Transcranial magnetic stimulation · Grey Matter

Transcranial magnetic stimulation (TMS) makes neurons fire from outside the head by inducing an electric current in the cortex with a brief, strong magnetic pulse, and it is used both to probe the brain (what happens if this area is disturbed?) and to treat depression and other disorders.

Transcranial magnetic stimulation. Transcranial magnetic stimulation (TMS) makes neurons fire from outside the head by inducing an electric current in the cortex with a brief, strong magnetic pulse, and it is used both to probe the brain (what happens if this area is disturbed?) and to treat depression and other disorders.

A coil held against the scalp carries a large current for a fraction of a millisecond. The magnetic field it creates, of about 2 to 3 tesla near the coil, passes through the skull without resistance, and its rapid change induces an electric field in the tissue below, strong enough to fire axons in the outer few centimetres of cortex. Over the motor cortex a single pulse makes a muscle on the opposite side twitch; repeated pulses (rTMS) at a given rhythm change the excitability of the stimulated area for minutes to hours.

It writes, it barely reads. TMS is a way to perturb a region and watch the effect, combined with EEG or a measured muscle response; it gives a causal test that recording methods cannot.

Its reach is shallow and broad. The induced field falls quickly with depth and spreads over about a centimetre or more, so deep structures and fine targets are out of reach without special coils that trade focus for depth.

Clinical use rests on repetition. Courses of daily rTMS over the prefrontal cortex are cleared for depression that resists drugs, and a single-pulse device is cleared for migraine with aura; evidence quality varies, and placebo responses are large in these trials.

It can provoke seizures. The risk is low, about one in a thousand patients and usually linked to protocol errors, and safety guidelines limit intensity and frequency.

TMS is the main routine way to make a healthy human cortex fire without surgery.

It does so coarsely, by inducing current in a region, never by choosing neurons.

Questions: Does stimulating the brain from outside fall under the proposed right to mental integrity? It does when it happens without valid consent or causes harm. TMS changes the excitability of the cortex for minutes to hours and is used with consent as a treatment, which is ordinary medicine, but the same capacity to alter brain activity is what the right to mental integrity was proposed to protect against when it is imposed or misused. The debate is less about clinics than about non-medical uses, such as stimulation sold to enhance performance or required in workplaces, where the person's freedom to refuse is weaker.