Transcranial direct current stimulation (tDCS) refers to a non-invasive brain-stimulation technique that uses electrical current to influence the activity of the brain’s cerebral cortex. The idea behind it is that by increasing or decreasing the activity of specific brain regions, certain brain functions could be enhanced or suppressed. If true, this could open up a number of new and interesting avenues for treating a variety of health conditions, or even enhancing certain cognitive functions – but what does the current science really say about the potential of tDCS? Read on to learn more about this intriguing brain-stimulation technique, how it might work, its effects, and what the potential risks and dangers behind it might be!
Transcranial direct current stimulation (tDCS) is a non-invasive technique for stimulating the brain. It involves using a low-intensity direct electrical current to modulate brain activity in certain regions of the cortex. The cortex is the outermost layer of the brain that plays key roles in many diverse cognitive functions related to memory, attention, abstract thinking, language, and more.
The practice of using electricity to stimulate the brain dates back further than you might think, although its inception took on a cruder form than the current protocols used in research today. The first evidence of transcranial stimulation comes from the Roman Empire, when Scribonius Largus, a Roman physician, described how placing live torpedo fish (a type of ray capable of emitting electricity) onto a patient’s head could (supposedly) relieve headaches .
The first person to use direct current stimulation in a clinical setting was Giovanni Aldini, who in the early 19th century supposedly used this technique to cure a patient of major depressive disorder – at least, according to accounts by Aldini himself .
tDCS later became popular with German psychiatrists in the late 19th century for the treatment of psychotic patients – but due to a lack of consistency among procedures, unclear descriptions of the treatment, and a misunderstanding of certain aspects of tDCS, results from these studies were either inconclusive or highly inconsistent. This led to the abandonment of tDCS in the 1930s until it made a brief reappearance in the 1960s before being abandoned once again – this time likely due to the emergence of new psychiatric drugs, which (it was believed) made tDCS unnecessary.
Finally, it wasn’t until the late 20th century that tDCS emerged once again, with a considerable number of new clinical studies being conducted over the past two decades or so.
tDCS involves placing one electrode on the head or scalp over the brain area to be stimulated, and another electrode on the head or neck of the opposite side – and then running a current through them in order to either increase or decrease the activity of the underlying brain region. Sometimes, the electrodes are first soaked in salty water in order to enhance their ability to conduct an electric current, but this isn’t always necessary.
Electrodes are most commonly placed above the motor cortex, the area of the cerebral cortex that is responsible for planning and executing voluntary movements. However, in recent years, more studies have been done with stimulation to the dorsolateral prefrontal cortex (DLPFC).
Although the exact strength of the current can vary from study to study, a somewhat typical range of settings is generally between 1-2 milliamperes (mA), applied for about 20 minutes or less at a time .
There are two types of stimulation: anodal or cathodal. The main difference between the two is their effects on the amount of brain activity (neuronal excitability) in the cortical area underneath the site of stimulation. In general, anodal stimulation increases the excitability of the neurons the current runs through, while cathodal stimulation decreases the excitability of the stimulated area.
One of the most highly-studied effects of tDCS is its purported ability to affect the “membrane potential” of neurons in specific parts of the brain . Membrane potential is the difference in charge between the inside of the cell and the fluid outside the cell, with a typical neuron having a resting potential of about -70 mV. When the membrane potential is increased (made more negative), the neuron fires more readily (often referred to as increased excitability) – and when it is decreased, the neuron is made less excitable.
Anodal stimulation will tend to increase the membrane potential, thus increasing the excitability of the neurons affected, whereas cathodal stimulation will, in general, decrease the membrane potential, thus decreasing the excitability of the region being targeted .
While the electrical and physical mechanisms of the stimulation are fairly well-understood, the precise manner in which these mechanisms might produce noticeable cognitive or psychological effects remains largely unknown. However, a few possibilities have been suggested.
In some cases, the effects of tDCS have been reported to last up to several months after the initial use of the therapy. This may suggest that at least some of the reported effects of tDCS may be partly mediated through neuroplasticity (the ability of the brain to reorganize connections between neurons over time).
However, most studies that have been done on tDCS haven’t done long-term follow-ups to see how long their effects lasted – and some other studies which have done this have reported only temporary or shorter-lived effects. In addition to raising questions about whether tDCS has any meaningful long-term impact on brain function, these conflicting results also call into question whether long-term neuro-plasticity might actually be involved in its potential effects at all.
