Key takeaway: The skull acts as an enormous resistor for electricity (which strongly limits tDCS), but the skull is completely transparent to magnetic fields. TMS exploits Faraday's Law of Induction: by discharging a massive, brief burst of electrical current through a wired coil sitting above the scalp, a powerful magnetic field is generated. Because this field is changing rapidly, it induces a secondary electrical field perpendicularly inside the brain tissue beneath it, strong enough to directly force neurons to fire action potentials.
Types of Stimulation
-
Single-Pulse TMS
Mapping excitability.
- Fires a single, brief magnetic pulse. Often used over the Primary Motor Cortex (M1) to evoke a physical muscle twitch (a Motor Evoked Potential, or MEP) in a specific hand or arm muscle.
- Clinicians and researchers use this to measure the baseline excitability of the corticospinal tract, or to precisely pinpoint the motor mapping representation of specific limbs before surgery.
-
Repetitive TMS (rTMS)
Inducing long-term plasticity.
- Delivers a train of pulses over several minutes (e.g., 10 pulses per second). Depending on the exact frequency, rTMS can induce Long-Term Potentiation (excitatory, usually high-frequency >5Hz) or Long-Term Depression (inhibitory, usually low-frequency ~1Hz) that lasts well beyond the stimulation session itself.
Clinical and Research Applications
-
Treatment-Resistant Depression
FDA-approved therapy.
- rTMS is widely used for major depression that has not responded to traditional pharmacology. The standard protocol involves directing high-frequency excitatory rTMS over the Left Dorsolateral Prefrontal Cortex (dlPFC) daily for 4 to 6 weeks, aiming to reactivate under-active frontal networks.
-
The "Virtual Lesion" Concept
Causal neuroscience research.
- Unlike fMRI (which can only show correlation), TMS allows researchers to briefly and safely disrupt a specific brain region to prove causation. For instance, holding the coil over Broca's area and firing brief pulses can temporarily stop a healthy subject from speaking, proving that region is causally required for speech production.
Hardware Geometry
-
The Figure-8 Coil
Achieving focal stimulation.
- Early TMS used circular coils, which stimulated a very broad, undefined area of the brain beneath the perimeter of the coil.
- The modern standard is the Figure-8 (or butterfly) coil. By looping the wire in an 8-shape, the magnetic fields from the two adjacent loops summate directly in the center of the cross under the coil, creating a highly focal, sharp peak of induced electrical current (roughly 1cm to 2cm wide) right at the target.