Transcranial Magnetic Stimulation

Inducing Currents in the Brain

TMS works on the principle of electromagnetic induction discovered by Michael Faraday. An insulated coil carrying a large, brief current is placed over the skull that overlies the part of the brain one wishes to stimulate. The current needs to be brief because induction partly depends on the rate of change of the electromagnetic field. The electrical current creates a brief magnetic field that passes through the scalp, and the magnetic field then induces an electrical current in the brain.

This simple method has many different uses. After over a century of effort the problem of using magnetic fields to stimulate the brain was solved by Anthony Barker in 1985, and it has since been used widely to stimulate both peripheral nerves and brain tissue in studies encompassing motor conduction in human development, motor control, movement disorders, swallowing, vision, memory, speech and language, epilepsy, depression, stroke, pain, and plasticity. It has proved to be a versatile technique and is now also being used in combination with electroencephalography (EEG), functional magnetic resonance imaging (fMRI), and single unit electrode recording in animals.

If the coil carrying the current is placed over the motor cortex, a region of the brain that controls movement, then the subject's finger, arm, hand, face, or leg may twitch because the magnetic field has induced a current in the corresponding region of the brain's body map. If the coil is placed at the back of the subject's head, that is, over a region controlling vision, the subject may see points of light or moving shadows. These are known as pho-sphenes. One use of TMS, then, is to map the organization of the body in the brain or to map visual space in the brain.

Modifying Brain Excitability

There are a few parameters under the experimenter's control, and varying them leads to TMS having varied uses beyond mapping. Pulses of TMS can be applied at different intensities; they can be applied singly or in repetitive transcranial magnetic stimulation (rTMS) of low or high frequency. The choice of stimulation parameters determines whether the effects of stimulation are excitatory or inhibitory. For example, two single pulses separated by less than 5 milliseconds (ms) can produce intracortical inhibition, that is, they make the stimulated area less sensitive to stimulation. Two single pulses separated by a gap greater than 10 and less then 30 ms can produce intracortical facilitation, that is, make the stimulated area more sensitive to stimulation. Repetitive TMS at a frequency of 1 hertz (Hz) has the effect of depressing cortical excitability for a period of time after the train of pulses has finished, whereas repetitive stimulation at 10 Hz or more may increase excitability. Theta burst stimulation, applying trains of 50 Hz stimulation in bursts every 200 milliseconds, has the effect of depressing cortical activity for a period following stimulation.

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