Modulatory mechanisms underlying high-frequency transcranial random noise stimulation (hf-tRNS): A combined stochastic resonance and equivalent noise approach

Abstract

27Background: High-frequency transcranial random noise stimulation (hf-tRNS) is a 28 neuromodulatory technique consisting of the application of alternating current at random intensities 29 and frequencies. hf-tRNS induces random neural activity in the system that may boost the 30 sensitivity of neurons to weak inputs. Stochastic resonance is a nonlinear phenomenon whereby the 31 addition of an optimal amount of noise results in performance enhancement, whereas further noise 32 increments impair signal detection or discrimination. 33Objective: The aim of the study was to assess whether modulatory effects of hf-tRNS rely on the 34 stochastic resonance phenomenon, and what is the specific neural mechanism producing stochastic 35 resonance. 36Method: Observers performed a two-interval forced choice motion direction discrimination task in 37 which they had to report whether two moving patches presented in two temporal intervals had the 38 same or different motion directions. hf-tRNS was administered at five intensity levels (0.5, 0.75, 39 1.0, 1.5, and 2.25 mA). 40 Results:The results showed a significant improvement in performance when hf-tRNS was applied 41 at 1.5 mA, representing the optimal level of external noise. However, stimulation intensity at 2.25 42 mA significantly impaired direction discrimination performance. An equivalent noise (EN) analysis, 43 used to assess how hf-tRNS modulates the mechanisms underlying global motion processing, 44 showed an increment in motion signal integration with the optimal current intensity, but reduced 45 motion signal integration at 2.25 mA. 46Conclusion: These results indicate that hf-tRNS-induced noise modulates neural signal-to-noise 47 ratio in a way that is compatible with the stochastic resonance phenomenon. 48 49 Keywords: global motion, high-frequency transcranial random noise stimulation, stochastic 50 resonance, internal noise, global sampling 51 52 53Transcranial random noise stimulation (tRNS) is a non-invasive electrical brain stimulation 54 technique characterized by alternating current delivered at random frequencies and intensities. This 55 technique can be applied at its full frequency spectrum between 0.1 Hz and 640 Hz, at the low-56 frequency range between 0.1 Hz and 100 Hz (lf-tRNS), or at the high frequency range (hf-tRNS), 57 between 101-640 Hz [1] . Early studies found that 10 mins of hf-tRNS applied over the primary 58 motor cortex (M1) induced an increment in cortical excitability with after-effects lasting up to 60 59 min [1][2][3][4]. In the last decade, several experimental procedures have been used to assess the effects 60 of tRNS on different cognitive and sensory abilities in order to understand its mechanisms [5]. For 61 example, it has been demonstrated that hf-tRNS improves behavioural performance on visual tasks 62 [6], attenuates visual motion adaptation [7], facilitates facial identity perception [8] and enhances 63 perceptual learning [9-14]. Moreover, five days of training with concomitant hf-tRNS over the 64 bilateral dorsola...

Transcranial Random Noise Stimulation acutely lowers the response threshold of human motor circuits

“…While our study provided proof-of-principle evidence of noise benefits at the neural population level in human cortex, we did not demonstrate the inverted U-shape function for higher tRNS intensities ( Figure 5) and can draw no inference about how much noise would be optimal for increasing the responsiveness of M1. Why did our neurophysiological study not reveal the expected inverted U-shape function for different tRNS intensities, although previous research has demonstrated this relationship when tRNS was applied while executing sensory detection tasks (Groen and Wenderoth 2016;van der Groen et al 2018;Pavan et al 2019)?…”

“…For example, the detection of low-contrast visual stimuli was significantly enhanced when the stimuli were superimposed with visual noise (Simonotto et al 1997). Recently, a similar enhancement of visual perception has been reported when noise was directly added to visual cortex via tRNS, which improved the detection of low contrast visual stimuli (van der Groen and Wenderoth 2016), visual decision making (Ghin et al 2018;Pavan et al 2019;van der Groen et al 2019), binocular rivalry (van der Groen et al 2018), and visual training in healthy participants (Fertonani et al 2011;Pirulli et al 2013) and patients (Moret et al 2018;Herpich et al 2019).…”

“…similar to the no noise condition) is currently unknown. Possible mechanisms might be that high intensity tRNS perturbs the sampling or processing of sensory information (Ghin et al 2018;Pavan et al 2019), the "sharpness" of neural representations of the external stimulus or the process of evidence accumulation (van der Groen et al 2018), which are part of perceptual decision making. Accordingly, high tRNS intensities might cause "false alarms".…”

Multi-session visual cortex transcranial random noise stimulation in adults with amblyopia

“…One possible explanation for the waning of within-session tRNS effects and the absence of a cumulative effect on contrast sensitivity relates to stimulation intensity. The relationship between tRNS intensity and visual function improvement during stimulation is an "inverted U", whereby stimulation that is weaker or stronger than an optimum level has limited effects [51,52]. Lasting changes in cortical excitability induced by prior sessions of tRNS might shift the optimal stimulation intensity towards lower levels, causing a waning of tRNS effects across sessions if stimulation intensity remains constant.…”

“…tRNS has also been reported to enhance visual perceptual learning in adults with cortical blindness [49]. Potential mechanisms for these effects include an acute enhancement of the signal-to-noise ratio within the visual cortex due to stochastic resonance [50][51][52] and longer-lasting alterations in neural membrane function due to repetitive opening and closing of sodium channels [53].…”