Finding the neural correlates of consciousness will not solve all our problems

EDITORIAL article Front. Psychol., 01 September 2011 | https://doi.org/10.3389/fpsyg.2011.00191

OPINION article Front. Hum. Neurosci., 19 July 2013Sec. Cognitive Neuroscience Volume 7 - 2013 | https://doi.org/10.3389/fnhum.2013.00387

Despite that leading theories of consciousness make diverging predictions for where and how neural activity gives rise to subjective experience, they all seem to partially agree that the neural correlates of consciousness (NCC) require globally integrated brain activity across a network of functionally specialized modules. However, it is not clear yet whether such functional configurations would be able to identify the NCC. We scanned resting-state fMRI data from 21 subjects during wakefulness, propofol-induced sedation, and anesthesia. Graph-theoretical analyses were conducted on awake fMRI data to search for the NCC candidates as brain regions that exhibit both high rich-clubness and high modular variability, which were found to locate in prefrontal and temporoparietal cortices. Another independent data set of 10 highly-sampled subjects was used to validate the NCC distribution at the individual level. Brain module-based dynamic analysis revealed two discrete reoccurring brain states, one of which was dominated by the NCC candidates (state 1), while the other state was predominately composed of primary sensory/motor regions (state 2). Moreover, state 1 appeared to be temporally more stable than state 2, suggesting that the identified NCC members could sustain conscious content as metastable network representations. Finally, we showed that the identified NCC was modulated in terms of functional connectedness and modular variability in response to the loss of consciousness induced by propofol anesthesia. This work offers a framework to search for neural correlates of consciousness by charting the brain network topology and provides new insights into understanding the roles of different regions in underpinning human consciousness.

Multiple Perspectives on Consciousness for Cognitive Science Richard A. Carlson (racarlson@psu.edu) Department of Psychology, Penn State University 613 Moore Building, University Park, PA 16802 USA The huge contemporary literature on consciousness spans multiple disciplines, including psychology, philosophy, and neuroscience. This tutorial will introduce participants to major proposals about consciousness, and their empirical and methodological implications. The goal is to prepare participants to explore the consciousness literature in greater depth. Our consideration of perspectives on consciousness will be organized by considering how these perspectives address core questions about consciousness, including: (a) How can subjectivity and agency be accommodated in a scientific theory of consciousness? (b) How can conscious and nonconscious or unconscious processes and representations be systematically distinguished? (c) How can conscious mental states be assessed or measured? (d) How can dissociations and impairments of consciousness be understood? The literatures to be considered address these questions in analytic, functional, computational, and implementational terms. Philosophical Perspectives Philosophers approach the problem of consciousness from a variety of analytic perspectives, some focusing on contemporary formulations of the mind-body problem and others on analyses of subjective experience. Among the philosophical perspectives we will consider are John Searle’s (1992) analysis of consciousness in terms of intentionality, David Chalmer’s (1996) distinction between “easy” and “hard” problems of consciousness, David Rosenthal’s (1993) “higher order thought” proposal, and Daniel Dennett’s (1991) “multiple drafts” theory of consciousness. Neuroscience Perspectives Neuroscientists have made a wide variety of proposals concerning the neural correlates of consciousness (NCC). A starting assumption is that a subset of current neural activity is correlated with current conscious experience. There is controversy, however, concerning how that subset is to be identified. For example, the NCC might be limited to particular types of cells or anatomical structures, or comprise global patterns of synchronized neural activity. We will consider recent proposals concerning NCC by Crick and Koch (1998), Damasio (2000), and Edelman and Tononi Psychological Perspectives Psychological perspectives on consciousness generally focus on functionally-defined aspects of cognition. For example, psychologists have identified consciousness with working memory (Baars, 1988), attention (Schneider & Pimm-Smith, 1997), metacognition (Nelson, 1996), and with the structure of mental states (Carlson, 1997). Cognitive research often focuses on distinguishing conscious and nonconscious influences on psychological processes such as learning (Dienes & Berry, 1997) and perception (Merikle, Smilek, & Eastwood, 2001). This research has generated a rich literature on methods for assessing consciousness. References Baars, B. J. (1988). A cognitive theory of consciousness. New York: Cambridge University Press. Carlson, R. A. (1997). Experienced Cognition. Mahwah, NJ: Lawrence Erlbaum Associates. Chalmers, D. (1996). The conscious mind. Oxford: Oxford University Press. Crick, F., & Koch, C. (1998). Consciousness and neuroscience. Cerebral Cortex, 8, 97-107. Damasio, A. R. (2000). A neurobiology for consciousness. In T. Metzinger (Ed.), Neural correlates of consciousness Cambridge, MA: The MIT Press. Dienes, Z., & Berry, D. (1997). Implicit learning: Below the subjective threshold. Psychonomic Bulletin and Review, 4, Dennett, D. C. (1991). Consciousness explained. Boston: Little, Brown and Company. Edelman, G. M., & Tononi, G. (2000). Reentry and the dynamic core: Neural correlates of conscious experience. In T. Metzinger (Ed.), Neural correlates of consciousness. Cambridge, MA: The MIT Press. Merikle, P. M., Smilek, D., & Eastwood, J. D. (2001). Perception without awareness: perspectives from cognitive psychology. Cognition, 79, 115-134. Nelson, T. O. (1996). Consciousness and metacognition. American Psychologist, 51, 102-116. Rosenthal, D. M. (1993). Thinking that one thinks. In M. Davies, & G. W. Humphreys (Eds.), Consciousness: Psychological and philosophical essays. Oxford: Blackwell. Searle, J. R. (1992). The rediscovery of the mind. Cambridge, MA: The MIT Press. Schneider, W., & Pimm-Smith, M. (1997). Consciousness as a message aware control mechanism to modulate cognitive processing. J. Cohen, & J. Schooler (Eds.), Scientific approaches to consciousness: The 25th Carnegie Symposium on Cognition. Mahwah, NJ: Erlbaum.

