In a nutshell: Brain structure and chemical concentration influence the effects of brain stimulation.

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Brain activity can be influenced by passing weak electrical currents through the skull. One of the most common methods for non-invasively stimulating the brain is transcranial direct current stimulation (tDCS). It works by sending a weak electrical current between two or more electrodes placed on the scalp. The current flows from a positive (anode) to a negative (cathode) electrode.

Some of this current reaches the cortex – the thick outer layer of the brain – where it can affect brain function and associated behaviours. This makes tDCS useful for studying the brain – by applying a current and measuring the effects. It is also used to treat some psychiatric conditions, such as depression.

However, tDCS, like many interventions, can have varied effects across individuals. For example, the effects can be different depending on whether the stimulated area of cortex is nearer to the anodal or to the cathodal electrode. A better understanding of what causes these differences could help to make tDCS more reliable and effective.

A group of researchers – led by Hannah Filmer and involving Brain Function CoE investigators Jason Mattingley and Paul Dux at the University of Queensland – investigated how the direction of current in tDCS influences its effects on the brain.

The researchers previously showed that the concentration of two chemicals in the brain, as well as the thickness of the cortex, can affect how people react to tDCS.

In their latest study, they took a closer look at the data from the previous studies to see if anodal and cathodal stimulation had different effects on participants’ performance on a simple decision-making task.

They found that anodal and cathodal stimulation had a similar effect: they both disrupted the improvements in response time that would normally be expected when people practice a task repeatedly.

However, anodal and cathodal stimulation had different effects on the brain processes underlying decision-making. Using mathematical modelling, the researchers showed that anodal stimulation affected the amount of evidence that participants needed to make a decision. By contrast, cathodal stimulation affected how quickly participants could discriminate between correct and incorrect responses.

The researchers also found that the susceptibility of people to each current direction was linked to distinct brain features. Differences in cortical thickness mattered more when using anodal stimulation, whereas a combination of cortical thickness and chemical concentrations was more important when using cathodal stimulation.

This work adds to the researchers’ understanding of what factors predict an individual’s response to tDCS. It also brings them a step closer to reducing the individual differences in how people respond to tDCS, potentially making it a more feasible treatment for conditions such as depression, chronic pain and tinnitus.

Next steps:
Now that the researchers have identified some of the biological factors responsible for the variability in tDCS effectiveness between individuals, they will examine how these factors interact with changes in the duration and intensity of brain stimulation.

Filmer, H. L., Ballard, T., Ehrhardt, S. E., Bollmann, S., Shaw, T. B., Mattingley, J. B., & Dux, P. E. (2020). Dissociable effects of tDCS polarity on latent decision processes are associated with individual differences in neurochemical concentrations and cortical morphology. Neuropsychologia, 141, 107433. doi: 10.1016/j.neuropsychologia.2020.107433

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