tDCS: Transcranial Direct Current Stimulation

yellow sponge with lightning symbolizing electricity and pink arrow showing direction of flow to brain illustration with a spot of light symbolizing stimulation

Transcranial direct current stimulation or tDCS is a non-invasive brain stimulation technique. The other one discussed on this site is cranioelectrical stimulation. Unlike it and audiovisual entrainment, tDCS has no frequency.


“The first report of therapeutic effect of low intensity galvanic current applied over the head was provided in 1804 by Giovanni Aldini, the nephew of Luigi Galvani, with a detailed description of a 27-year-old farmer who was completely relieved from melancholia after 6 weeks of stimulation.”

Jean-Pascal Lefaucheura,, Fabrice Wendling. Mechanisms of action of tDCS: a brief and practical overview. Editorial. Neurophysiologie Clinique Volume 49, Issue 4, Pages 269-275. September 2019.

Scientists have conducted approximately 1500 studies involving all kinds of clinical uses since Alberto Priori and colleagues in Italy in 1998 and Michael Nitsche and Walter Paulus in Germany in 2000 restarted clinical research on tDCS.

20th Century Research

With the development of higher-quality electronic stimulators in the 20th century, scientists have been able to demonstrate that tDCS excites neural activity within the sensorimotor cortex in anesthetized rats. Bindman et al, in 1964 documented the effects of stimulation polarity plus the length of time, which persisted for hours after the end of stimulation.

Later, Rush and Driscoll in 1968, Dymond et al in 1975, and Lolas in 1977, showed that a DC current flow could be introduced into the brains of both healthy subjects and patients suffering from psychiatric diseases. Unfortunately, mainly due to the lack of relevant tools to assess the global brain benefits of tDCS, this technique was nearly forgotten for 25 years.

Fortunately, around the turn of the 21st century, and with major advances in neuroscience, tDCS was re-discovered as a tool to modulate human brain activity and its physiological effects and benefits began to be systematically explored by Priori et al in 1998 and Nitsche and Paulus in 2000.

Introduction to tDCS

tDCS stimulates the brain underneath a wet sponge electrode(s); the resulting current flows through the neural networks being used during stimulation. Trained specialists use it in a clinical setting to modulate neuronal activity. Anodal stimulation inhibits delta and theta brain waves while exciting beta brainwave activity; cathodal stimulation does the opposite. it works particularly in the beta and gamma brainwave frequencies.

Neuromodulation uses technology to harness the brain’s ability to change and remold itself. tDCS acts on this ability through neurostimulation in order to effect reactivation of poorly functioning or underpowered areas. Properly done, neurostimulation guides the brain into improved functioning in people of all ages, from children to elders, at any time after brain injury. As far as I know, there is no time limit for restoring brain function after injury. Once the person performs as well as the standard population in IQ, memory, and other cognition tests, then they can end tDCS therapy. Basically, neurostimulation is a way to help a person with brain issues, big and small, get better.

But we’ve also seen those in the normal population perform above-and-beyond normal functioning following neurostimulation of all types.

Principles of Treating the Brain

Three principles guide most techniques of neurostimulation or brain training. Most take time to see results. tDCS works within 20 minutes.

The first principle is to reboot, repair, or rewire the brain.

The second principle is to train the brain to the edge of its ability through intensity and frequency of training over time with rest breaks. When training begins to become easier, increase the difficulty so as to continue to train to the edge of ability. Reassess regularly through feedback and progress discussions with the client and by using the same objective tests at the end of the treatment cycle. Exhaustion is a given, and so build in rest in between brain training sessions. Research shows learning works best with breaks in between.

The third principle is to engage in desired activity during or immediately after neurostimulation in order to stimulate rewiring of the neural networks involved in that activity.

tDCS and the Principles

TDCS neurostimulation follows these principles. It activates a targetted neural network, which in turn reinforces neural networks. Neurons that fire together, wire together. It’s often done over time once or twice per week and may also be done daily. However, rest periods allow the brain to recover and build up its (re)learning. From personal experience, pushing the injured brain too hard can have deleterious effects. The specialist must tailor the frequency to the person and monitor the effects closely.

small currents effect large changes in the delicate brain

Given that tDCS activates the damaged region, it’s important to engage that region with an activity while the neurons are excited and ready to work. Because these regions are excited, they re-establish communication between those regions that cooperate together when executing a particular task. One needs only a small amount of current to effect large changes in the delicate brain while engaging the networks one wants to work on. That’s why you do the activity you want to improve, during or just following stimulation. So, for example, if you want to improve language and speech for conversation, then place the tDCS sponge electrode over Wernicke’s Area as an anode. Then during 10 to 20 minutes of stimulation, engage in conversation. If there are mouth and tongue motor issues, where articulation of phonemes and verbal expression is difficult, then it would be a good idea to stimulate Broca’s Area as well.

tDCS units come with preset sessions that the specialist must “tune” to the size of treatment electrode being used. The ones used for brain injury should include a variety of stimulating currents and have a few different electrode sizes and configurations.

