“To demonstrate existential characters of neurons, any theoretically conceivable net embodying the possibility will serve.” (Pitts, McCulloch, 1948)
Consciousness is a multifaceted concept combining awareness and wakefulness. In everyday neurology it is usually equated to the waking state, and fluctuations in the level of vigilance are thought to reflect changes in brain (cortical) activation. It is believed to imply (subjective) experience and awareness of self (self-consciousness, self-detection, awareness of awareness, self-knowledge) and of environment (1). Jackson (2) restricted consciousness to the momentary relationship between the subject and the object or (in his anatomical-physiological terms) to the organism adjustment to the environment. However, perception and behaviour are possible without formal awareness (3–8) and some sustained (self)consciousness also exists during sleep, as we remain ourselves in the most unrealistic dreams and are also aware of our dreaming (9).
Today, research on consciousness is expanding, with a major focus on its understanding in relation to cortical/brain activation or functional complexity, long-range connectivity, neuronal synchronization in selected frequency ranges, uni/multimodal perception, motor activation, focused attention, etc. The major current theories about consciousness involve large-scale information processing, social processes, or neurobiological mechanisms (1). Distinctions between consciousness and attention have been documented (10–12), with implication in the cognitive neuroscience that consciousness could be distinct from other higher brain functions (13). The brain structures and processes thought to mediate in sustaining consciousness nevertheless are identified by the impairment of varying severity that results from local damage. Consciousness thus appears to be the result of a complex functional arrangement in which sustained sensory input, activation of non-specific ascending systems and primitive motor systems, activation of cortical neurones at due frequency, sensory-motor interaction, and balanced metabolism and neurotransmitters modulation are crucial (14, 15). This complex functional set-up conceivably also accounts for some specificity of the neurological signs predicting the outcome from the vegetative state (also referred to as unresponsive wakefulness syndrome) (16) and its evolution into a minimally conscious state (17–23).
Further investigation is needed to define the extent to which the reported electrophysiological, functional magnetic resonance imaging, positron emission tomography scan or autonomic changes imply some specificity of response or have clinical or prognostic relevance. This caveat notwithstanding, neuroimaging has documented retained connectivity in segregated networks in response to stimulus conditions in both minimally conscious and vegetative state subjects, with indication of the capability of the severely damaged brain to express surviving modular functions in the absence of the integrative processes necessary to consciousness (24–28). Although restricted to a relatively small portion of patients (29), this evidence further promoted research on the neuronal correlates of (un)consciousness (30) and expanded the clinical scenario. As a result, the vegetative and minimally conscious states appear today neither static nor homogeneous, and a tacit revision of the anatomo-functional set-ups underlying these conditions is de facto underway, warranting a formal nosographic revision of the current descriptive categories or accuracy of diagnosis (16, 31).
Regionally-mediated micro-consciousness processes have been proposed based on evidence of local neuronal organization in visual perception (32). On the other hand, increased synchronization between large neuronal populations of distinct areas related to perceptual dominance has been documented during conscious visual perception (33). The observation is consistent with evidence suggesting that neuronal activity synchronizes across cortical areas at conscious perception and with the theories of neural integration and complexity accounting for the properties of conscious experience and consciousness itself (13, 34–37). Long-range synchronization (e.g. in the gamma range) is thought to mediate in conscious perception (33) as it does in binding visual features and in all conditions in which neurones are selectively assembled to respond to any momentary functional requirement (38–44). However, its role in sustaining consciousness remains undocumented (45). In this respect, the major unsolved problem of biology is how billions of nerve cells work together to create consciousness (46, 47).
Consciousness is topical and is increasingly attracting scientists in neuroscience, medicine, neurocomputing, artificial intelligence, and robotics. Interest is increasing with the rapid progress in the investigation of higher brain function, advances in artificial intelligence, and diffuse perception of the inadequacy of traditional mind/body separations. The issue is also crucial in methodological and bioethical controversies pertaining to medicine and public or private healthcare (16, 31, 48). However, consciousness and related terms remain to a significant extent ambiguously defined and inadequately characterized. Peculiar conditions, such as epilepsy or the vegetative and minimally conscious states, may question the correlation between wakefulness and awareness and the available computational models of brain activity (30, 49, 50). Research attempting to correlate the contents of conscious experience with representations in specific neural populations relies to a relevant extent on the linguistic neutrality of “correlates” when the experimental paradigms and explanatory canons of neuroscience are not neutral about the mechanical relations with the brain and are supposed to investigate causes (51). A taxonomy of conscious, preconscious, and subliminal processing is still needed (52).
Neuroscience has advanced to the point that it appears that we can now treat consciousness as a scientific problem like any other (53), disregarding objections that it is epiphenomenal, not evolutionary in function, unaccountable by brain processes, unsuitable to objective investigated, etc. (53). To this end, a proper definition of consciousness and an up-to-date scrutiny of its descriptors are needed in order to be able to think scientifically about consciousness and to design experimental studies.
Key words: consciousness; descriptors; neuronal mechanisms; research.
Walter G. Sannita, MD
From the Department of Neuroscience, Ophthalmology and Genetics, University of Genova, Genova, Italy and Department of Psychiatry, State University of New York, Stony Brook, NY, USA