Man’s fascination with trying to alter his own way of thinking by means of electricity dates back to the introduction of electricity itself. However, it has only been in the past few decades that different instruments have made that dream possible for most people. Transcranial Direct Current Stimulation (i.e., direct current is current that moves in one direction as opposed to alternating current) came to the forefront in the year 2000 when two German researchers published the effects of low direct currents on the brain. From there on publications on the subject have increased exponentially and the technique itself has receiving wide coverage in the layman press. Thus far its beneficial effects have been studied for a large number of medical conditions and, in the «normal» state, as a way of enhancing our well-being, e.g. how we feel about ourselves. More recent studies have promoted the idea that the technique may be of use in autism spectrum disorders (ASD) claiming benefits in language acquisition as well as in different behavioral domains, e.g., social withdrawal hyperactivity and irritability.
The apparatus for tDCS is quite simple and its construction as a do-it-yourself (DIY) project may cost less than $10. It consists of two sponges soaked in a saline solution acting as electrodes and joined to a battery via wires. Depending on the power source and the way of administering the current, the device may be a tDCS machine or one used for electroconvulsive therapy. tDCS uses a low voltage (e.g., a 9 volt battery) and given the high resistance of skin delivers a low current between the electrodes, on the order of 1-2 milliamps. An electroconvulsive therapy device would use several hundred volts and deliver a larger current some 600 times greater than tDCS. Also in electroconvulsive therapy the current is administered in pulses whose duration and frequency can be controlled.
The major problem in the implementation of tDCS therapy is that the skin resistance changes both over time and with ambient conditions. Part of the circuit therefore is devoted to keeping a constant current output. Still the amount of current and how it is distributed may depend on the physical dimensions of the sponge acting as the electrode. This means that the amount of current delivered may easily vary in between different commercial devices and the physical state of the patient, e.g., whether the individual is anxious and sweating.
The working principle of tDCS is quite simple. Researchers naively assume on a push-pull type of model of brain physiology wherein the function of different cortical areas may be normalized by either activating or depressing the same. According to practitioners, the anode (positive electrode) provides for excitation of the cerebral cortex while the cathode (negative electrode) provides for inhibition. Since inhibition may be an undesired outcome for many applications, some protocols place the cathode outside of the head, e.g. on your shoulder. Although the model presumes that the current transverses the brain, it does not account for any subcortical effects.
The low current in tDCS provides for little, if any side effects. Patients may complain of a tingling or burning sensation over the electrodes, but usually become accustomed to the same after a few minutes. Since the sponges need to be kept wet, the occasional rinse may cause water to drip over the skin providing another pathway to conduct electricity and increased discomfort. TMS, another noninvasive electrophysiological device to which tDCS is often compared, has more salient side effects including the possibility of seizures. This is the case as TMS has direct physical effects on the brain, it causes groups of neurons to fire or become depressed. tDCS, in turn, changes the «probability» of neurons firing or becoming depressed, if it does anything at all.
As previously stated the low current generated by tDCS provides for little in terms of side effects but at the same time makes any claims at potential benefits questionable. In essence the minute current generated by the battery is presumed to transverse the skin, connective tissue, brain coverings and the brain itself in order to be effective. Modeling its action has not taken into account the fact that the brain provides multiple avenues to crisscrossing conductors leading to other parts of the brain, brainstem and spinal cord. These models also disregard the marked convolutions of the surface of the brain and rather treat it as an amorphous gel in the shape of a balloon. It seems rather unlikely that an electrical current would pursue a path directly across the brain while ignoring the multiple conducting pathways available. Also, most tDCS studies lack in terms of adequate active controls and base their report on small series of patients probably lacking in power. Future studies should perform comparisons using an active electrode over a different area of the brain than the one targeted and having enough patients to justify claims as to significant findings.
Thus far the fascination of tDCS researchers has been the dorsolateral prefrontal cortex (dPFC). When using this area as a target researchers have found improvement for increased numerosity, rare word usage, memory, addictive behaviors, learning, the acquisition of motor skills, etc (See Table). It seems improbable that stimulation of one area would provide an improvement to almost any imaginable mental problem. This is specially the case when we consider the low topographical resolution of the technique. In effect, in many cases, the area stimulated by a particular electrode appears to be more guesswork than science.
Table: Claims attributed to tDCS
Perception, reasoning, reading, numerical skills, addictive behaviors (smoking), decision making, depression, creativity, memory, schizophrenia (positive symptoms), motivation, migraine, visuomotor skills, epilepsy, aphasia, dystonia, stroke, multiple sclerosis, chronic pain, fibromyalgia, TBI rehabilitation, tinnitus, craving (obesity)
tDCS is now being marketed to the lay community without FDA regulation. As long as manufacturers abstain from making medical claims it may be considered a «cosmetic treatment» of neurology. Different forums have been created for DIY implementation and commercial ventures provide kits targeting gamers. The preliminary reports of benefits in autism and the inclination of these individuals towards video technology may entice the community to use the device without medical supervision. Personally, I believe that this may be a useless endeavor that may detract from more positive alternatives. Also the future potential outcomes of receiving current through the brain for prolonged periods of time, is in need of assessment.
The history of autism has been punctuated by positive claims from different therapeutic interventions only to later find out that the outcomes were never generalized to the real world. Many of the results reported by tDCS are based on carefully crafted experiments that veer markedly from real life situations. Would getting better at pressing a button at the laboratory make you better at anything else in real life? Would spending your time reading a good book, making new social contacts, or playing chess be better served than receiving tDCS? Also, we are not at a point to say whether the technique actually works. It may be that the bothersome aspects of having current shunted through your skin may increase alertness and this itself accounts for the positive results being reported.
I believe one of the problems is that TMS and TDCS only affect areas of the cerebral cortex about 6 cm deep if I remember correctly. This means it would be harder to stimulate limbic structures such as amygdala and hippocampus which have been found to have an association with autism, unless of course you assume that connections to the dorsolateral prefrontal cortex has a cascading effect, but not sure how you can tell limbic structures were truly stimulated.
I’m also curious how glial cells may play into this, as they may be affected in autism and other disorders also. Since Glia don’t have action potentials the way neurons do they won’t respond to TMS and TDCS. Of course maybe by stimulating neurons, you could affect glia also, I don’t know, and don’t know if glia are thought to be implicated in autism at all or not, but these are all thoughts also from a layperson with limited knowledge of neuroscience.
I guess there is also deep TMS where they use a helmet instead of a coil which could stimulate deeper structures below the cerebral cortex, but I think it has less specificity and can’t work in one area of the brain as easily, and not sure if there is an equivalent thing for TDCS, of course, again, these are just thoughts. I wish i knew more about TMS and neuroscience to comment further.
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The effect of TMS is rather superficial about 1 cm by 1 cm at the top of the cerebral cortex. It would affect the crest of gyri but not the depth of its convolutions. The deep brain stimulation with TMS is somewhat arguable. It is obtained with a different coil, the H-coil. The role of glia still has to be elucidated. At least for Deep Brain Stimulation, as in Parkinson’s disease, there is a prominent glial reaction at the site of the electrode implantation which may play a role on terminating any beneficial effects. Thanks
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