What We Know—And Don’t Know—About Brain Stimulation


Transcranial electrical stimulation (tES), or “brain zapping” as it is often (erroneously) described, is garnering a fair amount of attention, both from academics and the media. The technique has been used to boost mental functions such as attention and memory.

Manufacturers are now advertising and selling tES devices to the public. These devices provide transcranial direct current stimulation (tDCS) – the best-known form of tES – and are in some cases available online for less than $200. They sail through a loophole in regulatory requirements because they make no medical claims. But are they capable of fulfilling the promises that the media, researchers, and, most recently, manufacturers are making on their behalf?

Despite excited media and scientific speculation on the potential of tES, the public seems wary of using such technology. The fear factor certainly plays a prominent role. To the public, electricity and the brain are an unsettling combination. Films like One Flew Over the Cuckoo’s Nest and TV shows like Homeland depict electroconvulsive shocks that are difficult to watch without flinching.

This unpleasant association is both unfortunate and ironic, as the brain itself is an electrical system, performing many of its functions via electrical signals. It is equally ironic because many activities we engage in on a regular basis – like drinking coffee, for example – also have brain-stimulatory effects.

Even the words casually used to describe tES, such as “shocks” and “zaps,” contribute to negative and incorrect impressions. It is an extremely mild technique – usually 1–2 mA (at most 0.2 percent of the current entering a 100-watt lightbulb). Most important, tES is painless and very safe. As long as the screening criteria are met, such as excluding individuals with epilepsy, there are presently no causes for concern.

What makes scientists most nervous about tES is that the long-term effects have not been adequately studied, though published research that does discuss long-term effects reports only positive neurocognitive outcomes, such as more efficient oxygen consumption by the brain. However, in most research contexts, tES is applied for 10–30 minutes in one day, and the safety of excessive usage (several hours per day over several weeks) has not being examined.

Nearly two decades of research show that tES can improve several mental functions. For example, it enhances acquisition of new numbers and the quantities they represent.

Findings such as these are promising, but insufficient grounds for marketing tES to the public as a tool for cognitive enhancement. This kind of marketing is based on a one-size-fits-all principle, while a recent wave of research reveals that the effects of tES depend on psychological and physiological differences between individuals.

We recently examined how traits might lead to different responses to brain stimulation. In personality psychology, traits are patterns of emotions, cognitions, and behaviors that can be used, in broad terms, to characterize individuals. We chose mathematics anxiety as a trait for our experiment, and recruited individuals with high or low mathematics anxiety. Assessing mathematics anxiety is just a matter of scoring a questionnaire that individuals complete.

Participants performed a simple arithmetic task while receiving either real or placebo tES. The results formed what is referred to as a double dissociation: Participants with high mathematics anxiety who received real tES completed the arithmetic task faster than those who received placebo tES, with no loss of accuracy. This is taken as evidence of enhancement, because they achieved the same results in a shorter time.

We saw the opposite results for participants with low mathematics anxiety: Participants were faster, with no loss of accuracy, during placebo stimulation compared to real stimulation.

These findings illustrate the absence of a one-size-fits-all approach. The same stimulation led to opposed outcomes for participants depending on their level of mathematics anxiety. In this case, tES failed to assist participants with low mathematics anxiety in any way, suggesting that there are individuals who will not benefit from stimulation, and may indeed face impairments.

Research is consistently showing that tES is safe and without negative short-term consequences, and there is growing evidence that it is also safe in the long term. However, if consumers are wary of brain stimulation, safety is not the only factor they should consider. The issue is whether the product will provide the benefits consumers assume it will.

The technique is effective in laboratory settings, but the potential for individual differences to affect the outcome of tES means the brain stimulation community still doesn’t know enough to support its unregulated and unsupervised use by the public.

written by Amar Sarkar and Roi Cohen Kadosh, provided to the BBJ by The MarkNews

Amar Sarkar and Roi Cohen Kadosh are both affiliated with the Department of Experimental Psychology at the University of Oxford, where Amar Sarkar is a research assistant and Roi Cohen Kadosh is a cognitive neuroscientist and the director of the Cohen Kadosh Group. Roi Cohen Kadosh’s research has been supported by the Wellcome Trust, the European Commission, the Defence Science and Technology Laboratory, and the Intelligence Advanced Research Projects Activity, among others.

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