Transcranial magnetic stimulation and cognitive neuroscience

Walsh, V. & Cowey, A. (October 2000). Transcranial magnetic stimulation and cognitive neuroscience, Nature Reviews, Volume 1, 73-79.

Transcranial magnetic stimulation (TMS) is an investigative tool used in neuroscience to transiently interfere with brain functions, temporarily interrupting normal brain activity in a restricted region of the brain. This non-surgical technique introduces random activity as disorder into the information processing system, disrupting cognitive task performance. When coupled with a neuroimaging technique such as fMRI or EEG, TMS has proven very useful in experimentation to elucidate brain function.

Throughout the history of neuropsychology, patients with brain injuries ranging from mild to major have provided critical insights into the way the brain operates. As a mechanism of interference, TMS is unique in that it can be used to create 'virtual lesions', short-lived and reversible. Real lesioned brains will have undergone months if not years of neural reorganization following an accident to compensate for the deficit. Since TMS interference is typically not prolonged, no such opportunities exist for long-term reorganization processes to kick in. While this may seem like a poor approximation of classical lesioning, researchers believe that studying these types of temporary lesions may in fact be more useful to scientific examination without the introduction of the brain's compensatory coping strategies.

In addition to pure interference studies, the effects of TMS at the primary site of application have been shown to correspond very well with activation produced by self-induced behavior. For example, in one study activation was presented to the motor cortex above the motor threshold, creating behavior in the arm. When subjects were asked to reproduce the same arm movement voluntarily, there was great similarity between the two compared brain activations. However, one restriction of non-surgical TMS is that stimulation is limited to superficial cortical regions.