An overview of the neuroscience of consciousness

Alchemical Illustration from the Emerald Tablet of Hermes.
The Tablet had such an impact on the minds of histories greatest philosophers, esotericists and mystical thinkers, that it became the esoteric industry standard for every medieval and later renaissance system of alchemy.
Conscious experience in humans depends on brain activity, so neuroscience will contribute to explaining consciousness. What would it be for neuroscience to explain consciousness? How much progress has neuroscience made in doing so? What challenges does it face? How can it meet those challenges? What is the philosophical significance of its findings? This blogpost addresses these and related questions.

To bridge the gulf between brain and consciousness, we need neural data, computational and psychological models, and philosophical analysis to identify principles to connect brain activity to conscious experience in an illuminating way. This entry will focus on identifying such principles without shying away from the neural details. The notion of neuroscientific explanation here conceives of it as providing informative answers to concrete questions that can be addressed by neuroscientific approaches. Accordingly, the theories and data to be considered will be organized around constructing answers to two questions:
  • Generic Consciousness: How might neural properties explain when a state is conscious rather than not?
  • Specific Consciousness: How might neural properties explain what the content of a conscious state is?
A challenge for an objective science of consciousness is to dissect an essentially subjective phenomenon. As investigators cannot experience another subject’s conscious states, they rely on the subject’s observable behavior to track consciousness. Priority is given to a subject’s introspective reports as these express the subject’s take on her experience. Introspection thus provides a fundamental way, perhaps the fundamental way, to track consciousness. That said, consciousness pervasively influences human behavior, so other forms of behavior beyond introspective reports provide a window on consciousness. How to leverage disparate behavioral evidence is a central issue.
The term “neuroscience” covers those scientific fields whose explanations advert to the properties of neurons, populations of neurons, or larger parts of the nervous system. This includes, but is not limited to, psychologists’ use of various neuroimaging methods to monitor the activity of tens of millions of neurons, computational theorists’ modelling of biological and artificial neural networks, neuroscientists’ use of electrodes inserted into brain tissue to record neural activity from individual or populations of neurons, and clinicians’ study of patients with altered conscious experiences in light of damage to brain areas.
Given the breadth of neuroscience so conceived, an overview of sufficient depth must restrict breadth. On the neuroscience side, this review focuses on the central nervous system and the electrical properties of neurons, particularly in the cerebral cortex. On the side of consciousness, it focuses on perceptual consciousness, with emphasis on vision. This is not because visual consciousness is more important than other forms of consciousness. Rather, the level of detail in empirical work on vision often speaks more comprehensively to the issues that we shall confront.
A neuroscientific explanation of consciousness adduces properties of the brain, typically the brain’s electrical properties. A salient phenomenon is neural signaling through action potentials or spikes.A spike is a large change in electrical potential across a neuron’s cellular membrane which can be transmitted between neurons that form a neural circuit. For a sensory neuron, the spikes it generates are tied to its receptive field. For example, in a visual neuron, its receptive field is understood in spatial terms and corresponds to that area of external space where an appropriate stimulus triggers the neuron to spike. Given this correlation between stimulus and spikes, the latter carries information about the former. Information processing in sensory systems involves processing of information regarding stimuli within receptive fields.
Which electrical property provides the most fruitful explanatory basis for understanding consciousness remains an open question. For example, when looking at a single neuron, neuroscientists are not interested in spikes per se but the spike rate generated by a neuron per unit time. Yet spike rate is one among many potentially relevant neural properties. Consider the blood oxygen level dependent signal (BOLD) measure in functional magnetic resonance imaging (fMRI). The BOLD signal is a measure of changes in blood flow in the brain when neural tissue is active and is postulated to be a function of electrical properties at a different part of a neuron than that part tied to spikes. Specifically, given a synapse which is the connection between two neurons to form a basic circuit motif, spikes are tied to the presynaptic side while the BOLD signal is thought to be a function of electrical changes on the postsynaptic side (signal flow is from pre to post). Furthermore, neuroscientists are typically not interested in the response of a single neuron but rather that of a population of neurons, of whole brain regions, and/or their interactions. Higher order properties of brain regions include the local field potential generated by populations of neurons and correlated activity such as synchrony between activity in different areas of the brain (neural oscillations were postulated to be central to consciousness by Crick & Koch 1990).
The number of neural properties potentially relevant to explaining mental phenomena is dizzying. This review focuses on the facts that neural sensory systems carry information about the subject’s environment and that neural information processing can be tied to a notion of neural representation. How precisely to understand neural representation is itself a vexed question (Cao 2012, 2014; Shea 2014), but we will deploy a simple assumption with respect to spikes which can be reconfigured for other properties: where a sensory neuron generates spikes when a stimulus is placed in its receptive field, the spikes carry information about the stimulus (strictly speaking, about a random variable). Information as used in neuroscience is typically not a semantic notion, but bearing in mind that caveat, it will simplify matters to speak of a sensory neuron’s activity as representing the relevant aspect of the stimulus that drives the neuron’s response (e.g., direction of motion or intensity of a sound). This way of speaking is imprecise, so we shall return to neural representation in the final section when discussing how neural representations might explain conscious contents.
