Transcranial random noise stimulation (tRNS) has been shown to significantly improve visual perception. These performance improvements manifested as both (i) after-effects of visual training combined with tRNS (Fertonani et al., 2011; Pirulli et al., 2013; Contemori et al., 2019; Herpich et al., 2019) and (ii) acute effects during tRNS (van der Groen and Wenderoth, 2016; Ghin et al., 2018; van der Groen et al., 2018, 2019; Battaglini et al., 2019, 2020; Pavan et al., 2019). It has been postulated that the Stochastic resonance (SR) phenomenon underlie noise-induced signal enhancement in studies investigating the acute effect of tRNS on visual processing (van der Groen and Wenderoth, 2016; van der Groen et al., 2018, 2019; Battaglini et al., 2019, 2020; Pavan et al., 2019). SR describes the phenomenon that the response of nonlinear systems to weak, subthreshold signals can be enhanced by adding an optimal level of random noise (Moss et al., 2004; McDonnell and Abbott, 2009). One important feature indicative of the SR phenomenon is that noise benefits are a function of noise intensity and exhibit an inverted U-shape relationship. Thus, while the optimal level of noise benefits performance, excessive noise is detrimental (van der Groen and Wenderoth 2016; van der Groen et al. 2018; Pavan et al. 2019). Previous studies investigating the acute effect of tRNS on visual detection performance have shown that noise stimulation was particularly beneficial when the visual stimuli were presented with near-threshold intensity (van der Groen and Wenderoth, 2016; van der Groen et al., 2018; Battaglini et al., 2019). Currently, it is unknown whether signal enhancement via SR could be achieved separately within different neural substrates along the retino-cortical pathway. Inducing SR via tRNS offers a unique opportunity to investigate this question. Here we plan a series of three experiments which will be executed in three separate sessions. Experiment 1: TRNS over primary visual cortex (V1) has been shown previously to improve visual contrast detection (van der Groen and Wenderoth, 2016). Here we aim to replicate these results in a new cohort and to determine the optimal tRNS intensity for each individual participant.
Experiment 2: Previous research has suggested that the retina and the optical nerve are highly susceptible to alternating currents (Schutter and Hortensius, 2010). Improvement in vision was reported after repetitive transorbital alternating current stimulation over the retina, in studies involving patients with optic neuropathy or after optic nerve lesions (Gall et al., 2010; Fedorov et al., 2011). The retina and the optic nerve are an interesting target because they can be reliably reached even with low transcranial electrical stimulation intensities, since the eyeball is an excellent conductor. Here we will test whether tRNS applied to the retina and the optic nerve improves visual contrast detection in accordance with the SR mechanism. We will apply different tRNS intensities to determine the optimal intensity for each individual participant. Experiment 3: It is currently unknown whether SR effects are additive when noise is applied to connected yet anatomically remote neural populations. Here we will investigate whether simultaneous application of tRNS to the retina and V1 causes additive effects.