Abstract

Human mental life is accompanied by oscillatory signals that send information across distributed neural networks. Whether a stimulus reaches or escapes our conscious experience is influenced by the state of the brain in that moment, reflected in cerebral electrophysiology. Our understanding of this brain activity has grown vastly in recent years, thanks to leading advances in electro- and magneto-encephalography (EEG and MEG, or M-EEG, which enable us to monitor the electric brain signal) and recent developments allowing the direct modulation of endogenous oscillatory components that underlie cortical functions. Transcranial current stimulation, particularly the variant with alternating current (tACS), putatively lets us assess and gauge the role of oscillations on cognition. Several studies have confirmed that tACS can influence neural mechanisms and behavior, even conscious access. Until recently, cerebral activity during stimulation could not be assessed and observations were limited to the aftereffects. The aim of the project described in this dissertation is to investigate the validity of a pioneering procedure that can recover brain signal during simultaneous MEG and tACS. Then, exploiting this approach, we furthered our grasp of how the neural system is altered by transcranial stimulation and the complex relationship between the external current and the internal mechanisms of the brain. The overall goal is to explore our ability to manipulate neural signatures in ongoing activity and the conscious perception of an upcoming stimulus. Chapter 1 provides the reader with a general introduction of current studies and theories behind tACS influence on cognition and behavior. After a description of what tACS is and what it does, the focus is mostly on cutting-edge methods combining tACS and M-EEG, network connectivity and graph theoretical frameworks to study cognitive processes. At the end of the introductory chapter, we indulge on applications and consequences of these approaches, as well as open questions about our understanding of the prerequisites of conscious perception that drove the experiments described in the following chapters. Chapter 2 reports the first study in which we addressed the feasibility of concurrent tACS and MEG, the prerequisite for the rest of the project. Chapter 3 and 4 present studies that better delineated what happens in the brain in terms of oscillatory phase, connectivity and the dependency of tACS effects with the ongoing brain state during electrical stimulation. We addressed some key issues on the mechanisms of action of tACS and its sensitivity to in-vivo brain networks. Chapter 5 provides preliminary results of a study employing a near-threshold task paired with tACS and MEG in the context of conscious perception. We stimulated prestimulus brain rhythms in sensory cortices to see if their strength and connectedness with the rest of the brain could determine whether a stimulus will be perceived or not. Chapter 6, after a recapitulation of the main results in a broader perspective, discusses the meaning and the limitations of the experimental findings and how these extend our current knowledge.