Neural Correlates of Consciousness Research
Tracking the correlations between brain processes and states of phenomenal consciousness, such as feelings of pain, seeings of blue, hearings of trumpet sounds, is the basic method of scientific consciousness research. Searching for such correlations with the help of modern brain imaging techniques has produced, since its inception in the 1990s, a body of remarkable results and a number of competing hypotheses regarding the neural correlates of conscious experience within different sensory modalities (Frith et al., 1999; Gallace and Spence, 2008; Ionta et al., 2011; Fleming and Dolan, 2012; Merrick et al., 2014; Koch et al., 2016).
The notion of correlation leaves unresolved the precise nature of the relationship between subjective phenomenal states and neurophysiological processes. Francis Crick, one of the initiators of the search for the neural correlates of consciousness, emphasized that he used the word “correlate” as an ontologically neutral term. Such and such neural activity is detected, and it is accompanied by a certain subjective experience (Crick and Koch, 1990). As a first step this is acceptable but from the explanatory point of view, this cannot be the last word. The relationships between the two co-relata need be further clarified if we are to understand why the correlation exists in the first place. Most probably, the actual connection between mind and body is tighter than just a brute correlation of their states.
The theory of the identity of mind and body, formulated more than half a century ago by Place (1956) and Smart (1959), gives a straightforward answer: according to this theory, states of consciousness are brain processes and because of that, they systematically correlate with these processes. Thus, for example, a feeling of pain in my hip is identical to an activation of some area (or areas) in my brain, and this explains their regular correlation. In Smart’s version, the theory was a materialist attempt to discredit dualist accounts of mind-brain correlation, according to which the correlation is based upon a regular interaction between corporeal and incorporeal processes.
The theory was formulated before the heyday of the modern science of consciousness and the brain imaging technology it employs. However, we believe that even today the theory can be used to sufficiently clarify the relation between brain and consciousness. It is a simple, elegant tool for interpreting the results of neuroscientists’ lab-based mind-brain correlation measurements. There are other prominent metaphysical accounts of the brain/consciousness relationship. In this paper we shall show the advantages the theory of identity holds over those alternative accounts. Our second goal is to propose a working definition of a phenomenal and a neurophysiological type.
Measurements of the neural correlates of consciousness focus on particular instances of mind-brain correlations, the relata of which are individual phenomenal and neurophysiological “tokens.” However, the obvious goal of this enterprise is to generalize the token measurements into systematic type-type mind-brain correlations. Before delving into these questions, a word is needed about the very notion of the neural correlate of consciousness (or NCC, for short). In a widely cited definition, Chalmers (2000, p. 31) delimits the NCC as a minimal and sufficient neural system N whose activation leads to a conscious percept. This is basically correct, but needs to be somewhat modified.
We will follow Fink (2016) in taking the NCC to be not a neural system itself, but an activation of this system. Fink’s re-definition (“NCC 2.0”) is not only more precise than Chalmers’ but is congenial to our way of proceeding because it works with the distinction between tokens and types. “NCC2.0: An NCC2.0 of a phenomenal type P is a type of neural event or process N such that there is a mapping, where (i) each neural token ni of N is minimally sufficient for a phenomenal token pi of P, and (ii) where all and only neural tokens of N instantiate a feature-bundle 𝔽, such that 𝔽 is a (naturally) necessary condition for being an NCC of P.” (Fink, 2016, p. This definition is compatible with the sort of the theory of identity which identifies mental and neural phenomena at the level of types - and it is upon this version of the theory that we shall focus.
We have two things to add to Fink’s NCC2.0 definition. First, it requires “natural necessity” (the necessity of a natural law) at the level of types. We do not want to commit to such a strong requirement, for reasons which will emerge at the end of section “A Plea for Identity Theory in Consciousness Research”. Sufficiency at the type-level should be enough. Secondly, the neural “feature-bundle,” according to Fink, is a general mechanism of consciousness, such as, for instance, recurrent neural activity. This makes little sense even in the light of Fink’s own goals. The neural type-mechanisms need to be content-specific; they need to be able to articulate distinct phenomenal states.
