Transcranial direct-current stimulation
Transcranial direct-current stimulation, tDCS in short, is a neoromodulatory 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 can not directly create new neural activity, but influence the existing activity. tDCS may help patients suffering from strong depression and it may also relieve pain, help patients with neurodegenerative diseases, or enhance human cognition.
It is possible, however, that tDCS have no effect on the actual cognitive performance as it has been found by a quantitative review of past tDCS studies in 2015.
The tDCS is done by putting two 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 has opposite results. The efficacy of the technique closely depends on the strenght of the generated electrical field. Clinically, tDCS is usually administered via two sponge electrodes soaked in a saline solution for conductibility.
The effects of tDCS can be long-lasting. Prolonged sessions result in after-effects that may last hours. This is due to the brain neurotransmitters and receptors having polar properties and as such they react to electrical fields which results in lasting neurochemical changes in the brain. tDCS has a lasting effect on NMDA receptors and intracortical and corticospinal neurons. This type of stimulation also influences non-neuronal components of the nervous system, such as vessels and connective tissues, and can influence the widening of the blood vessels in the brain.
The ability of the electrical current to influence human cognition, especially the perception of pain, has been observer since the 48 CE by the Roman court physician Scribonius Largus. He observed that placing a live fish of the order Torpediniformes, able to deliver electric discharge of up to 220 V, can ease the pain of the headaches his patients suffered. Other great physicians such as Pliny the Elder, Claudius Galen, or 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. Galvani’s nephew Giovanni Aldini reported the successful treatment of patients suffering from melancholia. He noted, after testing his device on himself first, that the stimulation is rather unpleasant and it's 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
Experiments with the DC stimulation continued until the 1930' 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 the advances in pharmaceutics. However, the research into the effects of DC did not cease. From the 1960' and onward, there is a growing interest in this for of brain stimulation, with various papers reporting that there is indeed a measurable effect on the brain. These resulted in changes in mood and alertness. Nowadays, the practive of tDCS is being heavily researched. tDCS is also getting popular among the 'hacker' communities, such as reddit's /r/tDCS, since they are able to build the devices themselves.
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.
- 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.
- 1930' - tDCS is largely abandoned in favour of electroconvulsive therapy.
- 1960' - New interest in tDCS stimulation (and brain stimulation in general) is growing.
Enhancement & Therapy - Brain stimulation is used for effective treatment of epilepsy and depression. It is also used for enhancing cognitive abilities but the efficacy is disputed (see 2nd paragraph in summary above).
Ethical & Health Issues
The precise effect the 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, tDCS stimulation is deemed not to be harmful. Of course, more studies have to be conducted to say this with better certainty.
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 fatique. While this is beneficial for treatment of fatigue, it also raises concerns whether this could be used in healthy sportsmen as a form of doping that is undetectable by chemical tests.
Public & Media Impact and Presentation
With the 'brain hacking' community growing, tDCS techniques also got the attention of the media. Elif Batuman describes her experience with tDCS. She underwent stimulation in several sessions. She writes that the stimulation had some unpleasant but temporal effects on her skin and cognitive abilities, but that it was successful in treating her depression she usually takes medication for. An article in The Huffington Post written by an former NASA engineers also calls tDCS brain-hacking. He explains that he feels that 'brain hacking' is essential to making human lives better. Another article on TechCrunch, written by Daniel Chao, the CEO of Halo Neuroscience, talks about
Brain stimulation is especially popular among young, more tech-savvy adults. The community holds regular meetups and discussions, for example the Consiousness Hacking Meetup on which the enthusiasts share their experience and on which entrepreneurs introduce their brain stimulation products.
Related Technologies, Projects or Scientific Research
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- HORVATH, Jared Cooney, FORTE, Jason D. and CARTER, Olivia, 2015, Quantitative review finds no evidence of cognitive effects in healthy populations from single-session transcranial direct current stimulation (tDCS). Brain Stimulation [online]. 2015. Vol. 8, no. 3, p. 535–550. DOI 10.1016/j.brs.2015.01.400. Available from: http://dx.doi.org/10.1016/j.brs.2015.01.400
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- SAN-JUAN, Daniel, MORALES-QUEZADA, León, OROZCO GARDUÑO, Adolfo Josué, ALONSO-VANEGAS, Mario, GONZÁLEZ-ARAGÓN, Maricarmen Fernández, LÓPEZ, Dulce Anabel Espinoza, VÁZQUEZ GREGORIO, Rafael, ANSCHEL, David J. and FREGNI, Felipe, 2015, Transcranial direct current stimulation in epilepsy. Brain Stimulation [online]. 2015. Vol. 8, no. 3, p. 455–464. DOI 10.1016/j.brs.2015.01.001. Available from: http://dx.doi.org/10.1016/j.brs.2015.01.001
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