Other suggested mechanisms of tDCS include :
- Stimulating the release of brain-derived neurotrophic factor (BDNF), a protein that helps grow and create new neurons and connections in the brain (a process called neurogenesis).
- Stimulating the release of the neurotransmitter dopamine throughout the brain, and especially the prefrontal cortex.
- Stimulating the creation and activity of neural stem cells (NSCs), which could further support neurogenesis and synaptic plasticity throughout the brain.
However, it is important to note that these effects have not been directly studied or observed in humans, so they remain purely speculative until much more additional research is performed.
Although some studies may say otherwise, the truth is that the overall safety of tDCS is relatively unknown.
For example, nearly all of the individual studies on the safety of tDCS have looked only at very specific forms of stimulation (such as specific voltages, applied to specific parts of the brain) .
Another crucial limitation to be aware of is that the vast majority of studies only look at the “short-term” safety: usually, this means checking for side-effects only over a few hours or days from the time of use. However, this means that most studies don’t test for the possibility of subtler, “long-lasting” or “chronic” side-effects of tDCS.
Finally, another major limitation is that when it comes to clinical studies on tDCS, “safety” is very often defined simply as “doesn’t have any immediately obvious and major side-effects” . Note how this doesn’t mean quite the same thing that the average person probably thinks of when they think of something as being “safe,” which usually has more to do with both short-term and long-term risks and dangers!
Therefore, while you can find some studies that claim that tDCS is “safe,” the reality is these studies don’t apply as broadly as they might seem at first glance. At the end of the day, unless you can find a study that specifically tested the safety of the exact same stimulation protocol that you yourself are using – and if it was a long-term study that also did follow-up testing for any potential long-lasting side-effects – then the safety of any particular tDCS protocol should be considered unknown.
Another very important thing to keep in mind is that tDCS should only be administered by a qualified professional under a properly-controlled setting.
Unfortunately, many people attempt to use tDCS on themselves by building their own devices by themselves. This is particularly the case within the “nootropics” community, where people often use experimental supplements, compounds, and techniques to try to enhance their own cognitive abilities or potential. This “home-brew” or “D.I.Y.” (“Do-It-Yourself”) approach can come with many potential risks and dangers.
One of these dangers includes not controlling the voltage and/or current levels precisely enough. This means that a person might not actually know for sure what level of stimulation they are giving to their brain, which could open up the risk of unwanted effects or other adverse reactions. In especially severe cases, using a home-made tDCS device could even lead to electrical shocks, burns, and other harmful consequences.
Another major drawback of using “D.I.Y.” tDCS devices is a lack of appropriate electrode placement. The scientists who design and run experiments on tDCS are extremely knowledgeable in the anatomy and function of the brain – and therefore they are very deliberate about exactly where they target their tDCS stimulation, as well as how strong it is and exactly how long the stimulation sessions last.
People who try to apply tDCS to themselves often lack this knowledge, which essentially means that they’re just randomly stimulating some part of their brain, hoping to get a specific cognitive effect! Clearly, this is extremely unlikely to work out, except by pure chance.
Therefore, the only way to ensure proper tDCS administration is to seek out a qualified professional. While this practice is not extremely widespread yet, it is often possible to find local groups of medical practitioners or other mental health research professionals who offer this service to the public. Therefore, if you are interested in trying tDCS out for yourself, we recommend doing some research to find local professionals – otherwise, we do not recommend trying this out on your own at home.
In the sections below, we’ll outline some of the early research that has been done on tDCS and its potential effects on a variety of physiological and psychological processes.
However, it is important to keep in mind that the vast majority of this scientific research is still in a very early stage, and a lot more research will be needed before any solid conclusions can be made about the effects of tDCS in healthy human users.
As such, we are not officially recommending or endorsing any of the potential applications of tDCS below, as the science behind them is simply much too preliminary to come to any firm conclusions yet.
As always, none of the information below should be used to replace conventional medical care. If you believe that you might be experiencing any of the symptoms or health conditions discussed below, it is extremely important to talk to your doctor first to obtain an official medical diagnosis and develop an appropriate treatment plan.
It is also important to note that all of the findings below involve tDCS protocols and sessions that were administered by trained professionals, under very specific and controlled settings. In other words, there is no reason to assume that similar effects would be seen by someone using a “home-made” or “D.I.Y.” tDCS device on their own, since any of these particular effects would be highly dependent on exactly where and how the stimulation is being targeted (which only trained experts have the background knowledge to do properly).