Global gamma band synchronisation is perhaps the most extensively studied candidate for a Neural Correlate of Consciousness (NCC) . Yet despite numerous studies confirming its association with consciousness, it seems to be neither sufficient nor necessary for the presence of all subjective experiences. Analysis of gamma synchronisation studies suggests that it is a correlate of the initial binding of expected, attended, task-dependent contents of consciousness, whereas task-irrelevant contents do not seem to require gamma synchronisation. While discovery of such a content-related NCC is a remarkable achievement for the neurophysiological research of consciousness, it does not fully explain some of the fundamental structural properties of consciousness, namely the temporal and spatial integration of all available experiences into a coherent stream of consciousness. As an alternative, instead of focusing solely on the selected contents of consciousness, the neural mechanisms of the fundamental properties of consciousness could be studied by contrasting states of (un)consciousness. Recent research into the states of consciousness suggests that, for instance, informational complexity is a highly sensitive predictor of the presence of consciousness, possibly reflecting background structural properties of the unity of subjective experiences. As a limiting factor, though, such a state-related NCC does not seem to reflect the phenomenal diversity of the contents of consciousness. Arguably, these limitations could be overcome by devising experimental setups that would simultaneously probe the neural correlates of the contents and the state of consciousness.

'Self' and the Problem of Consciousness

Neural correlates of conscious processing of emotional faces: Evidence from event-related potentials

