Kat Dyke – Brain stimulation beyond the lab

What if there was a way to improve your memory, enhance your focus and change your mood using a technique which claimed to be safe, simple and effective. Would you be tempted, and if so what should you know?

Transcranial direct current stimulation (tDCS) is a non-invasive stimulation technique known to be able to temporarily alter states within the brain. It has been reported to be effective in everything from enhancing memory performance [1] and maths abilities [2], to improving symptoms of depression [3] and schizophrenia [4], but how much of this is evidence based and how does it work?

KD

The technology behind tDCS is fairly simple, you place two electrodes on the head (an anode and cathode) and run a low voltage current between them; some of this current is able to travel through the skull and to the surface of the brain known as the cortex. The direction in which the current flows from one electrode to the other has been found to influence the effects, and can be used to either increase or decrease the excitability of a brain region.

In addition to being relatively simple, tDCS is also generally considered safe and low risk. Unlike its better known cousin electro convulsive therapy (ECT), tDCS uses very low electrical currents, so any side effects appear to be mild and transient, such as an itching sensation under the electrode [5]. However, it should be noted that the available safety information comes from controlled studies in which trained professionals apply stimulation using devices with a number of safety features. Until recently this wasn’t too concerning as brain stimulation outside of research environments was uncommon. However, this is slowly changing and it is now possible to find devices and DIY tutorials online. A number of manufacturers now offer a quick ‘brain boost’ at an affordable cost, but the research doesn’t necessarily back up their claims and individuals may not always be aware of relevant safety issues. Beyond the obvious concerns of using a non-regulated product, there are a number of more subtle issues potential users should be aware of.

Reasons to be cautious

When applying brain stimulation some basic neuroanatomy is required, you need to know where to put the electrodes to target a specific symptom or behaviour. Some available devices try to solve this problem using fixed electrode arrangements; however everyone’s head shape is different so the electrodes won’t necessarily be over the desired location. This isn’t the only problem, as even when electrodes are located correctly you could end up stimulating areas you weren’t expecting to influence. This is because the brain is a non-uniform conductor and every brain is different. Recent work using computational modelling has shown that the electrical current which reaches the cortical surface can spread to a wide area and that this is variable between individuals [6]. In non-research environments where it’s not possible to monitor the direct effects of the stimulation (using computational modelling, neuroimaging or alternative stimulation techniques) this could be problematic, particularly when stimulation is used regularly.

In research contexts tDCS is typically applied at intensities of 1-2mA, for up to 20 minutes no more than once a day. In home environments it may be tempting to increase the intensity or duration of the stimulation, but this isn’t necessarily a good idea. One study found that increasing the current intensity from 1-2mA changed the direction of the expected effects [7] and increasing the duration of stimulation on 26minutes has also been found to alter results [8]. The frequency at which stimulation is applied is another potential issue. Although the effects of a single session of tDCS are thought to last up to a few hours, when stimulation is applied more regularly (one daily for 5 days) it has been found to lead to more long lasting results [9, 10]. This could be critically important when used to help people with psychiatric and neurological illness, however, in healthy individuals aiming to boost their brain power at home this could lead to unexpected and unwanted changes. For example, one study reported that after 6 days of stimulation participant’s abilities in some cognitive tasks improved but other were negatively affected [11].

Another factor to be aware of is that not everyone responds to stimulation in the same way. Some people may show no response to stimulation, where as other may even show the opposite to expect pattern, for example a protocol which is typically used to reduce cortical excitability can actually increase it in some individuals [12]. This isn’t always clear from tDCS research as effects are often explored at a group level which means that individual variability is often hidden in the data; despite this variability is a real issue and should be given due consideration.

The future of home use tDCS:

At present some of the claims made by avocets of home tDCS use may not be fully founded in the scientific literature, and there are a number of practical and safety issues which should be addressed. Despite this, home stimulation itself may not be a bad thing and with proper information and guidance could become a useful technique; particularly in therapeutic contexts in which individualized treatment plans and proper monitoring could occur. For now the future of tDCS remains bright, but maybe not quite as clear cut as those marketing it would have you believe.

Kat Dyke (lpxksd@nottingham.ac.uk)

PhD Student, School of Psychology

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References

  1. Richmond, L.L., et al., Transcranial direct current stimulation enhances verbal working memory training performance over time and near transfer outcomes. J Cogn Neurosci, 2014. 26(11): p. 2443-54.
  2. Hauser, T.U., et al., Enhancing performance in numerical magnitude processing and mental arithmetic using transcranial Direct Current Stimulation (tDCS). Front Hum Neurosci, 2013. 7.
  3. Brunoni, A.R., et al., Transcranial direct current stimulation (tDCS) in unipolar vs. bipolar depressive disorder. Progress in Neuro-Psychopharmacology & Biological Psychiatry, 2011. 35(1): p. 96-101.
  4. Mondino, M., et al., Transcranial direct current stimulation for the treatment of refractory symptoms of schizophrenia. Current evidence and future directions. Current Pharmaceutical Design, 2015. 21(23): p. 3373-3383.
  5. Poreisz, C., et al., Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients. Brain Research Bulletin, 2007. 72(4): p. 208-214.
  6. Datta, A., et al., Inter-Individual Variation during Transcranial Direct Current Stimulation and Normalization of Dose Using MRI-Derived Computational Models. Front Psychiatry, 2012. 3: p. 91.
  7. Batsikadze, G., et al., Partially non-linear stimulation intensity-dependent effects of direct current stimulation on motor cortex excitability in humans. The Journal of Physiology, 2013. 591(7): p. 1987-2000.
  8. Monte-Silva, K., et al., Induction of late LTP-like plasticity in the human motor cortex by repeated non-invasive brain stimulation. Brain Stimulation, 2013. 6(3): p. 424-432.
  9. Alonzo, A., et al., Daily transcranial direct current stimulation (tDCS) leads to greater increases in cortical excitability than second daily transcranial direct current stimulation. Brain Stimulation, 2012. 5(3): p. 208-213.
  10. Galvez, V., et al., Transcranial direct current stimulation treatment protocols: should stimulus intensity be constant or incremental over multiple sessions? International Journal of Neuropsychopharmacology, 2013. 16(1): p. 13-21.
  11. Iuculano, T. and R. Cohen Kadosh, The mental cost of cognitive enhancement. The Journal of Neuroscience, 2013. 33(10): p. 4482-6.
  12. Wiethoff, S., M. Hamada, and J.C. Rothwell, Variability in response to transcranial direct current stimulation of the motor cortex. Brain Stimulation, 2014. 7(3): p. 468-475.
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