Acute Benefits of Transcranial Random Noise Stimulation on Sensory and Motor Processing

In everyday life countless stimuli are delivered to our central nervous system from the environment. The processing of incoming information is not free of noise. Even though it is counterintuitive at first, some level of background noise can have a positive impact on signal processing in the central nervous system. It has been hypothesized that neural processing can benefit from added noise via a Stochastic Resonance (SR) phenomenon. SR is a general mechanism that enhances the response of nonlinear systems to weak subthreshold signals by adding an optimal level of random noise. Using transcranial random noise stimulation (tRNS), electrical noise can be added centrally to cortical circuits to modulate brain function. In this thesis, we investigated the immediate online effects of noise on the central nervous system. We probed the responsiveness of motor and visual systems in the presence and absence of electrical noise. Additionally, we explored the effects of high-frequency non- stochastic electrical stimulation on sensory processing. We began by reviewing the current literature and delineating the effects of electrical noise at the cellular, systems, and behavioral levels. We discussed the putative mechanism underpinning the effect of electrical noise stimulation on neural processing and how electrical noise might be utilized to augment sensory and motor function. In the first study, we investigated the acute effects of noise on the neural population level in awake human participants. We showed that the responsiveness of the primary motor cortex (M1) increases immediately when electrical noise is added via tRNS. In the second study, we explored the acute effects of tRNS delivered to two connected yet anatomically remote neural populations within the visual system, namely the primary visual cortex (V1) and the retina. We observed a significant decrease in the visual contrast detection threshold compared to baseline during tRNS delivery to V1 but not to the retina, suggesting that tRNS affects these neural populations differently. In the third study, we empirically tested the theoretical prediction that in addition to stochastic signals, non-stochastic signals can also cause resonance effects. We found that non-random high-frequency transcranial alternating current stimulation (hf-tACS) applied to V1 lowers the contrast detection threshold, reflecting enhanced visual detection performance. Altogether our work addresses the potential use of acute electrical noise added to cortical circuits to modulate physiology and enhance brain function. Our findings are consistent with the hypothesis that neural processing can benefit from added noise via a SR phenomenon, but also demonstrate the potential use of alternative waveforms to induce resonance-like effects. More broadly, our work sheds new light on a possible mechanism underpinning the effect of acute electrical noise stimulation on neural processing and provides a new paradigm for testing SR theory predictions in the human brain. Our results highlight the general importance and relevance of SR-like mechanisms in the human brain and will potentially lead to new developments and applications across various disciplines, including basic neuroscience, neurophysiology, computational biology, and clinical research.