With all that in mind, let’s see what some of the recent science has to say about the possible effects of tDCS.
Some early evidence suggests that tDCS may affect a person’s ability to learn and train new skills.
For example, according to one study, applying tDCS to individuals trying to memorize symbols was reported to improve number processing and numerical abilities, with effects lasting for up to six months after the initial treatment .
One systematic review of data from 13 different studies reported that 3-5 daily sessions of anodal tDCS (applied to the motor cortex) significantly improved motor sequence learning .
According to a study on 104 subjects, tDCS reportedly improved learning rates when applied to the right inferior frontal and right parietal cortex .
Stimulation of the left or right dorsolateral prefrontal cortex (DLPFC) was reported to improve driving abilities in a simulator in 24 volunteers .
While these early results are promising, they also suggest that tDCS probably has to be applied to very specific areas in order to target different particular types of learning and cognitive abilities. More research will be needed to explore these potential effects further.
Many conditions involving chronic pain – such as fibromyalgia, for example – are often difficult to treat. In part, this is because some of the more “common” treatments, such as opioids, often cause undesirable side-effects, and can even have a high risk of leading to addiction.
However, there are some promising early findings that may suggest that tDCS could potentially be used to treat (or at least better-manage) pain in some conditions and circumstances.
For example, according to one randomized controlled trial (DB-RCT) in 48 fibromyalgia patients, five twenty-minute sessions of anodal tDCS applied over the primary motor cortex (M1) reduced pain intensity ratings 30 days after the last treatment .
Similarly, another DB-RCT study in fibromyalgia patients reported that 10 daily sessions of anodal stimulation over the motor cortex resulted in improvement in pain scores and quality of life at both 30 and 60 days post-treatment .
In a smaller crossover study in seven patients with difficult-to-treat chronic pelvic pain, two days of 20-minute tDCS sessions were reported to result in significantly reduced pelvic pain two weeks after treatment .
Finally, one study in patients with chronic pain (due to traumatic spinal cord injury) reported that five consecutive days of tDCS treatment reduced pain by up to 58% in comparison to control treatment .
Nonetheless, while these early results are promising, much more research will be needed before tDCS could become an officially-approved and widespread medical treatment for managing pain.
tDCS was also reported to improve performance in language tasks in 3 people with language impairment (aphasia) and improved word retrieval (DB-RCT study) [19, 20]. Additionally, tDCS improved the detection of mismatches in 36 subjects, and enhanced grammar ability in another 50 subjects [21, 22].
Some preliminary studies have suggested the potential of tDCS to help treat or alleviate some of the symptoms of depression.
For example, one study (DB-RCT) in 22 patients with treatment-resistant major depressive disorder applied anodal stimulation to the dorsolateral prefrontal cortex for two weeks. While overall depression scores did not differ between tDCS and placebo groups, the tDCS group reported increased subjective ratings in positive emotions compared to the control group .
Another randomized controlled trial reported that 3 weeks of anodal stimulation in patients with depressive symptoms resulted in significant improvements in mood, attention, and working memory compared to “placebo” tDCS (also known as “sham” treatment, which basically just means fake treatment that doesn’t actually do anything) . However, one patient in this study was reported to develop a mild form of mania, which may suggest some potential negative side-effects of using tDCS to try to treat mood disorders.
While these early findings might sound promising, they are still very preliminary, and much more research will be needed to fully verify the effectiveness and safety of this potential application of tDCS. In the meantime, it is unlikely that tDCS will become a “mainstream” medical treatment for mood disorders anytime soon.
In one small study of six individuals with insomnia, tDCS applied to the dorsolateral prefrontal cortex during sleep was reported to improve “sleep efficiency” – meaning that it decreased the amount of time the subjects spent in lighter stages of sleep, while also increasing the amount of time spent in stages of sleep associated with “deeper” sleep .
In another study of 32 patients with post-polio syndrome (a condition developed after suffering from polio and characterized by deteriorating muscle strength endurance), daily anodal stimulation of the premotor cortex for 3 weeks was reported to result in significantly improved sleep quality, vitality, and social functioning in comparison to control treatment .
