Transcranial direct current stimulation (tDCS) is the use of an electrical current to enhance certain brain functions, with benefits such as increased blood flow, faster motor learning, and improvement of depressive symptoms.
Read more to see how tDCS affects your brain and who benefits from it.
What is tDCS?
Transcranial direct current stimulation (tDCS) is a non-invasive technique that uses a low-intensity direct current to modulate brain activity in certain regions in the cortex. The cortex is the outermost layer of the brain that plays key roles in the memory, attention, cognition, language, and consciousness.
The practice dates back further than you would believe, although its inception took on a cruder form than the current protocol used in research today. The first evidence of transcranial stimulation comes from the Roman Empire, when Scribonius Largus, a Roman physician, described how situating live torpedo fish (a type of ray capable of emitting electricity) over a patient’s head could relieve headaches .
The first person to use direct current stimulation in a clinical setting was Giovanni Aldini, who in the early 19th century used the technique to cure a patient of major depressive disorder, according to detailed accounts by Aldini .
TDCS became popular with German psychiatrists in the late 19th century in the treatment of psychotic patients, but due to a lack of uniformity among procedures, unclear descriptions of treatment, and a misunderstanding of certain aspects of tDCS, results from studies were either inconclusive and/or inconsistent. This led to the abandonment of tDCS in the 1930s until the 1960s where it once again reappeared, only to be abandoned again, likely due to the emergence of new psychiatric drugs.
It wasn’t until the late 20th century that tDCS emerged once again, with a large number of clinical studies being conducted in the last two decades that have attested to its effectiveness and lack of serious side effects.
How Does it Work?
TDCS involves placing one electrode on the area to be stimulated and the other on the head or neck of the opposite side and running a current through them in order to either increase or decrease the activity of neuronal activity. The electrodes are first soaked in salty water in order to conduct an electric current.
Electrodes are most commonly placed above the motor cortex, the area of the cerebral cortex (outer layer of the brain) that is responsible for planning and executing a voluntary movement. However, in recent years, more studies have been done with stimulation to the dorsolateral prefrontal cortex (DFC).
The strength of the current is between 1-2 mA and is usually applied for a duration of up to 20 minutes .
There are two types of stimulation: anodal or cathodal. The main difference between the two is their effects on neuronal excitability, or the ability of a neuron to react to stimuli. In general, anodal stimulation increases the excitability of the neurons the current runs through, while cathodal stimulation decreases the excitability of the stimulated area.
The effects of tDCS have been shown to last up to months after initial use of the therapy, indicating that the benefits attributed to tDCS are mediated in some part through neuroplasticity, or the ability of the brain to reorganize connections between neurons.
Mechanism of Action
One of the most studied effects of tDCS is its ability to affect the membrane polarity of the neuron . 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 potential of -70 mV. When membrane potential is increased (made more negative), the neuron fires more readily (referred to as excitability), and when decreased, the neuron is less excitable.
Anodal stimulation will tend to increase the membrane potential, thus increasing excitability of the neurons affected, whereas cathodal stimulation will, in general, decrease the membrane potential, decreasing the excitability of the region .
Long term effects from tDCS are attributed to its ability to enhance neuroplasticity
Other suggested mechanisms of tDCS include the release of brain-derived neurotropic factor (BDNF, a protein that helps grow and create new neurons and connections in the brain), the release of dopamine, and an increase in neural stem cells . However, these effects have not been observed in humans so they remain unsubstantiated.
Side Effects and Risks
Surprisingly, tDCS has been shown to have very few observed side effects, with the most prominent being a tingling sensation, fatigue, and itching under the electrodes during stimulation, and nausea and headaches after stimulation .
However, some researchers have noted that there may be indirect consequences of tDCS on regions next to the ones being stimulated. Stimulation of one area may provide benefits in certain cognitive capabilities while decreasing one’s cognitive abilities in other aspects.
Moreover, we also do not know of the long-term effects of multiple daily sessions over periods of years or even a lifetime as it has never been studied.
Caution is also warranted for those considering making their own DIY tDCS device. Users who pursue this route should make sure they applying the correct amount of electric current, positioning the electrodes in the appropriate areas to achieve the desired effects, and using the correct stimulation.
Benefits of tDCS
1) Improves Learning and Skill Acquisition
When tDCS was applied to individuals trying to memorize symbols, it improved number processing and numerical abilities, with effects lasting for up to six months after .
A systematic review of 13 studies found that daily sessions for 3 or 5 days of anodal tDCS applied to the motor cortex significantly improved motor sequence learning .
According to a study on 104 subjects, tDCS improves learning rate when used on the right inferior frontal and right parietal cortex .
Stimulation of the left or right dorsolateral prefrontal cortex improved the driving ability in a simulator in 24 volunteers .
2) Decreases Pain in Chronic Pain Conditions
The pain involved with chronic pain conditions such as fibromyalgia is often difficult to treat, with conventional options like opioids posing undesirable side effects and a high risk for addiction.
In a randomized controlled trial (DB-RCT) in 48 fibromyalgia patients, five 20-minute sessions of anodal tDCS applied over the primary motor cortex (M1) reduced pain intensity ratings 30 days after the last treatment .
Another randomized, controlled (DB-RCT) study in fibromyalgia patients found that 10 daily sessions of anodal stimulation over the motor cortex resulted in improved in pain scores and quality of life at both 30 and 60 days post-treatment .
A smaller crossover study (study design where every subject is first exposed to treatment, and then control) in seven patients with difficult-to-treat chronic pelvic pain, two days of 20-minute tDCS sessions resulted in significantly reduced pelvic pain two weeks after treatment .