David Chalmers presents the hard problem as follows:
It is undeniable that some organisms are subjects of experience. But the question of how it is that these systems are subjects of experience is perplexing. Why is it that when our cognitive systems engage in visual and auditory information-processing, we have visual or auditory experience: the quality of deep blue, the sensation of middle C? How can we explain why there is something it is like to entertain a mental image, or to experience an emotion? It is widely agreed that experience arises from a physical basis, but we have no good explanation of why and how it so arises. Why should physical processing give rise to a rich inner life at all? It seems objectively unreasonable that it should, and yet it does. If any problem qualifies as theproblem of consciousness, it is this one. (Chalmers 1995: 212)
The Hard Problem can be specified in terms of generic and specific consciousness (Chalmers 1996). In both cases, Chalmers argues that there is an inherent limitation to empirical explanations of phenomenal consciousness in that empirical explanations will be fundamentally either structural or functional, yet phenomenal consciousness is not reducible to either. This means that there will be something that is left out in empirical explanations of consciousness, a missing ingredient (see also the explanatory gap [Levine 1983]).
Modern neuroscience of consciousness has attempted to explain consciousness by focusing on neural correlates of consciousness or NCCs (Crick & Koch 1998, 2003). Identifying correlates is an important first step in understanding consciousness, but it is an early step. After all, correlates are not necessarily explanatory in the sense of answering specific questions posed by neuroscience. That one does not want a mere correlate was recognized by Chalmers who defined an NCC as follows:
An NCC is a minimal neural system N such that there is a mapping from states of N to states of consciousness, where a given state of N is sufficient under conditions C, for the corresponding state of consciousness. (Chalmers 2000: 31)
Similarly, Christof Koch and others speak of “the minimal neural mechanisms jointly sufficient for any one specific conscious experience” (Koch et al. 2016: 307). One wants a minimal neural system since, crudely put, the brain is sufficient for consciousness but to point this out is hardly to explain consciousness even if it provides an answer to questions about sufficiency. There is, of course, much more to be said that is informative even if one does not drill down to a “minimal” neural system which is tricky to define or operationalize (see Chalmers 2000 for discussion; for criticisms of the NCC approach, see NoĆ« & Thompson 2004; for criticisms of Chalmers’ definition, see Fink 2016).
The emphasis on sufficiency goes beyond mere correlation, as neuroscientists aim to answer more than the question: What is a neural correlate for conscious phenomenon C? For example, Chalmers’ and Koch’s emphases on sufficiency indicate that they aim to answer the question: What neural phenomenon is sufficient for consciousness? Perhaps more specifically: What neural phenomenon is causally sufficient for consciousness? Accordingly, talk of “correlate” is unfortunate since sufficiency implies correlation but not vice versa. For example, correlation does not imply causal sufficiency, so not every correlate will be explanatory in the sense of answering Chalmers’ and Koch’s question. After all, assume that the NCC is type identical to a conscious state. Then many neural states will correlate with the conscious state: (1) the NCC’s typical effects, (2) its typical causes, and (3) states that are necessary for the NCCs obtaining (e.g., the presence of sufficient oxygen). Thus, some correlated effects will not be explanatory. For example, citing the effects of consciousness will not provide causally sufficient conditions for consciousness.
While many theorists are focused on explanatory correlates, it is not clear that the field has always grasped this, something recent theorists have been at pains to emphasize (Graaf, Hsieh, & Sack 2012; Aru et al. 2012; Koch et al. 2016). In other contexts, neuroscientists speak of the neural basisof a phenomenon where the basis does not simply correlate with the phenomenon but also explains and possibly grounds it. However, talk of correlates is entrenched in the neuroscience of consciousness, so one must remember that the goal is to find the subset of neural correlates that are explanatory, in answering concrete questions. Reference to neural correlates in this entry will always mean neural explanatory correlate of consciousness (on occasion, I will speak of these as the neural basis of consciousness). That is, our two questions about specific and generic consciousness focus the discussion on neuroscientific theories and data that contribute to explaining them. This project allows that there are limits to neural explanations of consciousness, precisely because of the explanatory gap (Levine 1983).
  • Aru, Jaan, Talis Bachmann, Wolf Singer, and Lucia Melloni, 2012, “Distilling the Neural Correlates of Consciousness”, Neuroscience and Biobehavioral Reviews, 36(2): 737–746. doi:10.1016/j.neubiorev.2011.12.003
  • Cao, Rosa, 2012, “A Teleosemantic Approach to Information in the Brain”, Biology and Philosophy, 27(1): 49–71. doi:10.1007/s10539-011-9292-0
  • –––, 2014, “Signaling in the Brain: In Search of Functional Units”, Philosophy of Science, 81(5): 891–901. doi:10.1086/677688
  • Crick, Francis and Christof Koch, 1990, “Toward a Neurobiological Theory of Consciousness”, Seminars in the Neurosciences, 2: 263–275.
  • Graaf, Tom A. de, Po-Jang Hsieh, and Alexander T. Sack, 2012, “The ‘Correlates’ in Neural Correlates of Consciousness”, Neuroscience & Biobehavioral Reviews, 36(1): 191–197. doi:10.1016/j.neubiorev.2011.05.012
  • Levine, Joseph, 1983, “Materialism and Qualia: The Explanatory Gap”, Pacific Philosophical Quarterly 64(4): 354–361. doi:10.1111/j.1468-0114.1983.tb00207.x
  • Shea, Nicholas, 2014, “Neural Signalling of Probabilistic Vectors”, Philosophy of Science, 81(5): 902–913. doi:10.1086/678354
  • –––, forthcoming, Representation in Cognitive Science, Oxford, New York: Oxford University Press.