Issues with Do-It-Yourself and Incomplete Studies

People can purchase do-it-yourself tDCS units, perhaps thinking because the voltages are small and the effect initially lasts only hours, that it can do no harm even when used improperly. However there are two issues with that assumption.

  • The current must be properly controlled using a technique called “current sourcing.” Do-it-yourself tDCS “devices” don’t employ current sourcing. So, at the onset of stimulation, when the skin resistance is high, there won’t be enough current to produce a clinical effect. But as the skin begins conducting better and the connection improves, the current could end up being much too high, burning the skin and causing neural over-stimulation. As mentioned before, the electrode also must be tuned to the current used to ensure a proper clinical effect without doing harm.
  • It takes a trained specialist who understands brainwaves, neurophysiology, brain injury, and electrical physics to use tDCS properly to increase electrical activity in the injured brain or inhibit overactivity without causing undue harm. A trained specialist conducts tDCS while sitting in close proximity to the person so as to watch them for any sign of overwork and to participate in activating the desired network.

In first-generation studies on stroke, “the negative effects of extra-orbital (prefrontal) cathodal stimulation were largely ignored.

“In second-generation studies, the language, motor and speech areas of the left brain have homologous or “mirror” regions in the right hemisphere. Following damage from stroke or injury, these areas typically try to take over functionality of the lesioned/injured left side to restore some functionality. Under normal circumstances, this is a good thing. However, given we have the ability to restore much of the lesioned areas of the left hemisphere using tDCS, those attempts of the right-side to take control interfere with therapy. So, the practice of bilateral stimulation involving cathodal DC stimulation over the homogolous areas proved to be an even more effective means of inducing improvements in speech, language and motor control. It does so by inhibiting the Broca’s, Wernicke’s and motor homogulous areas, which attempt to take over control of the lesioned area and thus impede rehabilitation.”

Dave Siever. Transcranial DC Stimulation. Neuroconnections. Spring 2013.

Potential Consequences

In my book Lifeliner: The Judy Taylor Story, I relate what happened to an earlier patient because studies didn’t acknowledge problems, which lead to the treating physician being unaware of what could go wrong. My experience with tDCS also highlights the importance of listing negative effects in research studies. These clinical experiences also reinforce how important it is to have responsive specialists closely monitoring treatments. By responding immediately to the same issue in me as happened in first-generation stroke studies and changing location, they stopped negative effects and achieved the radical success in language as seen in the better-designed stroke studies. A lay person relying entirely on studies or preset sessions would be unable to assess why negative effects happened and adjust.

tDCS versus ECT and rTMS

Powered by a 9V battery, tDCS directly stimulates the brain underneath its sponge electrode from 0.25mA to up to 2mA (milliamperes or milliamps). for a short period of time, which is usually 20 minutes but as short as 10.

In contrast, ECT — the electroconvulsive therapy of movies and still used in hospitals — and rTMS or repetitive transcranial magnetic stimulation are both plugged into the electrical power grid as they require and deliver more current. ECT is meant to induce seizures. ECT is 800 mA versus tDCS’s maximum of 2mA. rTMS uses a magnetic coil to generate electrical current that then stimulates the brain.

The magnetic coil of “TMS requires high currents [several thousand milliamperes] to be pulsed through the coil, which generates a loud acoustic impulse whose peak sound pressure level (SPL) can exceed 130 dB.” [rTMS is TMS applied over a specific region.]

Lari Koponen, Stefan Goetz, Angel V Peterchev. Double-containment coil with enhanced winding mounting for transcranial magnetic stimulation with reduced acoustic noise. July 2020. Preprint.

