Modulation of Visual Contrast Sensitivity with tRNS across the Visual System, Evidence from Stimulation and Simulation

Abstract

Transcranial random noise stimulation (tRNS) has been shown to significantly improve visual perception. Previous studies demonstrated that tRNS delivered over cortical areas acutely enhances visual contrast detection of weak stimuli. However, it is currently unknown whether tRNS-induced signal enhancement could be achieved within different neural substrates along the retino-cortical pathway.In 3 experimental sessions, we tested whether tRNS applied to the primary visual cortex(V1) and/or to the retina improves visual contrast detection. We first measured visual contrast detection threshold (VCT; N=24, 16females) during tRNS delivery separately over V1 and over the retina, determined the optimal tRNS intensities for each individual(ind-tRNS), and retested the effects of ind-tRNS within the sessions. We further investigated whether we could reproduce the ind-tRNS-induced modulation on a different session (N=19, 14females). Finally, we tested whether the simultaneous application of ind-tRNS to the retina and V1 causes additive effects. Moreover, we present detailed simulations of the induced electric field across the visual system.We found that at the group level tRNS decreases VCT compared to baseline when delivered to the V1. Beneficial effects of ind-tRNS could be replicated when retested within the same experimental session but not when retested in a separate session. Applying tRNS to the retina did not cause a systematic reduction of VCT, irrespective of whether the individually optimized intensity was considered or not. We also did not observe consistent additive effects of V1 and retina stimulation.Our findings demonstrate significant tRNS-induced modulation of visual contrast processing in V1, but not in the retina.Significance statementOur findings confirm previous evidence showing acute online benefits of tRNS of V1 on visual contrast detection in accordance with the stochastic resonance phenomenon. We further extend it, demonstrating that the optimal tRNS intensity varies among participants, but when individually tailored it can improve visual processing when re-tested within the experimental session. The tRNS-induced enhancement in visual sensitivity was observed for cortical contrast processing, but stimulation of the retina did not lead to systematic effects.

Contrast detection is enhanced by deterministic, high-frequency transcranial alternating current stimulation with triangle and sine waveform

“…The required sample size was estimated using an a priori power analysis [G*Power version 3.1 ( 49 )] based on the effect of maximum contrast sensitivity improvement with tRNS shown by Potok et al ( 50 ) ( = 0.165, effect size f = 0.445). It revealed that 28 participants should be included in an experiment to detect an effect with repeated-measures analysis of variance (rmANOVA, 4 levels of stimulation condition), α = 0.05, and 90% power.…”

“…In each experiment, three tACS intensities and a control no tACS condition were interleaved in random order. Our main outcome parameter in all experiments was a threshold of visual contrast detection (VCT) that was determined for each of the different tACS conditions ( 50 ). The experimental procedure to estimate VCT followed a previously used protocol to assess the influence of tRNS on contrast sensitivity ( 50 ).…”

“…Our main outcome parameter in all experiments was a threshold of visual contrast detection (VCT) that was determined for each of the different tACS conditions ( 50 ). The experimental procedure to estimate VCT followed a previously used protocol to assess the influence of tRNS on contrast sensitivity ( 50 ). In brief, VCT was estimated twice independently, in two separate blocks within each session ( Fig.…”

“…In both experiments, tACS (80 Hz) stimulation (tACS triangle in Experiment 1 or tACS sine in Experiment 2 ) was delivered with 0.75 mA, 1 mA, and 1.5 mA amplitude (peak-to-baseline), resulting in maximum current density of 60 , which is below the safety limits of 167 for transcranial electrical stimulation ( 57 ). These intensities were selected based on previous studies investigating the effects of tRNS on contrast sensitivity ( 39 , 50 ).…”

“…To target V1, we used an electrode montage that was previously shown to be suitable for visual cortex stimulation ( 39 , 50 , 60 ). The electrodes were placed on the head at least 20 min after the application of an anesthetic cream.…”