Neural Plasticity and Consciousness

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

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.

The problem of consciousness is mostly regarded as identical to the mind-body problem. According to Chalmers’ philosophical arguments, the hard problem of consciousness lies in establishing and explaining the link between physical processes and conscious experiences, via psychological processes. A brief history of various theories of consciousness is given and a selection of theories are tested against Zeman’s three fundamental intuitions and Chalmers’ controversial zombie argument. The hard problem of consciousness is further described using Levine’s notion of an explanatory gap between physical matter and conscious experience, through the first and third persons. Various states, contents, levels and processes of consciousness are summarised, including Damasio and Meyer’s dual perspective for defining consciousness. Tart’s three definitions do not entirely describe altered states of consciousness. While the challenge of finding the core function of human and animal sleep remains unknown when tested under the null hypothesis, studies on the neural correlates of consciousness during meditation have revealed neuroplasticity effects. The synchrony of gamma brain oscillations reflecting various styles of meditation or attention, also known as the binding problem, may be related to conscious experiences. This binding problem with gamma brain oscillatory synchronization also arises in relation to sensory awareness or perception, affecting the perception of time and hallucinatory experiences in various disorders of consciousness such as severe schizophrenic and deja vu (in healthy or epileptic) patients. In conjunction with medication treatments, music therapy is often useful in accelerating the healing process in most such disorders of consciousness. It is still unknown how this sensory awareness to music is perceived in medicated patients suffering from disorders of consciousness. More clinically elusive are near death experiences, in which consciousness persists independently of brain function, where there is no scientific basis for such consciousness to exist and no physiological or psychological model that can explain it. Near death experiences can be regarded as a special state of consciousness, which provides further evidence that the consciousness problem may be very close to the mind-body problem that originates in Descartes’ classic theory of dualism and is transformed into Chalmers’ contemporary theory of natural dualism. The final section of this chapter offers an overview of all invited chapters.

The Equations for Consciousness: A Reply to “Tracking the Travels,” a Review of <i>Journey of the Mind</i>

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.

One of the greatest challenges of consciousness research is to understand the relationship between consciousness and its implementing substrate. Current research into the neural correlates of consciousness regards the biological brain as being this substrate, but largely fails to clarify the nature of the brain-consciousness connection. A popular approach within this research is to construe brain-consciousness correlations in causal terms: the neural correlates of consciousness are the causes of states of consciousness. After introducing the notion of the neural correlate of consciousness, we argue (see Against Causal Accounts of NCCs) that this causal strategy is misguided. It implicitly involves an undesirable dualism of matter and mind and should thus be avoided. A non-causal account of the brain-mind correlations is to be preferred. We favor the theory of the identity of mind and brain, according to which states of phenomenal consciousness are identical with their neural correlates. Research into the neural correlates of consciousness and the theory of identity (in the philosophy of mind) are two major research paradigms that hitherto have had very little mutual contact. We aim to demonstrate that they can enrich each other. This is the task of the third part of the paper in which we show that the identity theory must work with a suitably defined concept of type. Surprisingly, neither philosophers nor neuroscientists have taken much care in defining this central concept; more often than not, the term is used only implicitly and vaguely. We attempt to open a debate on this subject and remedy this unhappy state of affairs, proposing a tentative hierarchical classification of phenomenal and neurophysiological types, spanning multiple levels of varying degrees of generality. The fourth part of the paper compares the theory of identity with other prominent conceptions of the mind-body connection. We conclude by stressing that scientists working on consciousness should engage more with metaphysical issues concerning the relation of brain processes and states of consciousness. Without this, the ultimate goals of consciousness research can hardly be fulfilled.

Consciousness is difficult to pin down. Most human beings go about their days with full and more or less uninterrupted consciousness, without contemplating their own (or other peoples’) conscious states. To be in the world, and accomplish great acts takes little metaawareness of consciousness, but in the study of consciousness our inability to think outside of our conscious states creates controversies at the conceptual and methodological levels. As Victor Lamme states (2006), even when we set aside the more difficult (or more poorly defined) questions about conscious experience to focus on finding the neural correlates of consciousness (NCC), we face immense difficulties (Lamme, 2006, p. 494). Experiments designed to find the NCC often involve the manipulation of conscious states through anesthesia, the study of sleep, or brain lesion studies (Lamme, 2006, p. 494). However, even in the case of anesthesia, where we can voluntarily induce a reversible altered state of consciousness there does not seem to be a clear dividing line between consciousness and unconsciousness with any of the processed electroencephalogram (EEG) signals (Guzeldere, 1998, p. 1) such that the conscious and unconscious states are still confirmed behaviorally (Lamme, 2006, p. 494). This leads to a problem, as it must be decided what "behavioral measures 'count' as evidence for the subject having conscious experience (p. 494)" a problem that is not so simple as the ability to speak and respond, as will be more clear in a later discussion of intraoperative awareness. Furthermore, Guven Guzeldere points to the difficulty of defining what "the problem of consciousness" is, within and "across disciplinary boundaries (Guzeldere, p. 7)." The problems that philosophers of consciousness, cognitive scientists and neuroscientists address when they study consciousness are not inevitably going to be identical, but are shaped by disciplinary perspectives, methods and technologies. Therefore, in this paper I am going to contrast two similar models of consciousness, Giulio Tononi’s Integrated Information Theory and Daniel Dennett’s Multiple Drafts Model, and evaluate them against the mechanisms of several anesthetics (Propofol, ketamine, and the inhalation anesthetics, including xenon), which will be summarized by a review of the literature. I have two goals in mind with this project: first, I have chosen two very similar models in order to demonstrate how small differences- such as Tononi’s engagement with the concept of qualia and Dennett’s deconstruction of it-- have large implications for what types of knowledge are possible when these models are