The rTMS’s magnetic coil is significantly larger than the tDCS’s sponge electrodes. tDCS’s active sponge electrode is 20cm2 (though in some studies it can be as large as an ineffective 49cm2), The reference sponge electrode is 60cm2. Being larger in size, rTMS stimulates a greater square area of the brain than tDCS. This is not a desirable treatment modality, to extend the range of stimulation beyond the dysfunctioning or damaged area. The specialist can target specific areas of the brain vastly better with tDCS than rTMS. That leads to desired outcomes with fewer unwanted effects in uninjured areas of the brain. (See the video under Resources for why head and reference sponge electrode sizes are important for treatment success.)

What Is tDCS?

Transcranial direct current stimulation or tDCS creates a current between an electrode and the targetted area of the brain the electrode sits over. Like all electrical current, for current to flow, one end needs to be positive, the other negative. Anodes are positive; cathodes are negative. Think of a battery. One connector has a plus sign. That’s the positive connector or anodal; the other is negative or cathodal. When the wet sponge electrode is an anode, it makes the brain the cathode and current flows toward it, that is, increasing negativity goes through the skull and brain toward the cathode. That increases electrical activity in the part of the brain underneath the electrode and in its related neural network as current flows along those pathways, too.

If there’s too much brain power or activity in a certain area, then the sponge electrode is the cathode, making that part of the brain an anode. Thus, electrical activity decreases in that brain area as current flows toward the sponge electrode.

Preset sessions on tDCS units are based on research involving hundreds of studies on conditions such as anxiety, depression, language, speech, sight, hearing motor control, plus all types of processing speeds and proficiencies. The specialist must have knowledge of locations of brain functionality and their associated networks. Given these complexities, the specialist must have knowledge of Brodmann Areas and EEG 10-20 system locations.

The Electrodes

tDCS is a hand-sized unit powered by a 9V battery (and an internal voltage multiplier up to 24V) with two sponge electrodes, the larger being the reference electrode.

The sponges are like the prongs of an electrical plug. Each sponge on the head corresponds to a different wire; the larger sponge electrode on the shoulder is the reference like the neutral third prong of a plug. It’s important for success that the sponge electrode on the head is smaller than that of the reference one on the shoulder.

The reference sponge electrode goes on the opposite shoulder to that of the treatment electrode. If the treatment electrode is on top of the head, then the reference electrode is placed under the chin. If the treatment electrode is centred over the front of the head, then the reference is placed over the bottom-end of the neck on the back. And so the reference can be on the shoulder, neck, or chin, depending on where the head electrode is located. The aim is to direct the stimulation at right angles to the neurons being stimulated.

The Procedure


tDCS is a treatment that’s informed by qEEG. However, DTI (diffuse tensor imaging) will tell the specialist the state of your neural networks and help them predict any potential unexpectedly negative outcomes with preset sessions. Specialists should, in any case, customize sessions for people with brain injury because of the possibility of unexpected results with preset sessions. When the specialist reviews with you your test results, they’ll explain why they’re offering tDCS, what they hope to achieve, and the frequency and expected duration of the treatment.

The Treatment

The specialist will sit near you and explain what will happen so you can be reassured this isn’t ECT at all. The qEEG tells the specialist what area of the brain to treat and thus where to place the sponge electrode. The specialist will measure your head to ensure it’s placed correctly. A very experienced specialist will know where to place it without using a measuring tape. They’ll soak the sponge in water (or saline) and will secure it to your head with a soft headband. They’ll then place the soaked ground sponge on the shoulder opposite to the head placement in order to complete the circuit. Your sleeve may be all that’s needed to secure the shoulder sponge in place. Or you may need a soft weight. Your head and shoulder may get a little wet from the sponges. It’ll dry off.

When you’re ready, the specialist will turn the unit on and select the desired session. They’ll ensure the electrodes have good contact and that you don’t feel the current aside from a mild tingling. You must speak up if you feel burning or pain because it means there isn’t good contact. Also, that isn’t a good thing. During the session, the specialist will engage in desired activity, such as for example conversation if you need to regain conversational skills.

Since every person is unique, the specialist may learn that one person requires 10 minutes while another 20. You must thus provide feedback during and after the tDCS sessions about what you’re feeling physically, mentally, cognitively, and emotionally. Keeping a diary during the first few sessions will help the specialist. And the specialist must begin these sessions with caution and close monitoring in the following hours and days.

tDCS affects functioning within 15 to 20 minutes. And the effects will last hours. Upon repeated use, the effects become permanent, and tDCS can then be discontinued. For more on what tDCS is like, see my posts on being a test case for treating aspects of brain injury.

