Transcranial magnetic stimulation (TMS) is a procedure in which electrical activity in the brain is influenced by a pulsed magnetic field. The field is generated by passing current pulses through a conducting coil, held close to the scalp so that the field is focussed in the cortex, passing through the skull. Magnetic induction dictates that the changing field acts on charges in the tissue it passes through, causing small local currents to flow. When this stimulation is delivered at regular intervals, it is termed repetitive TMS, or rTMS. Recently, improvements in electronics (especially capacitors) have enabled machines capable of alternating these strong magnetic fields at physiologically interesting rates (up to 25 Hz), called high frequency rTMS.
The stimulation provided by TMS is illustrated in the diagrams on this page.
The early clinical uses of TMS were restricted to the field of neurology, where it was used to examine conduction in the central and peripheral nervous system by stimulating neurons. More recently, TMS and rTMS have been used to investigate aspects of cortical processing, including sensory and cognitive functions. The use of rTMS to excite local areas of cortex, combined with knowledge of how local cortical activity can change during various disorders, has raised the possibility of the use of rTMS as a therapeutic tool for psychiatric and neurological disorders. An introductory article covers this aspect in slightly greater depth.