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).

Stochastic Resonance

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).

(van der Groen et al., 2022)

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.