Transcranial magnetic stimulation (TMS)

TMS uses a brief, strong magnetic pulse from a coil held over the scalp to induce, by Faraday's law, an electric field in the underlying cortex that depolarizes neurons noninvasively. A single suprathreshold pulse over motor cortex produces a measurable motor evoked potential; paired-pulse and single-pulse protocols probe cortical excitability, inhibition, and connectivity. Repetitive TMS (rTMS) and patterned protocols such as theta-burst stimulation induce lasting, plasticity-like changes in excitability and are cleared for major depression and obsessive-compulsive disorder.

Spatial resolution is on the order of a centimeter and depth is limited: the induced field falls off with distance, so deeper targets require larger, less focal coils, and focality and depth fundamentally trade off. Individual variability in coil placement, brain anatomy, and state at the time of stimulation makes dosing imprecise. Closing the loop by triggering pulses on a real-time EEG oscillatory phase or brain state is an active research direction aimed at reducing this variability.