In patients with chronic pain due to traumatic spinal cord injury, five consecutive days of tDCS reduced pain by 58% in comparison to control treatment (sham) .
3) Enhances Language Learning and Abilities
TDCS improved the performance of language tasks in 3 people with aphasia (language impairment) and improved word retrieval (DB-RCT) [18, 19]. Also, tDCS improved detection of mismatches in 36 subjects and grammar ability in 50 subjects [20, 21].
4) Improves Symptoms of Depression
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 depression scores did not differ between tDCS and placebo, however, subjective mood ratings showed an increase in positive emotions compared to controls .
Another randomized controlled trial found 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 . Interestingly, one participant in the treatment group became a mild form of mania, a condition characterized by increased energy and elevated mood and an absence of functional impairment.
5) Improves Sleep Quality
In a small study of six individuals with insomnia, tDCS applied to the dorsolateral prefrontal cortex during sleep improved sleep efficiency, and decreased the amount of time the subjects spent in lighter stages of sleep while increasing the amount of time spent in stages of sleep associated with deeper sleep .
In 32 patients with post-polio syndrome (a condition developed after suffering from polio and characterized by deteriorating muscle strength endurance), anodal stimulation of the pre-motor cortex daily for 3 weeks resulted in significantly improved sleep quality, vitality, and social functioning in comparison to control treatment .
6) Improves Symptoms and Task Performance in ADHD Patients
ADHD is a condition characterized by the inability to control actions in a goal-directed manner, control impulsivity, and focus attention on relevant tasks.
In 21 male adolescents with ADHD, anodal stimulation with tDCS applied to the right inferior frontal gyrus significantly improved the ability of to ignore irrelevant and competing information on a task designed to test selective attention and information processing (Flanker Task) .
Another study found that in nine ADHD individuals, tDCS on the prefrontal cortex improved the speed of processing information as well as the ability to between activities .
7) Improves Recovery in Stroke Patients
Both anodal stimulation to the motor cortex of the affected hemisphere and cathodal stimulation to the motor cortex of the unaffected hemisphere resulted in improved motor function in stroke patients (DB-RCT) [R].
When ten stroke patients with an inability to either understand or express language (aphasic) due to stroke were treated with 20 minutes of anodal stimulation over their left prefrontal cortices for five days, there was a significant improvement in the ability to name objects . The effect lasted up to a week after treatment was stopped.
Another study examined sham (fake) stimulation in comparison to anodal and cathodal stimulation of the right and left superior temporal gyrus (Wernicke’s Area, a region of the brain associated with speech comprehension), respectively, in stroke patients with apahasia. After daily tDCS session five times a week for 2 weeks. every group (control group included) saw an increase in verbal comprehension, indicating the presence of a placebo effect . However, the cathodal stimulation group saw a much large increase in verbal comprehension in comparison to the other two groups.
8) Reduces Risk-Taking Behavior
Cathodal, but not anodal, stimulation to the dorsolateral prefrontal cortex (DLPFC) reduced risk-taking behavior in a double-blind randomized controlled trial (DB-RCT) .
In another study, anodal, but not cathodal stimulation of the DLPFC stimulation resulted in a significantly lower risk preference on a risk game 
9) Improves Verbal Reaction Times
TDCS applied to the left dorsolateral prefrontal cortex improves verbal reaction time, suggesting that it may enhance neuroplasticity, according to a study in 12 healthy volunteers .
10) Improves Working Memory
Multiple studies have shown that stimulation of the dorsolateral prefrontal cortex improves working memory [33, 34, 35]. Working memory is a variation of short-term memory that allows for the manipulation information in order to achieve a goal.
Interestingly, anodal stimulation to the cerebellum did not have any effect on working memory .
11) Improves Visual Attention
12) Reduces Cravings
One study found that tDCS applied to the prefrontal cortex reduced cravings for sugar and high carbohydrate food in 19 people, although it had no effect on the amount of food consumed .
TDCS using electrodes on the dorsolateral prefrontal cortex has also been shown to reduce food cravings in 30 individuals with binge eating disorder (BED), significantly reducing cravings for sweets and savory proteins . Moreover, tDCS also decreased total food intake by 11%.
Another study (DB-RCT) in 27 smokers found 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 the number of cigarettes smoked in comparison to the control group .
13) Improves Symptoms of Parkinson’s
In a double-blind, randomized trial study on eight patients with Parkinson’s undergoing physical training, the inclusion of tDCS on the primary motor and premotor cortex improved walking speed and balance (DB-RCT) .
The premotor cortex plays a role in movement, in understanding the actions of others, and in using abstract rules to perform specific tasks.
A user experimented with using tDCS for a year and found that it improved their visual perception, alertness, and strength and endurance. They also noted that the benefits lasted after ceasing use of tDCS.
Another blogger found that 130 daily sessions of tDCS significantly increased their working memory (Dual N-Back), applying statistical analysis to their results to ensure they were statistically significant.
Other users have reported experiencing headaches, fatigue, and a decrease in their ability to focus, but that the effects were temporary and stopped after discontinuation with tDCS.
The current evidence strongly supports the use of tDCS to enhance cognitive functions in healthy individuals and as a clinical treatment in neuropsychiatric disorders and decreasing pain in chronic pain conditions. Further studies are needed to elucidate the mechanisms by which tDCS exerts its effects as well as optimal treatment times, intensity, type of stimulation, and frequency of treatment.