![]() For example, when a low-intensity visual stimulus is repeatedly presented at perception threshold, the participant consciously perceives it on some but not on all trials. It can help reveal endogenous neural mechanisms underlying conscious perception, particularly if the physical stimuli remain identical in both conscious states. This basic approach has been referred to as ‘contrastive analysis’ ( Baars, 1989 Aru et al., 2012). To study NCCs, one generally tries to induce minimally two different conscious experiences using ‘consciousness paradigms’, to then measure and compare brain activity in both (with neuroimaging techniques). The neural correlates of consciousness (NCCs) have been defined as the minimal set of neuronal mechanisms that are jointly sufficient for a conscious experience ( Crick and Koch, 1990b). In this article, we focus on content consciousness, specifically in the visual modality. ‘Content consciousness’ refers to moment-by-moment experiences of a conscious being, such as the experience of seeing blue, hearing a trumpet, or the famous ‘what-it-is-like’ to momentarily be a bat ( Nagel, 1974). ‘State consciousness’ determines consciousness on a global level, for example distinguishing the extent of consciousness in coma, wakefulness, or anesthesia (e.g., Koch, 2004 Laureys and Tononi, 2010). Generally, it is useful to separate minimally two concepts of consciousness. ‘Consciousness’ can be defined in many ways (for our own taxonomy, see de Graaf et al., 2012 de Graaf and Sack, 2014). Naturally, these techniques have been tried on the oldest problem of all: the nature of consciousness. As such, our understanding of the brain has proceeded at a staggering pace. In the last few decades, progress in technology and signal analysis have resulted in new neuroimaging and electrophysiology techniques, greatly enhancing the range and resolution of brain research applications. But such results also showcase an increasingly acknowledged difficulty in NCC research the challenge of separating neural activity necessary for conscious experience to arise (prerequisites) from neural activity underlying the conscious experience itself (substrates) or its results (consequences). This review of oscillatory correlates of consciousness suggests that, for example, activity in the alpha-band (7–13 Hz) may index, or even causally support, conscious perception. We also discuss how increasingly sophisticated techniques allow us to study the causal role of oscillatory activity in conscious perception (i.e., ‘entrainment’). We provide an overview of oscillatory measures that appear to correlate with conscious perception. In this paper, we focus on spontaneous brain oscillations rhythmic fluctuations of neuronal (population) activity which can be characterized by a range of parameters, such as frequency, amplitude (power), and phase. Empirical research on these NCCs covers a wide range of brain activity signals, measures, and methodologies. 2Maastricht Brain Imaging Centre, Maastricht, NetherlandsĬonscious experiences are linked to activity in our brain: the neural correlates of consciousness (NCC).1Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands.
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