Effects of non-invasive electrical brain stimulation and modulated neural mechanisms: a combined behavioural, computational and electrophysiological approach

Transcranial electrical stimulation (tES) is a non-invasive neuromodulatory techniques used to induce transient alterations of the normal cortical activity and to study the related behavioural and physiological outcomes. Transcranial direct current stimulation (tDCS) delivers low intensity direct electrical current and its effects are influenced by the polarity used (anodal or cathodal). High-frequency transcranial random noise stimulation (hftRNS) is a tES regime that delivers alternating current at random intensity and random frequencies and its underlying mechanisms are still unclear. The aim of this research is to investigate the effects and mechanisms of action of different tES regimes on the visual cortex probed using well-known paradigms of visual motion perception. In a first series of experiments (Chapter 2) we explored the effects of anodal and cathodal tDCS and hf-tRNS over the left hMT+ on a global motion direction discrimination task. The results showed that hf-tRNS reduced the motion coherence threshold, whereas anodal and cathodal tDCS did not have any effect. To further investigate hf-tRNS effects on global motion processing, we measured observers’ performance in estimating local motion directions and the number of estimates that can be polled together (Chapter 3). The results showed that hf-tRNS only increased the integration of local motion direction cues, suggesting an effect on the pooling of directional signals rather than an alteration of the selectivity for specific directions. To further explore hf-tRNS mechanisms of action on the visual cortex, we investigated whether its modulatory effects could be explained within the stochastic resonance framework, whereby the addition of an optimal level of stimulation-induced noise could increase the discrimination of weak stimuli (Chapter 4). Hf-tRNS was administered at five different intensities while observers performed a motion direction discrimination task. Significant improvements in performance for hf-tRNS at 1.5 mA were found, whereas further increments of the intensity (2.25 mA) significantly impaired the performance. These results indicate that hf-tRNS modulates motion direction discrimination in a way that is compatible with the stochastic resonance phenomenon. In Chapter 5, we explored if hf-tRNS is able to induce behavioural and electrophysiological aftereffects. Before and after 20 minutes of bilateral hf-tRNS, EEG activity was recorded during a period of resting state and during the execution of a motion direction discrimination task. The results showed that offline hf-tRNS did not affect poststimulation direction discrimination accuracy and VEPs measure of amplitude and latency elicited by the moving stimulus. Moreover, mean power spectra density for alpha and beta oscillations at rest increased between pre and post-stimulation, but not specifically for the hf-tRNS. Overall, these results suggest that offline hf-tRNS has limited behavioural and electrophysiological long term effects on the visual cortex. In conclusion, the results presented here show that online hf-tRNS is able to improve motion direction discrimination performance in agreement with the stochastic resonance framework when delivered over hMT+ . We suggested that this could be the results of an increased probability of weak neurons coding for the signal direction to generate an action potential consequently increasing the overall signal-to-noise ratio