Transranial Magnetic Stimulation

Transranial Magnetic Stimulation

What is TMS?

Transcranial Magnetic Stimulation (TMS) is a neurophysiological technique that induces a current in a small area of the brain, using a magnetic field to pass the scalp safely and painlessly. In TMS, a current passes through a coil of copper wire that is wound inside a plastic-insulated casing and held over the participant’s head. This coil resembles a paddle or large spoon, and is held in place either by the investigator, or by a mechanical stabilisation device similar to a microphone stand or metal frame. As the current passes through the coil it generates a magnetic field that can pass through the scalp and skull, and in turn induces a current in a small area of the participant’s brain.

Why do we use TMS?

TMS is used in many laboratories to study the effects of localised brain stimulation on perception, attention, movement control and higher thought processes (such as executive control and memory): The magnetic field induces currents stimulating the neurones in a small area beneath the stimulation coil, and temporarily altering their normal function. By measuring the behavioural consequences of TMS, we can infer which brain regions are required for specific behaviours.

Is TMS safe?

The technique is considered to be generally safe for use in neurologically healthy individuals.You can find more information about safety in our TMS Information booklet (.pdf).

Which TMS protocols do we use?

In our lab, we primarily use 'online' single-pulse TMS and 'offline' continous theta burst stimulation (cTBS).

Single-pulse TMS. As the name suggests, one TMS pulse is delivered per trial, at one of several possible times after the onset of task-relevant stimulus. By comparing the behavioural effects of TMS at different times, experimenters can thus infer the time-course of neural processing in the stimulated region.

cTBS: In this protocol, behavioural performance is compared before and after a sustained period of repetitive TMS. A series of TMS pulses is delivered for <1 min before we test performance. Rather than interrupting time-critical processing in the stimulated cortex, the logic of this approach is to modulate cortical excitability for a period of 15-60 minutes following the offset of the TMS. Behaviour is measured during the period of the TMS-induced aftereffect.

TMS and EEG

TMS-EEG is now possible due to recent technical developments in EEG amplifiers, and opens new avenues in cognitive neuroscience. One of the main limitations of TMS is that it cannot distinguish between effects due to interfering directly with the function of the stimulated brain area (e.g. a frontal area), and indirect effects on functionally connected brain areas (e.g. an occipital area). By combining TMS with EEG, we can examine such indirect effects. Furthermore, with ‘offline’ TMS protocols (i.e. a series of pulses delivered before a task), it is difficult to determine which processing stages are influenced; EEG can clarify this.

Would you like to know more?

Hallett, M. (2007). Transcranial magnetic stimulation: A primer. Neuron, 55(2), 187-99. doi:10.1016/j.neuron.2007.06.026

O'Shea, J., & Walsh, V. (2007). Transcranial magnetic stimulation. Current Biology, 17(6), R196-R199. doi:10.1016/j.cub.2007.01.030