Transcranial magnetic stimulation
From Psy3242
Contents |
What It Is
Transcranial Magnetic Stimulation (abbreviated to TMS) is a technique used in a wide variety of neurological experimental tests, which creates a temporary lesion in a specific brain area. It has been extremely influential in demonstrating neurological causality (seeing what damage to which areas of the brain produce what effect). First used in 1985 in England, TMS has proven its worth in numerous neurological studies not only in determining causal relationships, as previously stated, but also for being a relatively painless procedure, in contrast to earlier electrical stimulative methods. However, a distinction should be made between TMS and (repetitive) rTMS; the latter of these produces longer-lasting lesions. rTMS, also, has been shown in some studies to be an effective treatment for some psychiatric and neurological disorders, including strokes, Parkinson's Disease, and depression.
How It Works
TMS uses rapidly fired and changing magnetic fields (called electromagnetic induction) delivered by way of a metallic, figure-eight "coil" over the scalp, to disrupt the neurons in the intended area. The electromagnetic pulse in single TMS causes the neurons in the intended area to "depolarize and discharge an action potential". The subject's behavior and perception are then slightly altered--she or he may notice phosphenes, or flashes of light, if the pulse was directed at his/her primary visual cortex, or (s)he may demonstrate an increased reaction time at a cognitive task. The effects are short-lived, normally gone very shortly after the neurons are stimulated.
rTMS produces a longer-lasting effect. It is thought to affect the synaptic abilities of the neurons in contact with the pulses, relating to both long-term potentiation (LTP), where the connections between neurons (neurotransmission) are strengthened, and long-term depression (LTD), where the neurotransmission is weakened.
In research, two techniques for delivering TMS pulses are commonly used. The first, called "Online TMS", involves delivering a pulse while a subject is completing the intended task. If the participant's performance of the task is then altered, then the part of the brain subjected to the pulse is almost positively involved in the performance of the task. The other type, "Offline repetitive TMS" occurs when a subject first completes a task without TMS and his/her performance is analyzed. Then the TMS is given, and the task is completed again. Any changes in performance are noted and measured.
Where You've Seen It
The 1996 study carried out by Cohen et al. (as seen in the plasticity symposium) used TMS to create a temporary lesion of the primary visual cortex in blind subjects, who were then shown to have increased difficulty in reading Braille and determining embossed Roman letters, indicating that the visual cortex is put to use, even in the blind, for processing Braille and other tactile information.
Sources
Wikipedia
Cohen et al.: Functional relevance of cross-modal plasticity in blind humans.
