Various models of tDCS devices

Transcranial direct-current stimulation (tDCS) is a neuromodulatory technique in which a small and constant direct current is delivered through the skull in order to inhibit or excite neurons in the brain, that is to say to change the threshold at which the neurons will fire. Therefore it does not directly create new neural activity but influences the existing activity.[1] tDCS probably helps patients suffering from major depression and relieves pain (especially in fibromyalgia) and symptoms of craving in addictions.[2] tDCS could also help patients with neurodegenerative diseases, or enhance human cognition.[3] It is possible, however, that tDCS has no effect on the actual cognitive performance as it has been found by a quantitative review of past tDCS studies in 2015.[4]

Main Characteristics

tDCS is done by putting two or more electrodes on the scalp and running weak direct current between them so the current passes through the brain. Different areas of the brain can be affected based on the position of the electrodes. Depending on whether the stimulation is anodal or cathodal, the neuronal resting membrane potential is either depolarized or hyperpolarized, respectively. Anodal stimulation enhances excitability, while cathodal stimulation produces opposite results. The efficacy of the technique closely depends on the strength of the generated electrical field. Clinically, tDCS is usually administered via two sponge electrodes soaked in a saline solution for conductibility.[5][6]

The proponents of the method argue that prolonged sessions of tDCS result in long-lasting after-effects that may last for hours. The mechanism of tDCS outlasting effect is predominantly caused by the induction of synaptic changes, especially in glutamatergic and gamma-aminobutyric acid (GABA) neurons. tDCS also modulates resting membrane potential generally along the whole axons, this may result in non-synaptic mechanism, which might contribute to the long-lasting effect of tDCS in intracortical and corticospinal neurons.[7] tDSC might also elicit changes in non-neuronal tissues in the brain, including vessels and connective tissues.[3]

Historical overview

A schematics showing how tDCS works.

The ability of electrical current to influence human cognition, especially the perception of pain, has been observed since 48 CE by the Roman court physician Scribonius Largus. He observed that placing a live fish of the order Torpediniformes, able to deliver electric discharges of up to 220 V, on patients could ease the pain of the headaches his patients suffered. Other great physicians such as Pliny the Elder, Claudius Galen, and Ibn-Sidah noted the beneficial effects of these animals in their works as well. The study of electric rays inspired Luigi Galvani and Alessandro Volta to experiment with the effects of electrical current on human physiology.[8] Galvani’s nephew Giovanni Aldini reported the successful treatment of patients suffering from melancholia.[9] He noted, after first testing his device on himself, that the stimulation is rather unpleasant and its effects lasted for several days:

First, the fluid took over a large part of my brain, which felt a strong shock, a sort of jolt against the inner surface of my skull. The effect increased further as I moved the electric arcs from one ear to the other. I felt a strong head stroke and I became insomniac for several days. - Giovanni Aldini, 1803 [10]

Experiments with DC stimulation continued until the 1930s, when its usage was abandoned in favour of more reliable electroconvulsive therapy which, for a while, overshadowed the practice of brain polarization, and later by advances in pharmaceutics. However, the research into the effects of DC did not cease. In 1964, Joe Redfearn stimulated volunteers with tDCS. He reported that the participants became more 'giggly'. Then he also treated patients with depression. However, the results were not conclusive, mainly due to the use of low current. The method was rehabilitated recently by Michael Nitsche and Alberto Priori.[11][8][12] tDCS is also becoming popular among the 'hacker' communities, such as reddit's /r/tDCS[13] or DIY tDCS[14]. 'DIY' means 'Do-It-Yourself' since the proponents are able to build the devices themselves.

Purpose

Transcranial direct-current stimulation is a tool used to treat neuropsychiatric diseases and to influence human cognition without the use of invasive methods or pharmaceutics.[6]

1st century CE: Roman physician Scribonius Largus helps patients with headaches by placing electric rays over their heads.

18th century: Galvani and Volta experiment with the effects of electric current on human physiology and the brain.

1930s: tDCS is largely abandoned in favour of electroconvulsive therapy. [1]

1964: Joe Redfearn conducted research on tDCS.

