How can a coil held against the head influence neurons beneath the skull without sending current through an implanted wire? The key is induction: a magnetic field that changes over time can create an electric field, which in turn nudges charged particles. In repetitive transcranial magnetic stimulation, or rTMS, short magnetic pulses produce weak currents in targeted brain tissue. The procedure is noninvasive, but its effects depend on where and how stimulation is delivered.
A changing field makes a current
Electricity and magnetism are linked. When current flows through a coil, it creates a magnetic field; when that field changes rapidly, it can induce an electric field in nearby material. In the brain, the induced field can drive small electrical currents that influence neuronal activity. This is the same broad physical principle behind a transformer, though the geometry and biological setting are very different. [2]
A steady magnetic field does not produce the same changing-field effect. rTMS instead delivers pulses that switch rapidly on and off. The magnetic field passes through the skull and induces weak currents in tissue; there is no need for an electrode to pierce the skull. NIMH describes rTMS as using repeated low-intensity pulses from an electromagnet to stimulate nerve cells and circuits. [1]
The coil sets a target
In a clinical rTMS session, a coil is positioned near a selected area of the head. Repeated pulses create local stimulation, and patients can feel tapping or knocking where the coil is placed. The targeted nature distinguishes rTMS from electroconvulsive therapy, which uses electrodes to deliver current and induces seizure activity under anesthesia. [1]
Targeting is not as simple as pointing to one neuron. Brain regions contain many interconnected cells, and the exact electric field depends on coil position, pulse parameters, and individual anatomy. NIMH notes that there is not consensus on the best coil positioning or pulse delivery for every use, and research continues on optimal targets and protocols. [1]
Clinical use has boundaries
NIMH describes rTMS as FDA-cleared for several specific uses, including treatment-resistant depression and obsessive-compulsive disorder, with other indications and protocols varying. It is generally considered after other treatments have been tried, and a trained medical team provides and monitors the procedure. The existence of an authorized use for one condition does not establish effectiveness for every disorder or every stimulation pattern. [1]
Fields used in rTMS are non-ionizing electromagnetic fields. The National Cancer Institute distinguishes these from higher-frequency x-rays and gamma rays, which are ionizing and can directly damage DNA or cells. Non-ionizing fields are not known to damage DNA or cells directly; that physical distinction does not, by itself, answer every question about a particular treatment’s benefits or risks. [2]
Mechanism is not outcome
Induction explains how a magnetic pulse can create a current in brain tissue. It does not alone explain why a course of pulses may improve symptoms for some patients, which circuits change over time, or which schedule will work best. Those are clinical and biological questions that require evidence from people, not just a diagram of a coil and field lines.
NIMH reports that rTMS is generally well tolerated, while noting possible short-term effects such as scalp discomfort, headache, or lightheadedness; seizures are possible but uncommon. Long-term effects have not been fully determined. Research into pulse parameters, targets, and combinations with other care continues, so mechanistic plausibility and established clinical use should remain distinct. [1]
Sources & further reading
- Brain Stimulation Therapies NIMH
- Electromagnetic Fields and Cancer National Cancer Institute
Prepared for the independent PSI Center reading library. About edition dates and source checking.
This explainer describes scientific principles. It does not assess individual treatment choices.
