Transcranial random noise stimulation (tRNS) involves a low intensity alternating current to the scalp. However, the intensity (in mA) and frequency change in a randomised way. The type of ‘noise’ that is administered can be changed by changing the frequencies that are 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.
The exact mechanism behind tRNS remains unknown (Antal & Herrmann, 2016) but there are some experimental phenomena that have been revealed. For example, the application of tRNS for 10 minutes over the motor cortex can increase the amplitude of motor evoked potentials (MEPs) for an hour and a half (Terney et al., 2008). 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 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 visual tasks has 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 (Harris et al., 2011; Schwarzkopf et al., 2011).
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. To conclude, although the mechanism behind tRNS remains not entirely certain, they are some interesting experimental phenomena that remain to be investigated.
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