2000s: Priori and Nietsche's experiments introduced a new era of tDCS research. [11]

2012: the first DIY tDCS GoFlow device was announced and the Kickstarter campaign began [15]

2013: the first tDCS which was produced by Foc.us was released[15]

Enhancement/Therapy/Treatment

Position of electrodes in accordance with international system

Several experiments have suggested that tDCS could enhance certain abilities and that it could be used as a treatment for various diseases.[16] In contrast, experiments conducted by Theresa Iuculano and Roi Cohen Kadosh have pointed out that the enhancement of certain abilities could lead to impairment of other abilities.[17] The efficiency of tDCS has also been doubted by certain researchers e.g.[4]

The effect of the tDCS method is based on neuroplasticity, which is the brain's ability to change. Min-Fang Kue et al. argue in their paper that tDCS could reverse pathological changes of the brain that are responsible for various neuropsychiatric diseases. They specifically mention the treatment of pain, tinnitus, depression, addiction, schizophrenia, anxiety disorder, and dementia.[18] In addition, Agnes Flöel, who listed several research studies concerning tDCS, claims that tDCS treatment could have positive impact on patients who suffer from mild cognitive impairment, Alzheimer's disease, movement disorders, epilepsy, or persistent deficits after stroke.[16]

Recently published evidence-based guidelines on the therapeutic use of tDCS [2] proposes the use of tDSC for patients suffering from major depression, pain (fibromyalgia, chronic lower limb neuropathic pain) and craving (concerning alcohol abuse, crack-cocaine abuse and smoking). The researchers also suggest a potential impact on tDCS on Parkinson's disease, but they do not confirm presumed effect of tDCS on tinnitus and drug-resistant depression, as well as they do not currently recommend tDCS as a treatment method for mentioned Alzheimer's disease, multiple sclerosis, or any type of epilepsy.

Anita Jwa points out various fields of cognitive enhancement that could be provided by tDCS. Namely, tDCS is supposed to improve language learning, numerical learning, and performance in video games.[19] Recently, it has also been suggested that tDCS could enhance sport performance due to improvement of motor learning.[20]

Ethical & Health Issues

Although tDCS is a non-invasive treatment and is deemed to be safe, several ethical issues have arisen. They are linked with the use of tDCS devices in general, with the do-it-yourself community, and also with the specific use and setting of certain devices.

The developers of tDCS devices usually point out certain conditions for which it is not recommended to use tDCS devices. One such circumstances is with individuals the age under 18, since the brain at that age is still developing. Another limitations are with regard to people with a history of seizures, epilepsy, pregnancy, brain lesions, bipolar depression, or severe heart disease. Patients with implants should also avoid the use of tDCS devices, since they could negatively interfere with their implant. In addition, sessions should not exceed a certain length of time (usually 30 minutes), and there should be a certain gap between sessions.[21]

The precise effect that direct current has on the brain itself is still not deeply understood. There is a possibility that long-term usage may be harmful and that some unwanted after-effects will occur. However, with observation of the proper experimentation protocol, especially the need to have a constant current density,[22] tDCS stimulation is deemed not to be harmful. More studies, however, have to be conducted to claim this conclusively.

Many of issues, linked to tDCS devices, arise from the DIY community. tDCS devices were originally used only in laboratory research, but the compelling results prompted certain tech enthusiasts to create their own tDCS devices. In addition, the components of tDCS devices are relatively inexpensive and the construction of the devices is not difficult.[15] These practices alarmed researchers, several of whom published an open letter in Annals of Neurology with various concerns concerning tDCS devices, primarily those used outside of laboratories. They point out that there are several issues that could influence session outputs and could lead to unintended effects. Firstly, electrodes usually extend beyond the regions that should be stimulated; thus the current could influence more regions. Second, the stimulation is influenced many of personal features and activities that precede and follow after it. Third, the enhancement of one skill could be accompanied with the impairment of other skills. Fourth, even a small change in the session could lead to a completely different output. Finally, the brain and physical features of each person are more or less different. The excitability of the brain or position of brain centres varies among the population. Factors such as handedness, gender, or different head anatomy could influence the output of the session. In addition, they argue that these potential risks are still acceptable in the case of treatment, but it is doubtful whether they are worth the risks in the case of enhancement.[23]