What To Do In Case of Negative Effects

Because we know little about the brain and even less about brain injury, a session may not have the expected effect and may, in fact, result in worsening of symptoms. The worsening may not occur until hours later.

This is why it’s important the specialist be accessible and responsive.

The person must be able to phone, email, or secure message in order to relate unwanted effects, receive reassurance and advice as to what to do, and feel secure that direct electrical stimulation will not damage them further. The specialist must then immediately change the protocol in light of any negative feedback. It may be that tDCS is not appropriate for one kind of symptom but highly effective for another. Thus upon review of the qEEG and, hopefully DTI, and in light of feedback, they’ll move the site to a more appropriate placement. Or it may be that the session is at too high a milliampere for the person and needs to be at a lower setting. Or that tDCS ought to be spaced out more.

Reported negative effects include:

  • Mild tingling
  • Moderate fatigue
  • Slight itching
  • Slight burning or mild pain
  • Headache
  • Trouble concentrating

Mild tingling would be consistent with the fact a battery-operated electrical current is being applied, but burning from bad contact with the skin. Fatigue is usually because the brain is being made to work, just like if you were exercising. Dizziness from lower blood pressure fades over time as the brain and body become used to the stress-releasing effects. (If a manufacturer or clinic does not warn against negative effects or claims none, avoid that model.)

Because treatments like tDCS are unknown to most specialists and the population, it’s important the specialist explain it and what to expect and to be available for any questions. As the person becomes used to this technology, their comfort with it will increase and questions will fade.

What Happens At The End

When the session ends and the electrodes removed, the person may feel a brief wave of dizziness and the need to have one big, good scratch where the head electrode was. Soft off technology can mitigate the dizziness. The person can then move on to the next part of their treatment such as brain biofeedback within the same appointment while enjoying the improvement tDCS has brought.


tDCS is a powerful tool in the brain-injury recovery kit. That’s why Johns Hopkins Physical Medicine and Rehabilitation is using it for traumatic brain injury and stroke. Like with other treatments, its effects last hours at first. But over time, they’ll become permanent. Unlike audiovisual entrainment, for example, its advantage is to target specific areas to recover specific abilities. Once achieved, it wouldn’t be needed anymore. Thus it’s an effective time-limited treatment even for those whose brain injury occurred years earlier.

For those without specialists who prescribe or use neurostimulation, audiovisual entrainment and/or cranioelectrical stimulation are safer at-home techniques to use.


“The average neuron contains a resting voltage of approximately 70 millivolts or 0.07 volts.’

Brain Battery. Knowing Neurons. 14 Dec 2012

Brainwaves are measured in microvolts.

“Popular theory suggests that anodal stimulation depolarizes the local neurons from their typical resting potential of 65 mv, by 5-10 mv, to 55 mv, which in turn will require less dendritic input to fire (depolarize) the neuron. The negative electrode, termed the cathode, hyperpolarizes the neuron slightly and it will require increased dendritic input to fire it”

Dave Siever. Transcranial DC Stimulation. Neuroconnections. Spring 2013.

Angel V. Peterchev, Timothy A. Wagner, Pedro C. Miranda, Michael A. Nitsche, Walter Paulus, Sarah H. Lisanby, Alvaro Pascual-Leone, and Marom Bikson. Fundamentals of Transcranial Electric and Magnetic Stimulation Dose: Definition, Selection, and Reporting Practices. Brain Stimul. 5(4): 435–453. Oct 2012. doi: 10.1016/j.brs.2011.10.001

Gottfried Schlaug, Vijay Renga, and Dinesh Nair. Transcranial Direct Current Stimulation in Stroke Recovery. Arch Neurol. 65(12): 1571–1576. Dec 2008. doi: 10.1001/archneur.65.12.1571

Ronak Patel, James Ashcroft, Ashish Patel, Hutan Ashrafian, Adam J. Woods, Harsimrat Singh, Ara Darzi, and Daniel Richard Leff. The Impact of Transcranial Direct Current Stimulation on Upper-Limb Motor Performance in Healthy Adults: A Systematic Review and Meta-Analysis. Systematic Review Article. Frontiers in Neuroscience. 15 Nov 2019. (I’m not a fan of meta-analyses, but this review lists numerous studies.)

Note: Some claim certain devices are the only one used in research. Untrue.

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