How cool would it be to have the ability to move someone else’s hand? Perhaps we could make our friends do some tasks for us or we could make kids finally clean their rooms. While the latter may truly require some rocket science, making someone’s hand move is much simpler.
Scientists have been achieving this by passing an electrical current through someone’s skull. Does this sound scary? To be clear, this is not like giving someone an electric shock. There is no need to fear it, as it is a common and non-invasive procedure called tES – Transcranial Electrical Stimulation. In this feature, we describe a particular brain stimulation technique from this TES group. Besides being used to move someone else’s hand, it has been used to study the region of the brain responsible for the movement of the human body, the motor cortex.
“tRNS allows scientists to modulate the neuronal activity in specific areas of the brain using alternating current.”
A recent paper published last October in the Scientific Reports of Nature Research shed some light on the parameters of this technique, called the Transcranial Random Noise Stimulation (tRNS). Despite its long and complicated name, tRNS is promising in the investigation of motor, sensory, and cognitive tasks. For example, it has been used successfully to improve the perception of facial identity. tRNS allows scientists to modulate the neuronal activity in specific areas of the brain using alternating current (the type of electric current we have in our houses) and random frequency too. In practice, this method is also efficient in the reduction of pain in multiple sclerosis as well as alleviating depressive symptoms and being effective in the treatment of other conditions, such as schizophrenia and Parkinson’s disease.
Despite these results and the acceptance of the technique in the scientific community, the way tRNS operates on a physiological level is still a bit mysterious. Some theories attribute the modulation phenomenon to the repeated opening of sodium channels in neurons (which promotes electrical impulses in the brain) or to the increased sensitivity of neuronal networks to modulation. Scientists have been trying to find out the role of tRNS in cognitive functions, but the mentioned published paper studied one stimulation parameter that might be the key to refine it: the high-frequency band.
To understand this basic parameter, one must explore how the overall technique works. tRNS was first applied in humans in 2008 by Turney and colleagues from Göttingen University. The difference between tRNS and other electrical stimulation techniques such as transcranial direct current stimulation (tDCS) is that the electrical current runs in random alternating frequencies, which means that although the range is controlled and is known, the brain cannot “predict” the order of the frequencies and doesn’t adapt to them, preventing habituation.
“A good example to explain the phenomenon of the random frequency is comparing [it] to when you smoke,” said Rita Donato, one of the study authors and current researcher at the Proaction Lab at the University of Coimbra in Portugal. “The first cigarette has a greater effect because you haven’t smoked for a while, but after two months of smoking, you don’t feel the smoking effect as much.” According to the PhD student, the same thing happens with the brain – that is, if you stimulate the brain with the same frequency, it adapts to it. Thus, a random frequency current stimulates the brain and prevents it from responding to the stimulus because of habituation.
“If you stimulate the brain with the same frequency, it adapts to it.”
This randomness effect allows the frequency to play an important role. Scientists wondered if the result on the motor cortex in previous studies was happening because of the use of a high frequency or if the effect was caused by the whole range of frequencies. The protocol for most tRNS studies was using a specific large frequency range, and not narrower subranges. “We decided to test if we could increase the brain excitability by using smaller scopes, these being 100-400Hz and 400-700Hz, while also testing a wider range of frequency similar to the standard protocol: 100-700Hz,” Rita Donato added.
The study of the motor cortex is a very noticeable way to study tRNS itself. By inducing this alternating current in that specific brain region, scientists can induce spontaneous movements of a hand, which is a very visible result, whereas studying other brain regions might not produce results so easy to see.
This study collected data from 14 female students that had their motor cortices stimulated. The generated hand movements were documented and then the data was analyzed. “We realized that a wide frequency range seems to yield a more pronounced effect,” Rita Donato explained. The study was done with a high-frequency band only, and the next step would be to repeat the study with a low-frequency band to see if it is possible to obtain similar results.
“The electrical stimulation improves the quality of life of amblyopic patients, so clinical application of electric stimulation is indeed possible.”
The Italian team from the University of Padova demonstrated that using a wider range of a high-frequency band is more effective than using narrower ones. This means that the efficiency of the technology should not be attributed to the use of specific frequencies but rather to the use of a range with many random, different frequencies.
According to the PhD student, the application of some of these techniques is already happening for patients with motor and mental disorders, but further research is still necessary in order to use tRNS in clinical projects. However, other similar procedures are already being used to treat certain conditions. The application of the Transcranial Direct-Current Stimulation is being used to enhance linguistic, mathematical and cognitive abilities. “It is exciting that the application of tDCS is improving visual acuity and also helping amblyopic patients [these patients have a problem with one eye that works less than the other],” the researcher stated. The electrical stimulation improves the quality of life of those patients, so clinical application of electric stimulation is indeed possible.
Moreover, transcranial electrical stimulation techniques are noninvasive. The most highlighted advantage is that these currents modulate the brain’s own electricity, by exciting or inhibiting its neurons, which means that they do not add unnecessary current. From different medical and physical treatments to the study of cognitive habilities, their use looks very diverse and promising. People sometimes fear what they do not know, but after all, a little bit of electrical current in the brain can go a long way.
What other amazing things could Transcranial Electrical Stimulation techniques be used for? We’d love to hear your ideas in the comments below!
Want to learn more about brain stimulation? Read about how brain stimulation can boost memory in this thought-provoking article.
— Written by Ana Sousa & Daniel Ribeiro. Illustrated by Alexa Erdogan.
— Edited by Alexa Erdogan, Marco Travaglio & Gabrielle-Ann Torre
- Brasil-Neto J., Learning, Memory, and Transcranial Direct Current Stimulation. Frontiers in Psychiatry 3, 80 (2012).
- Chan, H. N. et al. Treatment of major depressive disorder by transcranial random noise stimulation: Case report of a novel treatment. Biol. Psychiatry 72, e9–e10 (2012).
- Ghin, F., Pavan, A., Contillo, A. & Mather, G. The effects of high-frequency transcranial random noise stimulation (hf-tRNS) on global motion processing: An equivalent noise approach. Brain Stimul. 11, 1263–1275 (2018).
- Moret, B., Donato, R., Nucci, M. et al. Transcranial random noise stimulation (tRNS): a wide range of frequencies is needed for increasing cortical excitability. Sci Rep 9, 15150 (2019) doi:10.1038/s41598-019-51553-7
- Palm, U. et al. Effects of transcranial random noise stimulation (tRNS) on affect, pain and attention in multiple sclerosis. Restor. Neurol. Neurosci. 34, 189–199 (2016).
- Pavan, A. et al. Modulatory mechanisms underlying high-frequency transcranial random noise stimulation (hf-tRNS): A combined stochastic resonance and equivalent noise approach. Brain Stimul. (2019).
- Romanska, A., Rezlescu, C., Susilo, T., Duchaine, B. & Banissy, M. J. High-frequency transcranial random noise stimulation enhances perception of facial identity. Cereb. Cortex 25, 4334–4340 (2015).
- Stephani, C., Nitsche, M. A., Sommer, M. & Paulus, W. Impairment of motor cortex plasticity in Parkinson’s disease, as revealed by theta-burst-transcranial magnetic stimulation and transcranial random noise stimulation. Parkinsonism Relat. Disord. 17, 297–298 (2011).
- Terney D. et al. Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation. Journal of Neuroscience 28 (52) 14147-14155 (2008).