Transcranial magnetic stimulation

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.

TMS in research

TMS was originally developed as a tool in brain research, and has been used to stimulate or suppress brain activity in experiments on human subjects. This means researchers can effectively "turn off" a certain region of the brain and observe a subject's cognitive abilities. When used briefly, TMS is not believed to have long-term effects or be dangerous – it cannot interfere with critical brain functions (like breathing) because the magnetic fields reach only the forebrain, which does not perform functions critical for life support.

When properly combined with MRI and EEG, TMS becomes a brain mapping tool. 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 supressed 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.

TMS as therapy

TMS is currently under study as a treatment for severe depression and auditory hallucinations. It is particularly interesting as it may provide an 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.