Transcranial magnetic stimulation (TMS)

A magnetic field changing with time in both direction and strength or induces an electrical field.  In TMS (transcranial magnetic stimulation) this is exploited through using electromagnetic coils to induce currents in the brain that influence firing of neurons stately beneath the coil. The currents attenuate with distance from the coil. TMS is typically effectual to depths of 1.5–3 cm so the cerebral and cerebellar cortices are the usual goals, but more powerful coils can be used to affect subcortical structures. The shape of the induced electrical field is hard to model because of the nonuniform electrical properties of neural tissue but  it has a high spatial resolution (< 1 cm) if a figure-of-eight coil is used that concentrates magnetic flux at the node of the coil and  a temporal resolution of tens  of ms.

There are two modes of TMS, repetitive pulse and single pulse. Single pulse has the benefits which can be time-locked to the delivery of a stimulus so can allow precise timing of any effect. Moreover, single pulses may not always be effective and therefore repetitive TMS (rTMS) can be used. This delivers exponentially falling and rising magnetic pulses in Figure with a frequency up to 50 Hz for many seconds. TMS has effects on visual perception, movement control, language, attention, memory, and decision making. TMS can inhibit or excite depending on the stimulus characteristics. For example, 10 Hz rTMS to the motor cortex stimulates muscle contraction and improves performance in a motor learning task whereas I Hz rTMS impairs motor learning. When used to study cognition it is usually inhibitory so that it interferes with performance of cognitive tasks, either increasing reaction time or increasing the number of errors.

Figure: Transcranial magnetic stimulation (TMS). A rapidly changing magnetic field strength B (measured in tesla, T) produced by a single TMS pulse induces an electric current, I, in the brain.