Transcranial direct current stimulation (tDCS) is a non-invasive, painless brain stimulation method which uses electrical currents to modulate neuronal activity in specific parts of the brain. A constant, low intensity current is delivered through small electrodes attached to the scalp in order to either increase or reduce neuronal activity.
This is clearly a trending topic: although interest in tDCS dates back to the 1960s, a search in PubMed reveals that more than half of the articles on tDCS were published in the last two years.
Despite numerous studies on different applications for tDCS, its use is still not generally accepted in the clinical setting; tDCS is not an FDA-approved therapy, remaining mostly an experimental method. Although tDCS has been tested on numerous conditions such as depression, anxiety, schizophrenia, Parkinson’s disease, Alzheimer’s disease, chronic pain, fibromyalgia, and stroke, its efficacy is still largely inconclusive.
Many studies applying tDCS have already been published in 2015, as well as a few reviews analyzing its efficacy for different conditions. By gathering the available information for the application of tDSC in a specific context, reviews are particularly useful, allowing researchers to sort through all the conflicting data. And these have actually shown some promising applications for tDSC.
Learning and Memory
There have been claims that tDCS can enhance cognition in healthy adult populations, especially working memory and language production, spiking the interest in tDCS as a neuroenhancement tool.
tDCS seems to act as a neuromodulatory technique, inducing a long-term enhancement or reduction of signal transmission between neurons. By strengthening or weakening neuronal connections, it may facilitate learning and memory formation, as well as neural plasticity that contributes to functional recovery after stroke, for example.
However, a review on the effects of a single-session of tDCS showed that it did not have a significant effect on a variety of cognitive function such as language, episodic memory, working memory or mental arithmetic, just to name a few. Nevertheless, it did not exclude the possibility that tDCS may be effective after multiple sessions.
There are in fact many reports from studies in healthy subjects stating that tDCS enhances verbal performance and learning, improving such outcomes as verbal speed, fluency, and amount of verbal learning. These language enhancement outcomes could potentially be quite useful in treating language deficits associated with different pathological conditions. In fact, tDCS has been used to enhance treatment efficacy in post-stroke aphasia rehabilitation and the results seem promising, with tDCS being effective in increasing language skills despite a high variety of stimulation parameters and patient characteristics.
Language enhancement can also be applied to a word reading context. Repeated tDCS application to adults with developmental dyslexia has been shown to significantly improve reading speed and fluency.
Reports supporting a positive effect on memory enhancement can also be found. Different studies have demonstrated an improvement in working memory and episodic memory in healthy subjects, with an increase in accuracy and in response time. But again, the evidences are still considered insufficient for a clinical application.
These memory enhancement effects could be quite useful in both Alzheimer’s and Parkinson’s disease, and in post-stroke rehabilitation. Again, some promising outcomes in these pathologies have been reported, but there are still conflicting results.