Transcranial alternating current stimulation used to boost working memory

March 16, 2017

The fMRI scans show that stimulation “in beat” increases brain activity in the regions involved in task performance. On the other hand, stimulation “out of beat” showed activity in regions usually associated with resting. (credit: Ines Violante)

In a study published Tuesday Mar. 14 in the open-access journal eLife, researchers at Imperial College London found that applying transcranial alternating current stimulation (TACS) through the scalp helped to synchronize brain waves in different areas of the brain, enabling subjects to perform better on tasks involving short-term working memory.

The hope is that the approach could one day be used to bypass damaged areas of the brain and relay signals in people with traumatic brain injury, stroke, or epilepsy.

“What we observed is that people performed better when the two waves had the same rhythm and at the same time,” said Ines Ribeiro Violante, PhD, a neuroscientist in the Department of Medicine at Imperial, who led the research. The current gave a performance boost to the memory processes used when people try to remember names at a party, telephone numbers, or even a short grocery list.

Keeping the beat

Violante and team targeted two brain regions — the middle frontal gyrus and the inferior parietal lobule — which are known to be involved in working memory.

Ten volunteers were asked to carry out a set of memory tasks of increasing difficulty while receiving theta frequency stimulation to the two brain regions at slightly different times (unsynchronised), at the same time (synchronous), or only a quick burst (sham) to give the impression of receiving full treatment.

In the working memory experiments, participants looked at a screen on which numbers flashed up and had to remember if a number was the same as the previous, or in the case of the harder trial, if the current number matched that of two-numbers previous.

Results showed that when the brain regions were stimulated in sync, reaction times on the memory tasks improved, especially on the harder of the tasks requiring volunteers to hold two strings of numbers in their minds.

“The classic behavior is to do slower on the harder cognitive task, but people performed faster with synchronized stimulation and as fast as on the simpler task,” said Violante.

Previous studies have shown that brain stimulation with electromagnetic waves or electrical current can have an effect on brain activity, the field has remained controversial due to a lack of reproducibility. But using functional MRI to image the brain enabled the team to show changes in activity occurring during stimulation.

“The results show that when the stimulation was in sync, there was an increase in activity in those regions involved in the task. When it was out of sync the opposite effect was seen,” Violante explained.

Clinical use

“The next step is to see if the brain stimulation works in patients with brain injury, in combination with brain imaging, where patients have lesions which impair long range communication in their brains,” said Violante. “The hope is that it could eventually be used for these patients, or even those who have suffered a stroke or who have epilepsy.

The researchers also plan to combine brain stimulation with cognitive training to see if it restores lost skills.

The research was funded by the Wellcome Trust.

Abstract of Externally induced frontoparietal synchronization modulates network dynamics and enhances working memory performance

Cognitive functions such as working memory (WM) are emergent properties of large-scale network interactions. Synchronisation of oscillatory activity might contribute to WM by enabling the coordination of long-range processes. However, causal evidence for the way oscillatory activity shapes network dynamics and behavior in humans is limited. Here we applied transcranial alternating current stimulation (tACS) to exogenously modulate oscillatory activity in a right frontoparietal network that supports WM. Externally induced synchronization improved performance when cognitive demands were high. Simultaneously collected fMRI data reveals tACS effects dependent on the relative phase of the stimulation and the internal cognitive processing state. Specifically, synchronous tACS during the verbal WM task increased parietal activity, which correlated with behavioral performance. Furthermore, functional connectivity results indicate that the relative phase of frontoparietal stimulation influences information flow within the WM network. Overall, our findings demonstrate a link between behavioral performance in a demanding WM task and large-scale brain synchronization.