Transcranial Magnetic Stimulation (TMS) is a Cognitive Neuroscience technique that enables us to stimulate the human brain non-invasively. While this in itself is an impressive feat, the technique is not just a fascinating toy, but plays a huge role in informing our understanding of how the brain and different functions are related.
The technique was originally applied to the human brain by Anthony Barker and colleagues at the University of Sheffield and reported on in The Lancet in 1985. The technique was covered in the media here earlier this year for its role in alleviating depression, but it initially had, and still has, an important role in understanding how the brain works more generally.
Historically, knowledge of how the brain performed different functions relied on relating brain damage in patients with changes in behaviour. More recently, it has also been possible to measure changes in brain activity related to different tasks using functional MRI (fMRI) and Electro-encephalogram (EEG). The huge advantage of TMS is that it allows researchers to stimulate the brain where and when they choose, thus enabling us to relate changes in performance of a task with stimulation at a particular location and time.
So how does it work? TMS relies on electro-magnetic induction, which means that when an electric current runs through a wire, a magnetic field is created perpendicular to that wire. This magnetic field in turn induces a current in any nearby conductive material. In the case of TMS, a current runs through the TMS coil, around which a magnetic field is created. The nearby conductive material in this scenario is the brain and a current is induced in the cortex in the area lying under the centre of the coil.
This area of cortex is therefore activated and this activation can have different effects on a participant’s performance. The activation essentially creates noise in the normal signal and if the area of cortex underlying the coil is necessary for the task at hand, then it can lead the participant to slow down or to make errors. In the early days of TMS, this effect was referred to as a virtual lesion, which is a rather unfortunate term since the activation is nothing like a lesion, but rather is a very short-lived activation of the cortex.
Some examples of these effects include speech arrest, where stimulating the left lower frontal part of the brain (left inferior frontal cortex or Broca’s area) can temporarily prevent the participant from producing coherent speech. Stimulating the hand area of the motor cortex (responsible for controlling movement of the hand) can interfere with a participant performing an action such as grasping a cup. Stimulating cortex for vision at the back of the brain can slow or make the participant less accurate when performing a visual task such as target detection.
The advantage of this approach is that we can say an area of cortex is necessary for a particular task when stimulating it leads to a decline in performance. This is in contrast with fMRI where, for example, measuring activation in part of the brain during performance of a task means the area may be involved in the task in some way, but does not show that it is really necessary for performing the task.
TMS also has advantages over relating brain damage in patients with changes in behaviour. The first is that lesions are rarely, if ever, very specific to an area of cortex. It is much more common to have reasonably large areas of effected cortex, making it difficult to relate performance of the patient with damage to any particular part of the brain.
The second reason is that the brain can compensate for damage over time, which means that the pattern of activity across the brain can change in the months and years following damage. A third potential reason is that a patient themselves can use new strategies to perform tasks in order to get around their difficulties. This is a positive for the patient, but it can cloud efforts to relate function and brain areas.
Using TMS with so-called "normal" participants allows us to interfere with processing at a specific location for a specific time window allowing us to make stronger inferences about the contribution of a particular cortical area to a task.
TMS is a safe technique. All participants are screened before taking part in a session and individuals are excluded if they, or an immediate family member, suffer from epilepsy. Pregnant women are also excluded from participation. It is also usual to ask participants about drug use to ensure that those using a drug that could affect the central nervous system do not take part. For similar reasons, the amount of caffeine and alcohol intake in the hours before participation are also considered. As well as its potential role in treating psychiatric disorders, TMS is an important, safe and precise tool in elucidating some of the mysteries of how our amazing brains carry out everyday tasks.