The neural correlates of consciousness (NCC) are the minimal set of neuronal events and mechanisms observed to occur along with the mental states to which they are related.Koch 2004, p. 304. Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena; that is, neural changes which regularly correlate with a specific experience.See here for a glossary of related terms.
Neurobiological approach to consciousness Michio Kaku, a famous theoretical physicist, futurist and a science author famously said that "the human brain has 100 billion neurons, each neuron connected to 10 thousand other neurons. Sitting on your shoulders is the most complicated object in the known universe".
A science of consciousness must explain the exact relationship between subjective mental states and brain states, the nature of the relationship between the conscious mind and the electrochemical interactions in the body (mind–body problem). Progress in neuropsychology and neurophilosophy has come from focusing on the body rather than the mind. In this context the neuronal correlates of consciousness may be viewed as its causes, and consciousness may be thought of as a state-dependent property of an undefined complex, adaptive, and highly interconnected biological system.Squire 2008, p. 1223.
Discovering and characterizing neural correlates does not offer a causal theory of consciousness that can explain how particular systems experience anything, the so-called hard problem of consciousness,Kandel 2007, p. 382. but understanding the NCC may be a step toward a causal theory. Most neurobiologists propose that the variables giving rise to consciousness are to be found at the neuronal level, governed by classical physics. There are theories proposed of quantum consciousness based on quantum mechanics.
There is an apparent redundancy and parallelism in neural networks so, while activity in one group of neurons may correlate with a percept in one case, a different population may mediate a related percept if the former population is lost or inactivated. It may be that every phenomenal, subjective state has a neural correlate. Where the NCC can be induced artificially, the subject will experience the associated percept, while perturbing or inactivating the region of correlation for a specific percept will affect the percept or cause it to disappear, giving a cause-effect relationship from the neural region to the nature of the percept.
Questions that have been raised over the years include: what characterizes the NCC? What are the commonalities between the NCC for seeing and for hearing? Will the NCC involve all the pyramidal neurons in the cortex at any given point in time? Or only a subset of long-range projection cells in the frontal lobes that project to the sensory cortices in the back? Neurons that fire in a rhythmic manner? Neurons that fire in a synchronous manner?See Chalmers 1998, available online.
The growing ability of neuroscientists to manipulate neurons using methods from molecular biology in combination with optical tools (e.g., Adamantidis et al. 2007) depends on the simultaneous development of appropriate behavioral assays and model organisms amenable to large-scale genomic analysis and manipulation. The combination of fine-grained neuronal analysis in animals with increasingly more sensitive psychophysical and brain imaging techniques in humans, complemented by the development of a robust theoretical predictive framework, will hopefully lead to a rational understanding of consciousness, one of the central mysteries of life. Research has shown a correlation between significant measurable changes in brain structure at the end of the second trimester of the human fetus development, which facilitate the emergence of early consciousness in the fetus. These structural developments include the maturation of neural connections and the formation of key brain regions associated with sensory processing and emotional regulation. As these areas become more integrated, the fetus begins to exhibit responses to external stimuli, suggesting a nascent awareness of its environment. This early stage of consciousness is crucial, as it lays the foundation for later cognitive and social development, influencing how individuals will interact with the world around them after birth.
Level of arousal and content of consciousness There are two common but distinct dimensions of the term consciousness,Zeman 2001 one involving arousal and states of consciousness and the other involving content of consciousness and conscious states. To be conscious of anything the brain must be in a relatively high state of arousal (sometimes called vigilance), whether in wakefulness or REM sleep, vividly experienced in dreams although usually not remembered. Brain arousal level fluctuates in a circadian rhythm but may be influenced by lack of sleep, drugs and alcohol, physical exertion, etc. Arousal can be measured behaviorally by the signal amplitude that triggers some criterion reaction (for instance, the sound level necessary to evoke an eye movement or a head turn toward the sound source). Clinicians use scoring systems such as the Glasgow Coma Scale to assess the level of arousal in patients. Similarly, the NCC are delineated in research by content-specific and full types; both are distinct from the background conditions for consciousness.
High arousal states are associated with conscious states that have specific content, seeing, hearing, remembering, planning or fantasizing about something. Different levels or states of consciousness are associated with different kinds of conscious experiences. The "awake" state is quite different from the "dreaming" state (for instance, the latter has little or no self-reflection) and from the state of deep sleep. In all three cases the basic physiology of the brain is affected, as it also is in altered states of consciousness, for instance after taking drugs or during meditation when conscious perception and insight may be enhanced compared to the normal waking state.
