7th November 2017
Brain device boosts learning in monkeys by 40%
Scientists funded by DARPA have developed a non-invasive cap that stimulates parts of the brain via electrical currents and is shown to improve the learning abilities of macaques. In the not-too-distant future, this technology could be used by humans.
Researchers from HRL Laboratories (California), McGill University (Montreal, Canada), and Soterix Medical (New York) have collaborated on a study of transcranial direct current stimulation (tDCS) in monkeys. Funded by the Defense Advanced Research Projects Agency (DARPA), their work is published in the peer-reviewed journal Cell.
tDCS is a form of neurostimulation that uses constant, low direct currents delivered via electrodes on the head. When these electrodes are placed in the region of interest, the current induces intracerebral current flow. This can increase or decrease neuronal excitability in the specific area being stimulated, based on which type of stimulation is used. The change of neuronal excitability alters brain function, which can be used in therapies as well as to provide more information about the functioning of the human brain. In recent years, it has been used by neuroscientists to link specific brain regions to specific cognitive tasks or psychological phenomena. As of 2017, it has not yet been approved for medical use by the US FDA. It is, however, approved in Europe for treatment of major depressive disorder. The number of studies involving tDCS is growing exponentially.
In their experiments, the DARPA-funded team used tDCS on a group of macaques. They stimulated the prefrontal cortex at the front of the brain, an area known to be involved in complex processes like reason, logic, problem solving, planning and memory. The animals were made to perform a task based on associative learning. To obtain a reward, they had to learn associations between a visual cue and a location. The macaques would hunt for each reward after getting the visual cue.
The initial foraging trials took around 15 seconds, and once the animal learned the location of the reward, it took approximately two seconds to recall and find the target. Subjects in a control group (i.e. without neurostimulation) needed an average of 22 trials to learn to obtain the reward immediately. With tDCS, however, they required an average of just 12 trials.
The results of these trials showed that overall, learning was accelerated by 40% when 2 milliamps (mA) were sent noninvasively to the prefrontal cortex without increased neuronal firing. The study showed it was modulated connectivity between brain areas – not the neuron firing rates – that accounted for the increased learning speed.
“In this experiment, we targeted the prefrontal cortex with individualised non-invasive stimulation montages,” said Dr. Praveen Pilly, HRL's principal investigator on the study. “That is the region that controls many executive functions including decision-making, cognitive control, and contextual memory retrieval. It is connected to almost all the other cortical areas of the brain, and stimulating it has widespread effects. It is also the target of choice in most published behavioural enhancement studies and case studies with transcranial stimulation. We placed the tDCS electrodes on the scalp in both our control and stimulation conditions. The behavioural effect was revealed when they learned to find the reward faster.”
“The improved long-range connectivity between brain areas in the high frequency bands and reduced connectivity in the low frequency bands were the determining factors in our study that could explain the learning improvements with tDCS of the prefrontal cortex,” Pilly said. “Just because neurons can be more brisk in their firing may not lead to changes in performance. Boosting memory function likely requires better coordination of task-relevant information across the cortex.”
The team concludes: “These results are consistent with the idea that tDCS leads to widespread changes in brain activity and suggest that it may be a valuable method for cheaply and non-invasively altering functional connectivity in humans.”
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