NCC research and the problem of consciousness

One of the reasons why the Neural Correlates of Consciousness Program could appear attractive in the 1990s was that it seemed to disentangle theoretical and empirical problems. Theoretical disagreements could thus be sidestepped in order to focus on empirical research regarding the neural substrate of consciousness. One of the further consequences of this dissociation of empirical and theoretical questions was that fundamental questions regarding the Mind Body Problem or the “Hard Problem of Consciousness” could remain unresolved even if the search for the neural correlates had been successful.Drawing on historical examples, a widely held consensus in the philosophy of science, and actual NCC research we argue that there is no such independence. Moreover, as the dependence between the theoretical and the empirical level is mutual, empirical progress will go hand in hand with theoretical development. Thus, contrary to what the original NCC program suggested, we conclude that NCC research may significantly take advantage from and contribute to theoretical progress in our explanation and understanding of consciousness. Eventually, this might even contribute to a solution of the Hard Problem of Consciousness.

Brain structural complexity and consciousness

Structure shapes function. Understanding what is structurally special about the brain that allows it to generate consciousness remains a fundamental scientific challenge. Recently, advances in brain imaging techniques have made it possible to measure the structure of human brain, from the morphology of neurons and neuronal connections to the gross anatomy of brain regions, in-vivo and non-invasively. Using advanced brain imaging techniques, it was discovered that the structural diversity between neurons and the topology of neuronal connections, as opposed to the sheer number of neurons or neuronal connections, are key to consciousness. When the structural diversity is high and the connections follow a modular topology, neurons will become functionally differentiable and functionally integrable with one another. The high levels of differentiation and integration, in turn, enable the brain to produce the richest conscious experiences from the smallest number of neurons and neuronal connections. Consequently, across individuals, those with a smaller brain volume but a higher structural diversity tend to have richer conscious experiences than those with a larger brain volume but a lower structural diversity. Moreover, within individuals, a reduction in neuronal connections, if accompanied by an increase in structural diversity, will result in richer conscious experiences, and vice versa. These findings suggest that having a larger number of neurons and neuronal connections is not necessarily beneficial for consciousness; in contrast, an optimal brain architecture for consciousness is one where the richest conscious experiences are generated from the smallest number of neurons and neuronal connections, at the minimal cost of biological material, physical space, and metabolic energy.

The search for the NCC: Progress and challenges

Twenty years ago, Thomas Metzinger published the book "The Neural Correlates of Consciousness" amassing the state of knowledge in the field of consciousness studies at the time from philosophical and empirical perspectives. On the occasion of the 20th anniversary of this impactful publication, we review the progress the field has made since then and the important methodological challenges it faces. A tremendous number of empirical studies have been conducted, which has led to the identification of many candidate neural correlates of consciousness. Yet, this tremendous amount of work has not unraveled a consensual account of consciousness as of now. Many questions, some already raised twenty years ago, remain unanswered, and an enormous proliferation of theories sharply contrasts with the scarcity of compelling data and methodological challenges. The contrastive method, the foundational method used to study the neural correlate of consciousness (NCC), has also been called into question. And while awareness in the community of its shortcomings is widespread, few concrete attempts have been made to go beyond it and/or to revise existing theories. We propose several methodological shifts that we believe may help to advance the quest of the NCC program, while remaining uncommitted to any specific theory: (1) the currently prevalent “contrastive method” should lose its monopoly in favor of methods that attempt to explain the phenomenology of experience; (2) experimental data should be shared in centralized, multi-methods databases, transcending the limitations of individual experiments by granting granularity and power to generalize findings and “distill” the NCC proper; (3) the explanatory power of theories should be directly pitted against each other to overcome the non-productive fractioning of the field into insular camps seeking confirmatory evidence for their theories. We predict these innovations might enable the field to progress towards the goal of explaining consciousness.

You can't always get what you want

The predictive processing framework has gained significant popularity across disciplines investigating the mind and brain. In this article we critically examine two of the recently made claims about the kind of headway that the framework can make in the neuroscientific and philosophical investigation of consciousness. Firstly, we argue that predictive processing is unlikely to yield significant breakthroughs in the search for the neural correlates of consciousness as it is still too vague to individuate neural mechanisms at a fine enough scale. Despite its unifying ambitions, the framework harbors a diverse family of competing computational models which rely on different assumptions and are under-constrained by neurological data. Secondly, we argue that the framework is also ill suited to provide a unifying theory of consciousness. Here, we focus on the tension between the claim that predictive processing is compatible with all of the leading neuroscientific models of consciousness with the fact that most attempts explaining consciousness within the framework rely heavily on external assumptions.

A structural constraint on neural correlates of consciousness

Researchers on the neural correlates of consciousness (NCC) need to distinguish mere statisticalNCCs from NCCs proper. Some neural events may be co-occurrent, probabilistically coupled,or coincidental with a type of conscious experience but lack any deeper connection to it, whilein other cases, the relation between neural states and a type of experience hints at a strongmetaphysical relation, which distinguishes such NCCs proper from mere statistical NCCs. In orderto address this issue of how to distinguish NCCs proper from mere statistical NCCs, we proposea position we call neurophenomenal structuralism. The position hinges on the uncontroversialidea that phenomenal experiences relate to each other in degrees of similarity and difference.These complex structures are used to identify and individuate experiences in the methods ofneuroscience, psychophysics, and phenomenology. Such individuation by structure leads to phenomenalholism, which has implications for how to investigate consciousness neuroscientificallyand generates a constraint by which we can distinguish NCCs proper from mere statistical NCCs:the structural similarity constraint. Neural activation must preserve the structure governing thedomain of experiences it is associated with in order to count as that domain’s NCC proper. Anyactivation that fails to preserve phenomenal structure fails to be an NCC proper. We illustratehow this constraint works with a study by Brouwer & Heeger (2009) as an example.

