It sounds too good to be true: changing the brain’s activity simply by placing electrodes on the surface of the scalp. But that’s the idea behind transcranial direct current stimulation (tDCS). No scalpel required.

With this technique, the electrodes are simply damp sponges about 4 square centimetres in area, and they are used to deliver a current of just a couple of milliamps. “There may be some mild tinglings under the electrodes, but that’s about it,” says Leonardo Cohen at the National Institutes of Health in Bethesda, Maryland.

tDCS should not be confused with electroconvulsive therapy, where a much larger shock of 600 milliamps is applied to the whole brain. ECT is designed to trigger seizures and must be done under a general anaesthetic. It has side effects such as memory loss and confusion and so is only given to people with very serious depression.

So how does tDCS work? Neurons underneath the positively charged electrode are stimulated to fire more frequently in response to normal incoming signals. The negative electrode has the opposite effect, quietening the underlying cells. If one region needs to be boosted without muting another, the negative electrode can be placed over a thicker part of the skull, such as the area above the eye, to minimise impact.


As tDCS has only been used for about a decade its full potential is still unknown. Perhaps the most widely tested medical use has been to treat brain damage arising from a stroke. In a study on 10 people, five consecutive days of treatment led to improvements in language a week later (Stroke, vol 41, p 1229). The technique is also being investigated as a treatment for depression, chronic pain and migraine.

Because tDCS seems so safe it is being studied as a potential cognitive enhancer. For example, Cohen has shown it can help people learn a task involving precise hand movements: people who had tDCS during the training period performed better than their peers three months down the line (Proceedings of the National Academy of Sciences, vol 106, p 1590). Another group has shown it can boost mathematical skills (Current Biology, vol 20, p 2016).

VERDICT Equipment is simple and cheap, and the technique looks safe. Perhaps has the most potential for cognitive enhancement.

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