An important distinction in consciousness research is that between the contents and a state of consciousness (Chalmers 2000). The concept of the contents of consciousness refers to individual subjective experiences that occur in phenomenal consciousness, such as reading a word or hearing birdsong, and as such they are sometimes referred to as the phenomenal contents of consciousness (Revonsuo 2006). Most neural experiments on consciousness, especially in the dominant field of visual awareness studies, are concerned with the neural basis of such specific contents of consciousness, i.e., they select one or two subjective experiences within an overall stream of consciousness. In this type of experiment, participants may be presented with stimuli close to their perceptual threshold (Del Cul et al. 2009) or they may be instructed to observe ambiguous stimuli that may lead to perception of several alternating contents of consciousness (Kornmeier & Bach 2012). The brain responses are then contrasted between trials that differ in awareness of these stimuli. Notably, while participants in such experiments report being unaware of some contents of consciousness, they still maintain awareness of other experiences: such as seeing the edges of a computer screen, hearing the background noise of the Magnetic Resonance Imaging (MRI) scanner, or letting their thoughts wander away from the experimental task. Typically, such experiences are ignored as task-irrelevant, and consequently the so-called “unaware” or “unconscious” trials still bear very rich phenomenology.
Contrary to the selective contents of consciousness, the concept of the state of consciousness refers to an overall pattern of subjective psychological functioning that includes the totality of phenomenal contents of consciousness (Rosenthal 1986; Tart 1972). In addition to the relaxed waking state of consciousness in a healthy volunteer, which could be also regarded as a baseline state, altered, unconscious, and non-conscious states can be distinguished. In altered states of consciousness, such as dreaming or Lysergic Acid Diethylamide (LSD) psychomodulation, subjective experiences may undergo various perceptual and cognitive alterations, the neural basis of which can be studied by contrasting them with a baseline state of consciousness, e.g., by comparing brain activity before and after hallucinogen intake (Carhart-Harris et al. 2012). Given that there is no widely accepted definition and criterion for an altered state of consciousness (Móró 2010; Revonsuo et al. 2009), a rather common approach is to describe, classify, and study states that are traditionally called altered, avoiding a single definition that would grasp the core of all altered states of consciousness (Vaitl et al. 2005).
Contrary to the baseline and altered states of consciousness, unconscious states are deprived of subjective experiences, but they may still maintain the potential to become conscious. For instance, an unconscious state of dreamless sleep may turn into a conscious sleep once a sleeping participant begins to dream (for an alternative interpretation of dreamless seep, see Thompson this collection). Finally, nonconscious states are those completely deprived of a capacity to support phenomenal consciousness, such as an irreversible coma. In clinical neuroscience, the most extensively studied contrast between a pathological altered state of consciousness and an unconscious or non-conscious state is a comparison between minimally conscious and vegetative state patients (Sitt et al. 2014). When states of (un)consciousness are contrasted, neural representations of specific experiences are typically ignored, making it difficult or even impossible to assess the phenomenal specificity of findings, e.g., if participants were aware of particular external stimuli or what internally generated experiences they had. Nevertheless, research into these states may reveal neural patterns that are common to all subjective experiences without individuating them.
It is possible that these two lines of research may eventually reveal rather different, if not independent, NCC systems: a neural correlate of the state of consciousness and a neural correlate of the contents of consciousness (Chalmers 2000). If these exist, any neuroscientific program of consciousness research would be incomplete without searching for a state NCC. Furthermore, even if a separate state NCC did not exist, there is currently no evidence for this, and thus NCC research is incomplete if it does not investigate this possibility. This view is often dismissed on the basis that some of the most plausible candidates for a state NCC, such as the brainstem reticular formation (Merker 2007; Parvizi & Damasio 2001), are rather low-level neural systems, whereas converging evidence shows that consciousness is a cortical process (Singer this collection). Furthermore, it could be argued that a conscious state may be nothing more than the sum of individual experiences, in which case revealing the NCC of specific contents would automatically explain the state NCC. Yet a brief analysis of the fundamental structural properties of consciousness—see the following section—shows that the necessary and sufficient NCC cannot be revealed by an exclusive focus on the contents of consciousness. Notably, the arguments presented in this commentary will be confined to the biological nature of human and animal consciousness, and as such they are not applicable to the problem of machine, extraterrestrial, or silicon-brain consciousness.