Finding the NCCs will not solve all our problems

Subjective experience has often taken center stage in debates between competing conceptual theories of the mind. This is also a central object of concern in the empirical domain, and especially in the search for the neural correlates of consciousness (NCCs). By now, most of the competing conceptual theories of consciousness have become aligned with distinct hypotheses about the NCCs. These hypotheses are usually distinguished by reference to their proposed location of the NCCs. This difference in hypothesized location of the NCCs has tempted participants in these debates to infer that evidence indicating the location of the NCCs in one or the other brain region can be taken as direct evidence for or against a given conceptual theory of consciousness. We argue that this is an overestimation of the work finding the NCCs can do for us, and that there are principled reasons to resist this kind of inference. To show this we point out the lack of both an isomorphism and a homomorphism between the conceptual frameworks in which most theories are cached, and the kind of data we can get from neuroimaging. The upshot is that neural activation profiles are insufficient to distinguish between competing theories in the conceptual domain. We suggest two ways to go about ameliorating this issue.

The neural correlates of consciousness under the free energy principle: From computational correlates to computational explanation

How can the free energy principle contribute to research on neural correlates of consciousness, and to the scientific study of consciousness more generally? Under the free energy principle, neural correlates should be defined in terms of neural dynamics, not neural states, and should be complemented by research on computational correlates of consciousness – defined in terms of probabilities encoded by neural states. We argue that these restrictions brighten the prospects of a computational explanation of consciousness, by addressing two central problems. The first is to account for consciousness in the absence of sensory stimulation and behaviour. The second is to allow for the possibility of systems that implement computations associated with consciousness, without being conscious, which requires differentiating between computational systems that merely simulate conscious beings and computational systems that are conscious in and of themselves. Given the notion of computation entailed by the free energy principle, we derive constraints on the ascription of consciousness in controversial cases (e.g., in the absence of sensory stimulation and behaviour). We show that this also has implications for what it means to be, as opposed to merely simulate a conscious system.

Seeing the forest for the trees: Scene perception and the admissible contents of perceptual Experience

Debates surrounding the high-level contents of perceptual experience focus on whether weperceive the high-level properties of visual objects, such as the property of being a pine tree. Thispaper considers instead whether we perceive the high-level properties of visual scenes, such asthe property of being a forest. Liberals about the contents of perceptual experience have offered avariety of phenomenal contrast cases designed to reveal how the high-level properties of objectsfigure in our visual experience. I offer a series of equivalent phenomenal contrast cases intendedto show how the high-level properties of visual scenes also figure in visual experience. Thisfirst-person evidence of high-level scene perception is combined with third-person evidence fromthe extensive empirical literature on scene categorisation. Critics of liberalism have attempted todeflate existing phenomenal contrast cases by explaining the contrasts in terms of non-perceptualcontents or in terms of attentional changes. I show that neither response is applicable to mycontrast cases and conclude that we do indeed perceptually experience the high-level propertiesof visual scenes.

Exploring the range of reported dream lucidity

Dream lucidity, or being aware that one is dreaming while dreaming, is not an all-or-none phenomenon. Often, subjects report being some variant of “a little lucid” as opposed to completely or not at all. As recent neuroimaging work begins to elucidate the neural underpinnings of lucid experience, understanding subtle phenomenological variation within lucid dreams is essential. Here, we focus on the variability of lucid experience by asking participants to report their awareness of the dream on a 5-point Likert scale (from not at all to very much). Participants implemented a combination of mnemonic training lucid dream induction methods at home for one week and provided detailed reports about their dream experiences each morning. Consistent with previous research, cognitive induction methods led to about half of participants reporting at least one lucid dream and about half of all dreams including some level of lucidity. However, we also show that induction success rate varies significantly depending on the minimum criteria for lucidity. Participants also reported how much they adhered to specific components of each induction method, and the amount of mnemonic rehearsal during a brief early awake period was predictive of lucidity level. Furthermore, lucidity levels were positively correlated with dream control, dream bizarreness, and next-morning positive affect. Lastly, we asked participants open-ended questions about why they chose particular levels of lucidity. We focus a qualitative discussion on responses to those “semi-lucid” dreams (rated just a little, moderately, or pretty much lucid) to explore why participants rate their dreams as having intermediate levels of awareness. Together, the present study explores the frequency of semi-lucid dreams, what they are, why they might arise, their correlates, and how they impact methodological concerns in lucid dreaming research.

On the dangers of conflating strong and weak versions of a theory of consciousness

Some proponents of the Integrated Information Theory (IIT) of consciousness profess strong views on the Neural Correlates of Consciousness (NCC), namely that large swathes of the neocortex, the cerebellum, at least some sensory cortices, and the so-called limbic system are all not essential for any form of conscious experiences. We argue that this connection is not incidental. Conflation between strong and weak versions of the theory has led these researchers to adopt definitions of NCC that are inconsistent with their own previous definitions, inadvertently betraying the promises of an otherwise fruitful empirical endeavour.