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Individual responses to brain stimulation are related to two brain features

02.07.2019

In a nutshell: Two studies reveal that the differences in how people react to transcranial direct current stimulation are related to neurochemical concentrations and brain structure.

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Individual responses to brain stimulation are related to two brain features

Brain cells communicate with each other by sending and receiving electrical signals. This electrical activity enables the brain to control our behaviour, feelings, memories and thoughts.

Applying a weak electrical current to the brain – using non-invasive methods such as transcranial direct current stimulation (tDCS) – can affect behaviours such as motor learning, decision-making and concentration. It has also been used in brain training and to treat conditions such as depression.

One of the reasons that techniques like tDCS have not been used more broadly to treat brain disorders is that their effects vary significantly between individuals. Without knowing what causes these differences or how to predict them, the potential usefulness of tDCS is limited.

To try to answer these questions, Brain Function CoE researchers from the University of Queensland carried out two studies to investigate the role of different aspects of the brain: neurochemical concentration and brain structure. The studies were led by research fellow Hannah Filmer.

In both studies, participants carried out learning tasks before and after receiving tDCS. The electrical current was applied to the left prefrontal cortex, a part of the brain whose dysfunction is linked to many neurological and psychiatric conditions.

In these tasks, electrical brain stimulation normally affects behaviour by reducing or increasing reaction times, depending on when it is applied. By examining how the participants’ performance changed over the course of testing, the researchers could tell whether tDCS had worked as intended.

In the first study, the researchers looked at the presence of two neurochemicals in the brain that are important for behaviour: gamma-aminobutyric acid (GABA) and glutamate. Because brain stimulation can alter the concentrations of these chemicals, the researchers measured their levels before applying tDCS.

They found that baseline levels of GABA and glutamate were associated with differences in how individuals responded to tDCS. Neither chemical alone was responsible; rather, the ratio of GABA to glutamate was the deciding factor. Brain stimulation was most effective in participants who had more GABA than glutamate.

In the second study, the researchers looked at the structure of the cortex – the thick folded layer of cells that covers the brain’s surface. They measured the thickness of each participant’s cortex using magnetic resonance imaging (MRI).

The researchers found a significant link between the thickness of the left prefrontal cortex and the effectiveness of tDCS in an individual. In fact, more than one-third of the variation in tDCS efficacy between participants could be attributed to differences in their cortical thickness.

These studies are the first to show that particular features of the brain could be used on a case-by-case basis to predict who would benefit the most from non-invasive electrical brain stimulation.

Next steps:
The team plans to continue studying why the effects of brain stimulation vary between people and what other factors might be involved.


Reference:
Filmer, H. L., Ehrhardt, S. E., Bollmann, S., Mattingley, J. B., & Dux, P. E. (2019). Accounting for individual differences in the response to tDCS with baseline levels of neurochemical excitability. Cortex, 115, 324–334. doi: 10.1016/j.cortex.2019.02.012

Filmer, H. L., Ehrhardt, S. E., Shaw, T. B., Mattingley, J. B., & Dux, P. E. (2019). The efficacy of transcranial direct current stimulation to prefrontal areas is related to underlying cortical morphology. NeuroImage, 196, 41–48. doi: 10.1016/j.neuroimage.2019.04.026


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