Guest Post by Tessa Abagis, University of Michigan
In the 43 AD text Compositiones, Scribonius Largus, court physician to the Roman emperor Claudius, described a method to treat chronic migraines: placing torpedo fish on the scalps of patients to easing their pain with the electrical shocks the fish emit. This was well before the advent of modern-day neuroscience, yet Largus was on the right path to realizing something huge: Our brains are comprised of electrical signals that influence how brain cells communicate with each other and, in turn, affect our cognitive processes such as memory, emotion, and attention.
From torpedo fish to electroconvulsive therapy to transcranial direct current stimulation (tDCS), the science of brain stimulation – altering electrical signals in the brain – has changed greatly in the past 2,000 years. We now have a handful of brain stimulation devices that have been developed and marketed to various groups of people, ranging from online video gamers to professional athletes to people with depression (e.g. foc.us, Halo Neuroscience). Yet cognitive neuroscientists are still working to understand just how much and how precisely we can influence brain signals to effect cognitive change through these techniques.
tDCS, one of the more popular current forms of brain stimulation, is a noninvasive and inexpensive form of brain stimulation that delivers a low-grade electrical current through the skull and into the brain. Some scientists think it affects the likelihood of brain cells firing, altering the connections in the brain and potentially improving the cognitive skills associated with those brain regions. However, the effects of tDCS are inconclusive, particularly after a single session.
In fact, some findings have questioned whether enough electrical stimulation is passing through the scalp into the brain to even be able to alter connections between brain cells. Notably, at the 2016 meeting of the Cognitive Neuroscience Society, György Buzsáki of NYU presented research conducted with cadavers and concluded that very little of the administered electrical current with tDCS actually travels into the brain, maybe even under 10%.
Still others have found results that suggest the opposite, that the electrical stimulation induced by tDCS is reaching the brain in large enough amounts to incite neural effects. For example, recent neuroimaging studies have shown significant increases in neurotransmitter levels and blood flow at the site of tDCS stimulation occurring during a single-session of tDCS.
This research continues to be intriguing enough that the National Science Foundation and National Institutes of Health both have significant grant opportunities specifically intended to understand behavioral, cognitive, and neural effects of tDCS. In response to the growing concern that the limited amount of stimulation reaching the brain may not be enough for cognitive change, many researchers have begun to administer tDCS over a multi-day time period, in the hopes that multiple sessions have an additive effect.
Lately, this work has been combined with cognitive training tasks created to effect long-term improvements in cognition. Increasing neural activity with the addition of electrical stimulation during a daily task targeting one cognitive process could result in longer-lasting improvements. In a several-year collaboration between researchers at the University of Michigan and the University of California at Irvine, my lab has been investigating working memory training paradigms, in which participants have to hold progressively more information in their working memory, with concurrent tDCS. We, and other teams, have been researching whether tDCS can contribute to cognitive performance gains above and beyond what cognitive training provides on its own. While the results are still limited and somewhat mixed, they may be more compelling than those of just tDCS or cognitive training on its own.
Recent results from our collaboration have uncovered individual differences associated with cognitive improvement due to concurrent tDCS and working memory training. We have found that baseline performance on the working memory task strongly predicts task improvement with tDCS throughout the training period. In a randomized sham-controlled (i.e. placebo) study, we found that participants in the active tDCS condition who began training with lower baseline working memory performance, improved more than those who began with higher baseline performance. This was not found in the sham group and is potentially suggestive of tDCS enhancing effects of cognitive training only for people with initially lower working memory abilities.
Furthermore, participants who received active tDCS in addition to cognitive training had significantly larger improvements in working memory performance than participants who only completed the cognitive training with sham tDCS. Notably, these improvements in the active as compared to sham group even persisted up to an average of 12 months after the tDCS/cognitive training intervention. This suggests the tDCS-enhanced effects of cognitive training last for up to an entire year.
Ultimately, tDCS in combination with cognitive training as a means of cognitive enhancement is a technique that could have huge benefits, but each new article exclaiming that tDCS or cognitive training will solve your woes needs to be critically analyzed. The field needs much more research investigating the biological bases and neural mechanisms of tDCS, optimizing tDCS protocols, and determining how tDCS and cognitive training work together.
I am cautiously optimistic: I imagine that making strides using good science to understand brain stimulation, and tDCS specifically, could lead to improvements in attention and memory for people of all ages and some huge changes in society. Maybe we could help stave off cognitive decline in older adults or enhance cognitive skills, such as focus, in the people who need it the most, such as airline pilots and soldiers. Even though there are many more questions to ask and much more work to do to truly understand what is happening with tDCS, I am happy that we have at least moved on from torpedo fish.
Tessa Abagis is a PhD candidate in Cognition and Cognitive Neuroscience at the University of Michigan. She is currently investigating distractibility in healthy participants and those with ADHD as well as behavioral and neural correlates of working memory training and concurrent tDCS.
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