Lack of information is another issue connected with the sale of tDCS devices, especially for devices that are sold as DIY kits (e.g. Go Flow Pro, TDCS-KIT, TDCS Device Kit, and TDCS Transcranial Stimulation Kit), which tend to avoid any claim that could lead to FDA concerns. These concerns led to the regulatory denial of the GoFlow project and it ceased the sale of the TDCS Device Kit.[15] Anita Jwa points out that members of the DIY tDCS community regulate those who tent to make dangerous mistakes and advise them on user forums,[19] and Anna Wexler argues that this community is acquainted with the research.[24] However, the devices could be purchased by any customers, even those who are not involved in the DIY tDCS community. Therefore, an appropriate amount of information about the devices is needed.[25]

The use of tDCS in childhood is another ethical issue. As was mentioned previously, manufacturers of tDCS devices usually claim that their devices should be used by people under 18. There is, however, certain research that focuses on treatment through tDCS.[26] Roi Cohen Kadosh and his colleagues point out several ethical questions with regards to the use of tDCS in childhood. Specifically, they ask whether we should allow parents to let their children enhance one skill at the cost of another skill. Moreover, if tDCS proves to be efficient and safe even for children, they ask whether parents have an obligation to treat their children with tDCS.[25]

The relative availability and ease of use of tDCS, with the assumption that the effects on cognition are indeed present, raise questions about it being used as a form of 'cheating' or otherwise gaining unfair advantage over non-users. A study on tDCS on athletes resulted in the subjects having better muscle endurance and decreased muscle fatigue.[27] While this is beneficial for treatment of fatigue, it also raises concerns about whether it could be used in healthy sportsmen as a form of doping that is undetectable by chemical tests. WADA (the World Anti-Doping Agency) and the International Olympic Committee are aware of this issue. At the moment they are investigating whether tDCS is really efficient. If there is scientific proof that tDCS enhances sport performance of athletes, WADA will consider tDCS to be doping, when it harms health of athletes or violate 'the spirit of sport'.[28]

Anita Jwa, who conducted research among the DIY tDCS community, claims that 56 out of 121 responding users experienced certain side effects, including headache, discomforting feeling under electrodes, fatigue, nervousness or mood change, concentration difficulties, sleeping disturbance, change of visual perception, or nausea.[19]

Public & Media Impact and Presentation

With the 'brain hacking' community growing, tDCS techniques also have recieved the attention of the media. Elif Batuman describes her experience with tDCS. She underwent stimulation during several sessions. She writes that the stimulation had some unpleasant but temporary effects on her skin and cognitive abilities, but that it was successful in treating her depression for which she usually takes medication.[1] An article in The Huffington Post written by a former NASA engineer also calls tDCS brain-hacking. He explains that he feels that 'brain hacking' is essential to making human lives better.[29] Brain stimulation is especially popular among young, more tech-savvy adults. The community holds regular meetups and discussions, for example the Consciousness Hacking Meetup,[30] where the enthusiasts share their experience and entrepreneurs introduce their brain stimulation products.

The reactions to tDCS vary from enthusiasm to fear or scepticism. Enthusiastic declarations appear mainly on product reviews by tDCS users. Khrystyne Hastings on Halo Neuroscience's Facebook page maintains that 'This is so cool!! I want to try this!!'[31] However, there are also keen proponents of tDCS among scientists. Markom Bikson professor of Biomedical Engineering at The City College of The City University of New York, argues:

This is my optimistic (hopeful) look forward; one day soon, we will have “Tools for the Mind” (TEDx) that we can use to better ourselves. Certainly we need more research and the technology is early stage, but with something so transformative, the time to discuss its implications to society is now. I mention those issues as a call to action. Pay close attention for a sneak peak image at one technology thats exists today.‪[32]

Nonetheless, the fear of tDCS, especially those tDCS devices used outside laboratories, is more prevalent. Christopher Mims at MIT Technology Review warns:

Now, the first thing I have to say in this post about how to overclock your brain with a straightforward 20-minute application of electrical current is DO NOT TRY THIS AT HOME. The long-term effects of TDCS are unknown, and if you mess up and put orders of magnitude more current through your brain than is typically used in TDCS, obviously, you could kill yourself.[33]

Jamie Condliffe at Gizmodo claims:

