Transcranial random noise stimulation (tRNS) is a method of non-invasive electrical brain stimulation that, like tACS, involves the application of a low-intensity alternating current to the scalp of a subject between two electrodes. Unlike tACS, however, the intensity of the current (in mA) and frequency of the current change in a randomised way.
The type of ‘noise’ that is administered can be changed by adjusting the frequencies present within the ‘random noise’ waveform. The probability function - or the frequencies that are most likely to take place - is determined by a Gaussian curve or a rectangular distribution.
Mechanisms of Function
The exact mechanism behind tRNS remains unknown (Antal & Herrmann, 2016) but there are some experimental phenomena that have been revealed. For example, in one of the earliest research applications of tRNS, stimulation applied for 10 minutes over the motor cortex was seen to increase the amplitude of motor evoked potentials (MEPs) for an hour and a half (Terney et al., 2008). In addition, it appears that tRNS is polarity independent - where identical effects can be seen even when the polarity of the electrodes is reversed (Miniussi et al., 2013; Pirulli et al., 2016),
The effects of tRNS also change depending on the maximum amplitude of stimulation. If the amplitude is 0.4mA, tRNS reduces MEP amplitudes (Moliadze et al., 2012). In the visual cortex the application of tRNS tends to improve performance whilst participants perform a visual task (Pirulli et al., 2013).
The benefits of tRNS application whilst participants compete in visual tasks have led to stochastic resonance being discussed as a potential mechanism of action for tRNS (Antal & Herrmann, 2016). Under conditions where visual inputs are weak to exceed a threshold, the addition of noise (in the form of tRNS) can amplify weak visual inputs, similar to what has been observed with transcranial magnetic stimulation (Abrahamyam et al., 2011; Schwarzkopf et al., 2011).
Applications of tRNS
The application of noise to the frontal lobes also appears to promote learning under certain circumstances, although the mechanism is uncertain. For instance, Snowball et al. (2013) applied tRNS whilst participants completed calculation- and recall-based arithmetic and found they tRNS made them faster relative to sham.
Similarly, tRNS has been shown to improve learning in many other studies (Brem et al., 2018, Cappelletti et al., 2013, Contemori et al., 2019). tRNS has also been shown to modulate human attention (e.g., Lema et al., 2021, van Koningsbruggen et al., 2016), visual perception (e,g,m van der Groen and Wenderoth, 2016; Battaglini et al., 2019) and social perception (e.g., Penton et al., 2017)
To conclude, although the mechanism behind tRNS remains not entirely certain, there are some interesting experimental phenomena that remain to be investigated.
- Transcranial Alternating Current and Random Noise Stimulation: Possible Mechanisms. Andrea Antal and Christoph S. Herrmann. Neural Plasticity. 2016
- The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation. Cornelia Pirulli, Anna Fertonani, Carlo Miniussi. Brain Stimulation. July 2013
- Long-term enhancement of brain function and cognition using cognitive training and brain stimulation. Albert Snowball, Ilias Tachtsidis, Tudor Popescu, Jacqueline Thompson, Margarete Delazer, Laura Zamarian, Tingting Zhu, Roi Cohen Kadosh. Current Biology. June 2013
- Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities. Vera Moliadze, Deniz Atalay, Andrea Antal, Walter Paulus. Brain Stimulation. October 2012
- Improving Visual Sensitivity with Subthreshold Transcranial Magnetic Stimulation. Arman Abrahamyan, Colin W. G. Clifford, Ehsan Arabzadeh and Justin A. Harris. The Journal of Neuroscience. March 2011
- Stochastic resonance effects reveal the neural mechanisms of transcranial magnetic stimulation. Dietrich Samuel Schwarzkopf 1, Juha Silvanto, Geraint Rees. The Journal of Neuroscience. March 2011
- Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation. Daniella Terney, Leila Chaieb, Vera Moliadze, Andrea Antal and Walter Paulus. The Journal of Neuroscience. December 2008
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