Our aim in this paper is not to adjudicate between various proposals as to what constitutes an NCC. Whatever is the right NCC of the given type of a conscious mental state, it is identical with this state, we claim. Most contemporary attempts at accounting for brain-consciousness correlations are materialist. States of consciousness are not states of some mysterious immaterial substance; they are in some way anchored in detectable and measurable brain states. However, even within the materialist framework the correlation can be explained in various ways.
Regular mind-brain correlations form an evidentiary basis which can be used in support of different metaphysical accounts. The most straightforward approach is to try to explain the correlations in causal terms: NCCs are the causes of the states of consciousness. To “go from correlation to causation” is a move typical of the sciences and it might seem intuitively appealing to treat brain states as the causal sources of states of consciousness. No wonder, then, that we find advocates of the causal interpretation of NCCs. Neisser (2012); Sergent and Dehaene (2012), and Wessel (2012) put forward such causal accounts. Crick and Koch (2003), the founders of the field of modern consciousness studies, also want to “explain NCC in causal terms.” Still, we believe that this explanatory strategy is deeply problematic. It is far less plausible than the identity account of mind-brain correlation.
To see why, consider the picture put forward by the causal accounts. A neurophysiological process causes a phenomenal state of consciousness; therefore, it is different from that state, because causes and effects are always distinct.
Figure 1. Causal account of NCC. Phenomenal conscious state P is caused by neural process N.
However, materialist principles dictate that every conscious state must be implemented materially, i.e., by some brain state(s). Since the conscious state is different from the neurophysiological processes that are causing it, it must, on pain of psycho-physical dualism (cf. Fell et al., 2004), be implemented by a material process distinct from its neural cause. Thus we end up with two material processes involved in the production of the conscious mental state, not one. The first material brain process would be, according to the causal approach, the cause of a conscious state. The second neural process then would be the implementation of the phenomenal conscious state P, though it would not be its cause. Which of these two neural processes is to be considered the real NCC? Clearly it is the second kind. The first kind of process undoubtedly regularly correlates with occurrences of the state of consciousness at hand, because it is its regular cause and regular causes do correlate with their regular effects. Let us call this kind of brain process the Causal Neural Correlate of Consciousness (CNCC). Yet the CNCC is not what the cognitive neuroscientists empirically investigating consciousness are primarily searching for. They are searching for the brain processes of the second kind.
Take as an example the firings of C-fibers, beloved by philosophers of mind (see, e.g., Kripke, 1980). These firings are, in fact, the CNCCs of some kinds of pain sensations, because they are the causal antecedents of these sensations. Being a subset of sensory neurons projecting to the spinal cord, C-fibers are not even a part of the brain areas where these kinds of pain sensations arise. They are not its mechanism - that is, the activation of what is called by Swanson (2012, pp. Causal neural correlates of consciousness precede in time the real or “true” (Sandberg et al., 2016) NCCs (see Figure 2). They are sometimes labeled the “upstream” correlates of NCC, or NCCpre. Apart from them, one can also distinguish the NCCco or “downstream” correlates of consciousness. Both the neural antecedents and consequents are confounders of the true NCCs. Thus the true NCC is simply what remains after we cut off the various confounders; and, we suggest, the true NCC is what is captured by the mind-brain theory of identity.
Figure 2. Non-causal account of NCC. A phenomenal conscious state of pain is not caused, but non-causally implemented by its true neural correlate.
The preceding train of thought is, we believe, sufficient to demonstrate that the relation between states of phenomenal consciousness and brain states correlating with them cannot be causal, provided that a materialistic account of consciousness is correct. Empirical studies of consciousness, however, do not always distinguish between the causal and non-causal relations of phenomenal and neural states. For instance, Gallotto et al. (2017), after distinguishing “neural substrates” (i.e., NCCs) from both the neural prerequisites and neural consequences of a conscious experience, proceed to affirm that the neural substrates of experience “are directly causing, or are identical with, the phenomenal conscious experience” (p. 10; italics added). However, one cannot have both at the same time. These relations are fundamentally different.