Clinicians talk about impaired states of consciousness as in "the comatose state", "the persistent vegetative state" (PVS), and "the minimally conscious state" (MCS). Here, "state" refers to different "amounts" of external/physical consciousness, from a total absence in coma, persistent vegetative state and general anesthesia, to a fluctuating and limited form of conscious sensation in a minimally conscious state such as sleep walking or during a complex partial epileptic seizure.Schiff 2004 The repertoire of conscious states or experiences accessible to a patient in a minimally conscious state is comparatively limited. In brain death there is no arousal, but it is unknown whether the subjectivity of experience has been interrupted, rather than its observable link with the organism. Functional neuroimaging have shown that parts of the cortex are still active in vegetative patients that are presumed to be unconscious;Laureys, Trends Cogn Sci, 2005, 9:556–559 however, these areas appear to be functionally disconnected from associative cortical areas whose activity is needed for awareness.
The potential richness of conscious experience appears to increase from deep sleep to drowsiness to full wakefulness, as might be quantified using notions from complexity theory that incorporate both the dimensionality as well as the granularity of conscious experience to give an integrated-information-theoretical account of consciousness.Tononi et al. 2016 As behavioral arousal increases so does the range and complexity of possible behavior. Yet in REM sleep there is a characteristic atonia, low motor arousal and the person is difficult to wake up, but there is still high metabolic and electric brain activity and vivid perception.
Many nuclei with distinct chemical signatures in the thalamus, midbrain and pons must function for a subject to be in a sufficient state of brain arousal to experience anything at all. These nuclei therefore belong to the enabling factors for consciousness.Parvizi, J., & Damasio, A. (2001). Consciousness and the brainstem. Cognition, 79(1–2), 135–160. https://doi.org/10.1016/S0010-0277(00)00127-X. Conversely, it is likely that the specific content of any particular conscious sensation is mediated by particular neurons in the cortex and their associated satellite structures, including the amygdala, thalamus, claustrum and the basal ganglia.
Neuronal basis of perception The possibility of precisely manipulating visual percepts in time and space has made vision a preferred modality in the quest for the NCC. Psychologists have perfected a number of techniques – masking, binocular rivalry, continuous flash suppression, motion induced blindness, change blindness, inattentional blindness – in which the seemingly simple and unambiguous relationship between a physical stimulus in the world and its associated percept in the privacy of the subject's mind is disrupted.Kim and Blake 2004 In particular a stimulus can be perceptually suppressed for seconds or even minutes at a time: the image is projected into one of the observer's eyes but is invisible, not seen. In this manner the neural mechanisms that respond to the subjective percept rather than the physical stimulus can be isolated, permitting visual consciousness to be tracked in the brain. In a perceptual illusion, the physical stimulus remains fixed while the percept fluctuates. The best known example is the Necker cube whose 12 lines can be perceived in one of two different ways in depth.
A perceptual illusion that can be precisely controlled is binocular rivalry where two different static images are shown to each eye. Conscious observers see either one image or the other alternately; the brain does not allow for the simultaneous perception of both.
Logothetis and colleaguesLogothetis 1998 recorded a variety of visual cortical areas in awake macaque monkeys performing a binocular rivalry task. Macaque monkeys can be trained to report whether they see the left or the right image. The distribution of the switching times and the way in which changing the contrast in one eye affects these leaves little doubt that monkeys and humans experience the same basic phenomenon. In the primary visual cortex (V1) only a small fraction of cells weakly modulated their response as a function of the percept of the monkey while most cells responded to one or the other retinal stimulus with little regard to what the animal perceived at the time. But in a high-level cortical area such as the inferior temporal cortex along the ventral stream almost all neurons responded only to the perceptually dominant stimulus, so that a "face" cell only fired when the animal indicated that it saw the face and not the pattern presented to the other eye. This implies that NCC involve neurons active in the inferior temporal cortex: it is likely that specific reciprocal actions of neurons in the inferior temporal and parts of the prefrontal cortex are necessary.
A number of fMRI experiments that have exploited binocular rivalry and related illusions to identify the hemodynamic activity underlying visual consciousness in humans demonstrate quite conclusively that activity in the upper stages of the ventral pathway (e.g., the fusiform face area and the parahippocampal place area) as well as in early regions, including V1 and the lateral geniculate nucleus (LGN), follow the percept and not the retinal stimulus.Rees and Frith 2007 Further, a number of fMRIHaynes and Rees 2005Lee et al. 2007 and DTI experimentsShimono and Niki 2013 suggest V1 is necessary but not sufficient for visual consciousness.Crick and Koch 1995