be important in regulating moods. (Technology Review, March 2004 PDF)
TMS as therapy
TMS is currently under study as a treatment for severe depression, auditory hallucinations, migraine headaches and tinnitus. It is particularly interesting as it may provide a viable treatment to certain aspects of drug resistant mental illness, particularly as an alternative to electroconvulsive therapy. TMS is also under investigation for the treatment of drug-resistant epilepsy.
Although research in this area is in its infancy, there is now strong evidence that TMS is an effective treatment for both depression and auditory hallucinations, with more symptoms and disorders being researched. There is also some initial evidence that TMS may be used to temporarily induce particular mental skills, much like those of an autistic savant.TMS is currently being studied in a large, multi-center clinical trial being conducted in numerous sites in the U.S., Canada and Australia. This trial will involve approximately 300 patients and will be the most rigorous test of TMS in depression to date. Patients with depression who are interested in the trial can get more information at www.neuronetics.com
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Transcranial Magnetic Stimulation".