Transcranial electrical stimulation and the neural dynamics of consciousness

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The neural correlates of consciousness (NCCs) have been under serious investigation for about three decades. Perceptual consciousness has been found to be strongly associated with modulations of neuronal oscillations at various frequencies, but it has been difficult to ascertain whether the oscillations in a particular brain region are causal for consciousness of the content computed by that region. What is needed is a demonstration that modulation of the neural oscillations in a particular brain region either does or does not affect the likelihood of the content computed by that region of reaching consciousness. I propose electrical brain stimulation experiments in binocular flash suppression, metacontrast masking, Troxler vanishing, and manipulating exposure duration. Such paradigms help identify the NCCs by exposing what neural activity is present when the stimuli are perceived consciously compared with when they are not. For example, one metacontrast masking study showed that EEG oscillatory power, inter-(scalp)electrode phase synchrony, and ERPs were larger when the same masked target was visible versus invisible. My lab showed increases in gamma power and inter-regional phase synchrony, as well as theta-gamma cross-frequency coupling, in a network of relevant brain areas around the time when a conscious percept in binocular rivalry changed. In a study of Troxler vanishing from my lab, we found greater frontal-parietal phase synchronization and transfer entropy when a peripheral annulus that changed contrast remained in perception than when it vanished. I propose modulating the relevant oscillatory frequencies directly using transcranial electrical stimulation (tES) in various ways during performance in consciousness-varying paradigms. tES has been shown to modulate neural oscillations, both indirectly by increasing excitability (e.g., trans-cranial direct current stimulation, tDCS, trans-cranial random noise stimulation, tRNS) and by driving the oscillations directly (trans-cranial alternating current stimulation, tACS). Cross-frequency tACS has been used to demonstrate the dependence of spatial working memory on synchronization of theta (6 Hz) and high gamma (80-100 Hz) oscillations. Similarly, I propose to determine the dependence of conscious perception of visual stimuli on information flow (phase synchrony and transfer entropy) within the relevant brain networks. I have run an experiment in which I stimulated the precuneus with either anodal or cathodal tDCS during Troxler vanishing. We observe behavioural differences between the two conditions and will now use EEG to investigate whether the behavioural difference is accompanied by changes in phase locking value and/or transfer entropy between the frontal-parietal regions identified in the earlier EEG study. In other studies I will use high definition tACS to directly modulate the oscillatory frequencies involved, in order to make more direct and precise causal inferences.