The nature of the proper neural signature of conscious perception remains a topic of active debate. Theoretical support from integrative theories of consciousness is consistent with such signature being P3b, one of the main candidates in the literature. Recent work has also put forward a mid-latency and more localized component, the Visual Awareness Negativity (VAN), as a proper Neural Correlate of Consciousness (NCC). Early local components like P1 have also been proposed. However, experiments exploring visual NCCs are conducted almost exclusively using static images as the content to be consciously perceived, favoring ventral stream processing, therefore limiting the scope of the NCCs that have been identified. Here we explored the visual NCCs isolating local motion, a dorsally processed feature, as the primary feature being consciously perceived. Physical equality between Seen and Unseen conditions in addition to a minimal contrast difference between target and no-target displays was employed. In agreement with previous literature, we found a P3b with a wide centro-parietal distribution that strongly correlated with the detection of the stimuli. P3b magnitude was larger for Seen vs. Unseen conditions, a result that was consistently observed at the single subject level. In contrast, we were unable to detect VAN in our data, regardless of whether the subject perceived or not the stimuli. In the 200–300 ms time window we found a N2pc component, consistent with the high attentional demands of our task. Early components like P1 were not observed in our data, in agreement with their proposed role in the processing of visual features, but not as proper NCCs. Our results extend the role of P3b as a content independent NCC to conscious visual motion perception.

Despite the fact that blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) studies have become ubiquitous and are of ever increasing importance for clinical and basic neurosciences, the fundamental relationships between BOLD and the underlying neuronal physiology are not understood. This raises severe concerns about the validity of BOLD contrast per se, and the conceptual frameworks currently employed in interpreting cognitive neuroimaging data. In order to expand the explanatory power of functional MRI data, several crucial questions will have to be addressed. The two most important questions are: First, what is the ultimate spatial resolution of fMRI?, secondly, what is the "neural correlate" of functional MRI? This article attempts to compile a series of results from our and other laboratories, suggesting that both the questions of "spatial specificity" and "neural correlate" might be within the reach of a tentative solution, thus finally bridging the gap between functional neuroimaging and neuronal physiology.

Consciousness is a central issue in neuroscience, however, we still lack a formal framework that can address the nature of the relationship between consciousness and its physical substrates. In this review, we provide a novel mathematical framework of category theory (CT), in which we can define and study the sameness between different domains of phenomena such as consciousness and its neural substrates. CT was designed and developed to deal with the relationships between various domains of phenomena. We introduce three concepts of CT which include (i) category; (ii) inclusion functor and expansion functor; and, most importantly, (iii) natural transformation between the functors. Each of these mathematical concepts is related to specific features in the neural correlates of consciousness (NCC). In this novel framework, we will examine two of the major theories of consciousness, integrated information theory (IIT) of consciousness and temporospatial theory of consciousness (TTC). We conclude that CT, especially the application of the notion of natural transformation, highlights that we need to go beyond NCC and unravels questions that need to be addressed by any future neuroscientific theory of consciousness.

For a theory of phenomenal consciousness, the real issue is not that between vehicle and process, but between naturalistic and deconstructive theories. Most current naturalistic theories combine a hypothesis about the neural correlate of consciousness with a subsequent naturalistic proposal about how to close the explanatory gap. Deconstructive theories use theses about the neural correlate of consciousness only to motivate and support their claim that the “hard problem” of consciousness is a pseudo-problem which is not to be solved, but rather dissolved on non-naturalistic grounds. O'Brien & Opie present a hypothesis concerning the neural correlate of consciousness, but no genuine strategy to close the explanatory gap. Their theory can, however, contribute to the success of a deconstructive theory of PC.

Conscious perception of fear in faces: Insights from high-density EEG and perceptual awareness scale with threshold stimuli

The neural correlate of consciousness

SummaryWhen faced with ambiguous sensory input, conscious awareness may alternate between the different percepts that are consistent with the input. Visual phenomena leading to such multistable perception, where constant sensory input evokes different conscious percepts, are particularly useful for investigating the processes underlying perceptual awareness [1]. Understanding the role that high-level brain regions outside early visual cortex play in perceptual alternations could elucidate how top-down processes modulate conscious perception [2]. In two studies [3,4] published recently in Current Biology, different combinations of the present authors used repetitive transcranial magnetic stimulation (rTMS) to disrupt activity in human superior parietal cortex, and reported seemingly contradictory results [5] concerning the effect of disrupting the normal function of this area on bistable perception. Here we join forces to resolve this discrepancy.

An adaptive workspace hypothesis about the neural correlates of consciousness: insights from neuroscience and meditation studies