However, there are several important limitations to keep in mind with these early studies. For one, the sample sizes are still very small, and much larger samples will be necessary to fully verify these preliminary findings. Secondly, some of these studies were only done in patients with specific medical conditions (such as post-polio syndrome), and so it’s not yet clear if similar results would necessarily be seen in otherwise-healthy human users as well.
ADHD is a condition characterized by increased impulsivity and an inability to focus attention on relevant tasks. Some early research has investigated the potential of tDCS to target and potentially alleviate some of these core ADHD symptoms.
For example, according to one study in 21 male adolescents with ADHD, anodal stimulation with tDCS applied to the right inferior frontal gyrus was reported to significantly improve their ability to ignore irrelevant and competing information on a task designed to test selective attention and information processing (the “flanker” task) .
Another study reported that in nine ADHD individuals, tDCS on the prefrontal cortex improved the speed of processing information as well as the ability to between activities .
Once again, however, these studies have very small sample sizes, and so much more research will be needed to verify and extend these initial preliminary findings.
Both anodal stimulation to the motor cortex of the affected hemisphere and cathodal stimulation to the motor cortex of the unaffected hemisphere have been reported to improve motor function in one study (DB-RCT) of stroke patients .
Another study investigated the effects of tDCS in 10 stroke patients with stroke-induced aphasia (an inability to understand or produce language due to brain damage from the stroke). This study reported that five days of anodal stimulation of the left prefrontal cortex for 20 minutes significantly improved these patients’ ability to name objects. Although it is not known for sure how long-lasting or permanent these effects might be, these improvements were observed to last up to one week in this study .
Finally, another study examined “sham” (fake) tDCS stimulation in comparison to both anodal and cathodal stimulation of the right and left superior temporal gyrus in stroke patients with aphasia. This part of the cortex contains a region called Wernicke’s Area, a critical brain network involved in the ability to comprehend spoken language. According to this study, 5 weekly tDCS sessions for 2 weeks led to an improvement in verbal comprehension in all of the groups – including the “fake” (control) group! While the cathodal stimulation group showed stronger effects than the other two groups, this interesting pattern of results suggests that at least some of the effects of tDCS might simply be due to the placebo effect .
Cathodal, but not anodal, stimulation to the dorsolateral prefrontal cortex (DLPFC) was reported to reduce risk-taking behavior according to one early DB-RCT study .
In another study, anodal – but not cathodal – stimulation of the DLPFC resulted in significantly reduced preferences for risk in a risk-taking game .
However, while these early results are suggestive of a possible effect of tDCS on risk-taking behavior, much more research will be needed to verify and extend these preliminary findings, and to determine just how significant or relevant they might be.
One early study in 12 healthy volunteers reported that tDCS applied to the left dorsolateral prefrontal cortex improved verbal reaction times .
While the authors of this study proposed that this effect might be due to increased neuro-plasticity, more research (in much larger groups of participants) will be needed to confirm these findings, as well as to flesh out the underlying mechanisms involved in these potential effects.
Working memory is a type of short-term memory that refers to a person’s ability to hold a set of information in memory, while actively manipulating it.
An example of this would be having to listen to a phone number, then adding ‘1’ to each digit and repeating it back. “Short-term memory” usually refers just to the first part (of memorizing the phone number), whereas “working memory” includes this as well as the subsequent parts (i.e. the cognitive “manipulation” of this information).
Interestingly, another study reported that anodal stimulation of the cerebellum did not have any effect on working memory . This finding makes sense since the prefrontal cortex is the part of the brain most commonly linked to higher cognitive abilities (including working memory).
However, many of the sample sizes in these studies were quite small, and it’s also not known how long any of these early reported effects might last – in other words, these “enhancements” might just be temporary, and might not actually enhance overall cognitive performance in any long-lasting or “permanent” way. In any case, much more research (preferably in healthy human users of tDCS) will be needed to confirm these preliminary findings.
However, this research is in an extremely early stage, and so these early results will need to be followed up on by a lot more studies to confirm them.
Some preliminary research suggests that tDCS may be potentially helpful for managing problematic cravings, such as for junk food or even drugs.
For example, one study reported that tDCS applied to the prefrontal cortex reduced cravings for sugary and high-carbohydrate foods in 19 people, although it had no effect on the amount of food they actually consumed if they did decide to eat them .