To avoid this confusion, it is profitable to distinguish between horizontal and vertical NCCs (Hohwy and Bayne, 2015). Horizontal causal relations obtain between the proper NCC and its confounds, i.e., its causal antecedents and consequents. The notion of a vertical NCC concerns the very relation between a neural event and a phenomenal conscious state.
Figure 3. Horizontal and vertical NCCs. The diagram distinguishes between horizontal and vertical relations and isolates the real NCC - neural event N2 in the diagram. It is this event N2 that is to be identified with phenomenal conscious state P. The diagram is based on Hohwy and Bayne (2015, p.
Horizontally, neural events N1, N2, and N3 are related causally. The true NCC (N2) is thus causally involved in a horizontal structure of neural events. This paper is about the vertical relations between NCCs and their corresponding states of consciousness. These relations cannot be causal because attempts to treat vertical relations in a causal manner lead to the confusions described above. To accept only non-causal vertical relations means that not only the simple and straightforward causality, but also the more nuanced causal relations such as emergence6 cannot be admitted as vertical brain-mind relations. Identity, on the other hand, is not a causal relation, and thus can, in principle, serve as an appropriate candidate for the relation between conscious states and their NCCs.
In the following section we shall consider how a theory of identity for the mind-brain relation should look. Scientists searching for NCCs are measuring particular instances of mind-brain correlations, but this is hardly the end-point of their inquiry. They aim to generalize their findings. They want to know not only the neural correlate of a particular phenomenal token, say, the sensation of a sour taste. To put forward an infinite disjunction of particular phenomenal-neural correlations does not sound an attractive research project! The hope is that by accumulating a sufficient number of particular NCCs, the scientists will be in a position to generalize the data in some meaningful way. They will proceed by typing the phenomenal states into basic kinds and will try to assign to them broadly typed neurophysiological processes, carefully isolated during the experiments. This process needs to be repeatable both intrasubjectively and intersubjectively; hence it cannot stop at the level of individual tokens but must involve types. As Fink (2016, p. 3) put it, it is because science aims at generality that “we aim at types-NCCs.”
This process can begin very crudely, but a rough and ready form of phenomenal and neural type-taxonomy is at least a prerequisite of any empirical science of consciousness. Even if the consciousness scientist sets herself relatively unambitious goals - say, she just wants to compile a list of some law-like bi-directional correlations between phenomenal and neural states - she will need to rely on some notion of phenomenal and neural type, for it will be types of states and processes that eventually have to appear in this list of systematic correlations.
A solution to the vertical mind-body-relation problem, though, need not inevitably appeal to types. The so-called weak (or “token-token”) theory of identity postulates identity only at the level of individual tokens of mental and neural events. However, as the advocates of the weak theory of identity (such as Davidson, 1970/2012; Fodor, 1974) admit, the token-token theory of identity does not permit systematization into law-like psycho-physical generalizations: individual instances of mental and neural events cannot be regimented into correlated type-sets.7 The stronger, type-type theory of identity identifies types of conscious mental states with types of brain states and thus allows for systematic psycho-physical type-generalizations. Various forms of this stronger identity theory have been offered over the years (Place, 1956, 1988; Feigl, 1958; Smart, 1959; Lewis, 1966; Armstrong, 1968; Bechtel and Mundale, 1999; Polger, 2011; Polger and Shapiro, 2016; see also Gozzano and Hill, 2012).
Debates within the philosophy of mind are quite extensive as regards the facets of the identity theory, its pros and cons (see Polger, 2009, for an overview), but very little attention has been given to the pivotal question of what the neurophysiological and phenomenal kinds are. Most theorists participating in these debates use the notions of phenomenal and neurophysiological types only intuitively, without giving any explicit principles of individuation. This absence of a common understanding of what constitutes a type is striking, given the centrality of the notion of type within both philosophy and neuroscience. Is pain a type of phenomenal state ...