Firstly, while direct current brain stimulation has undergone scientific and military testing, its long term effects are unknown. In the short term it does bring improvements, but there's no way of knowing what the extra currents are going to do to your brain in the long run. Second, if you mess up and for whatever reason mange to shoot a large current through your head, that is a bad and dangerous thing. You only have one brain, and destroying your neurons by pumping too much electricity through them could leave you permanently damaged—or even dead.[34]

Certain journalists and experts have expressed scepticism concerning the efficiency of tDCS devices. For example, Gizmodo quotes professor Mark S. George, who argues:

This area is quite controversial, with positive studies getting published more frequently than failed trials, creating a publication bias [...] If there is an effect here in this study, the tDCS merely improved the ability of the subjects to learn. There was no transfer of information through the brain stimulation.[35]

Additionally, Nicole Lee from Engadget expresses at the beginning of her review of Foc.us that tDCS is not conclusively deemed to be efficient:

Originally used to help patients with brain injuries, tDCS has supposedly been found to increase cognitive performance in healthy adults. These claims haven't been proven yet though, and shocking your own cranium isn't exactly FDA approved.[36]

The reactions on the efficiency of the device differ among users. While some have reported that tDCS devices have a huge impact on their life, some customers claim that they do not observe any improvement. NickMason29 on Amazon says about ApeX Type A: 'Besides the quality of the unit, it helps to reduce my depression and anxiety symptoms. I feel better since I started to use it.'[37] However, there are also negative claims to this product. User 'tuthers' reports: 'No other effects noted. I was very excited and hopeful, but this did nothing good for me. I think it's a technology that will be improved upon in the future, and it's exciting to think of the possibilities. But I think we have a ways to go.'[38] Similar sorts of claims can be found concerning any available tDCS device. While positive claims tend to be prevalent, there is still the possibility that they are caused by a placebo effect.[39]

Several concern are linked with the DIY tDCS community. Certain researchers and journalists point out that it is not a good idea. For instance, Cara Santa Maria argues in an nterview conducted with her at Al Jazeera America:

I'm not surprised that people want to try tDCS at home, but I am concerned. tDCS is not yet a well-defined treatment protocol. Anybody receiving tDCS under a physician's care is doing so as a participant in a research study. That alone is not without risks. But at-home "brain hackers" are taking a dangerous gamble when they set out to change their brain activity via electrical stimulation. The lasting effects aren't fully tested. The voltage might not be well calibrated. The participant's tolerance may be different from that of the general population. I personally wouldn't want to do anything to my brain before there was scientific consensus about its safety and effectiveness.[40]

Elsewhere, it has been mentioned , that tDCS is not entirely supported due to concerns of pharmaceutical companies. Brent Williams argues:

As tDCS gains new significance as an effective depression treatment via web sites, blogs, and the work of research institutions, it will be interesting to watch the reaction of pharmaceutical companies. They stand to lose billions of dollars. Will they try to derail tDCS as a depression treatment via advertising or “sponsored” research? Will the FDA be paying attention? It will be interesting to watch.[41]

And Elijah Wolfson from Al Jazeera America reported:

But getting the research money that might help bring the devices out of the DIY shadows and into the commercial mainstream is still difficult. Because the technology is so simple and cheap, pharmaceutical companies see no possibility of revenue.[42]

Public Policy

tDCS, specifically Sooma tDCS™ is a CE-marked medical device, which is classified as a class IIa medical device approved for treatment of Major Depressive Disorder. [43] No tDCS device has yet been approved as medical device by FDA.[19] FDA actions have been reported against GoFlow and the TDCS Device Kit.[15] Therefore, tDCS could be regulated in accordance with certain currently existent regulations, but there is no regulation that focuses directly on tDCS devices. Maslen and her colleagues suggest that these devices should be regulated just like a medical devices, since in research they are used for treatment of neuropsychiatric diseases. They do not opt for a ban on tDCS devices, since they argue that tDCS devices are not more risky than many medical devices that are already approved.[44]

Related Technologies, Projects, or Scientific Research

The tDCS stimulation is dependant on electrodes. Certain devices such as TDCS-KIT, Foc.us, Go Flow Pro, and GoFlow include their own electrodes, while manufacturers of others (e.g., BrainStimulator,Cognitive Kit) ship their devices with sponge electrodes developed by Amrex.[45][46]

References