Another study applied tDCS to the dorsolateral prefrontal cortex, and reported reduced food cravings in 30 individuals with binge eating disorder (BED), significantly reducing cravings for sweets and savory proteins . Moreover, unlike the previous study described above, in this case, tDCS was also reported to decrease their total food intake by 11%, suggesting that tDCS may have a genuine effect on actual eating behaviors – at least, in some specific populations (such as patients with BED).
Finally, another study (DB-RCT) in 27 smokers reported that single sessions of tDCS applied to the dorsolateral prefrontal cortex over five days resulted in a significant decrease in cravings in response to smoking cues, and even reduced the total number of cigarettes smoked in comparison to the control group .
While these findings are quite interesting and promising, much more research will be needed to fully understand the potential applications of tDCS in managing cravings, and exactly who- and when it might help. In the meantime, it is unlikely that tDCS will become a standard form of treating eating disorders or drug-abuse disorders anytime in the near future.
Finally, some early evidence has looked at the potential of tDCS to possibly help treat or manage the symptoms of some major neurological disorders, such as Parkinson’s disease.
For example, in one DB-RCT study on eight patients with Parkinson’s undergoing physical training, adding tDCS stimulation on the primary motor and premotor cortex to these patients’ treatment protocols was reported to improve their overall walking speed and balance (two aspects of motor behavior that are often severely disrupted in Parkinson’s patients) .
However, this was only one study so far, and included just eight patients – so a lot more clinical research will be needed to see just how significant the potential effects of tDCS in Parkinson’s disease actually are.
Because tDCS is a broad category that applies to many different levels of voltage and current, and many different specific sites of stimulation in the brain, it’s hard to come to any general conclusions about the overall safety of tDCS as a whole.
On the plus side, the authors of one large-scale review have suggested that – in theory – most of the known dangers of applying electrical stimulation to the head and cortex would have to involve levels of electrical current that are generally much higher than those typically used during most tDCS protocols .
While this is slightly heartening news, the fact remains that the safety of the many different individual tDCS protocols out there has not been specifically or directly studied – and so at this time, it is not possible to come to any firm or definitive conclusion about the overall safety of this brain-stimulation technique as a whole.
This general lack of safety information also means that we don’t know as much as we should about the potential side-effects involved in tDCS.
- Insomnia / other sleep issues
- Lingering sensations of itchiness and/or “tingling”
- Skin burns, or persistent “burning sensations” (even in the absence of any “real” skin burn)
- Psychotic symptoms
Additionally, some preliminary findings suggest that people with pre-existing mood disorders (such as depression) or other psychiatric conditions may be at especially higher risk of experiencing these more-severe side-effects [5, 46].
Some researchers have also raised concerned that tDCS may damage pacemakers. While some studies have found no negative effects on pacemakers, only certain models and types of pacemakers have been tested ; therefore, caution is highly advised for anyone with a pacemaker.
Finally, some researchers have also suggested the possibility of “indirect” consequences of tDCS on regions immediately surrounding the ones being stimulated. The idea is that while the stimulation of one specific brain area could theoretically provide certain benefits in one particular cognitive function, it could also interfere with the cognitive functions of the other surrounding areas at the same time. While this possibility hasn’t been experimentally verified, it does raise additional questions about whether the purported effects of tDCS might actually be “benefits” per se, or if they might instead come with important negative trade-offs in other areas of cognition.
While the research on tDCS and its effects are still in a very early stage overall, some of the early results suggest some interesting future potential. Nonetheless, most – if not all – of these purported effects and applications will still require extensive additional research to fully confirm and validate them. By extension, at this point, it is relatively unlikely that tDCS will become an officially-approved and widely-used medical treatment in the near future.
Furthermore, all of these purported effects only apply to tDCS that is administered by trained and qualified professionals. Unfortunately, some people attempt to give themselves tDCS treatments “at home” using “home-made” or “D.I.Y.” tDCS machines. This approach carries substantial risks and potential dangers and is not recommended.
Additionally, the overall safety of tDCS in healthy human users – especially its potential long-term effects – has not been well-established, and many important questions still remain. Much more research will be needed to determine exactly which stimulation parameters and techniques are safe, as well as if any of their purported “benefits” are actually significant and long-lasting, rather than being merely temporary or based in the “placebo effect.”
In conclusion, tDCS offers many promising future avenues of scientific and medical investigation, but is not yet at a stage where it can be adopted for widespread use as a conventional medical treatment.