For many years, since Baars (1988) explicitly formulated it, contrastive analysis has been the key methodological approach in experimental studies of consciousness. When certain properly chosen psychological experimental setups (allowing an invariant target stimulus either to be consciusly experienced or not) were combined with brain-imaging methods, contrastive analysis became a quite powerful tool of research (Crick, 1994; Koch, 2004). By subtracting markers of brain processes recorded in the conditions without conscious experience of the target from the markers recorded in the conditions where the same target is consciously experienced it was believed that the markers of neural correlates of consciousness (NCC) can be obtained. However, as it turned out in the subsequent theoretical and experimental analysis, the picture is not so clear and simple (Bachmann, 2000, 2009; Miller, 2007; Aru et al., 2012; de Graaf et al., 2012). For example, when in the invariant conditions of independent variables a masked visual stimulus was consciously perceived or not (consciousness of the target standed as a dependent variable), NCC which were measured as a spectral perturbation of EEG was present already before stimulus presentation (Aru and Bachmann, 2009). Thus, the neural correlate of consciousness of a stimulus was present earlier than the stimulus itself was presented. Now, a reader must not get excited here because instead of some paranormal explanations brain-science based explanations can be comfortably used. In order to overcome the conceptual crisis hitting the traditional contrastive analysis based NCC research it was suggested that unconscious prerequisite processes (NCCpr) emerging as a result of contrastive analysis of brain-process markers of consciousness and similarly unconscious consequent processes (NCCco) must be differentiated from the constitutive processes directly associated with conscious experience (Aru et al., 2012; de Graaf et al., 2012). Thus, new experimental approaches were in need to avoid the trap of distilling prerequisite, direct, and consequent processes as mutually confounded and empirically inseparable. Despite some first attempts in this direction (Aru and Bachmann, 2015), the specialist landscape in this domain has remained obscure and no breakthrough solutions have been in sight. Moreover, there seems to be a number of additional uncertainties when we try to disentangle the various sub-types of NCC. Even NCCpr and NCCco are not unitary in terms of their theoretical meaning and associated neural processes. First, as the contents on which the perceptual report is founded can be selective, the markers of unused conscious contents may be erroneously neglected as markers of unconscious processes. They actually belong to consciousness level processes, but related to contents of consciousness qualitatively different from the ones specified by NCC. Second, in measuring NCC we must be able to disentangle contributions of the general consciousness enabling mechanisms and the selective contents representing mechanisms because their markers can be different and thus confused. In what follows I will substantiate these two issues.

Consciousness or conscious experience is a mental phenomenon that is familiar to all of us, but the way in which it is produced escapes us to a large extent. Each person has a vague idea of what it means to be conscious, but consciousness is rather hard to define, albeit easy to identify. It is that function of the brain that makes us conscious of external or internal stimuli and of our thoughts regarding these subjective experiences. Conscious experience is a first-person perspective of mental states and events tracking as they unfold. It includes mental phenomena such as a perception, emotion, memory, idea, continuous temporal sequence of events. A mental process and its adjoining neurophysiological phenomena represent two aspects of the same event. We have direct access to the mental aspect, while we can observe the neurophysiological aspect only when we study the event as a biological process. The psychological study of consciousness describes the special properties of this brain function, its origin and utility in the global economy of an animal organism. The neurobiological study aims to find the neural correlates of consciousness, aims to establish causal relations between the neural phenomena and the different conscious states. Lastly, the formulation of an explanatory theory can provide a satisfactory understanding of the phenomenon. This review aims to bring some clarification in the field of consciousness, selecting the hypotheses which mostly fulfill the requirements, in order to be confirmed as explanatory theories. A valuable test for confirming an explanatory hypothesis is its predictive power. Using this criterion we have evaluated comparatively, some of the proposed explaining hypotheses.

<i>Human and Animal Minds: The Consciousness Questions Laid to Rest</i>

A Rediscovery of Consciousness

Do neural correlates of consciousness cause conscious states?

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.

More than half of the activities of daily living rely on upper limb functions (Ingram et al., 2008). Humans perform upper limb movements with great ease and flexibility but even simple tasks require complex computations in the brain and can be affected following stroke leaving survivors with debilitating movement impairments. Hemispheric asymmetries related to motor dominance, imbalances between contralateral and ipsilateral primary motor cortices (M1) activity and the ability to adapt movements to novel environments play a key role in upper limb motor control and can affect recovery. Motor learning and control are critical in neurorehabilitation, however to effectively integrate these concepts into upper limb recovery treatments, a deeper understanding of the basic mechanisms of unimanual control is needed. This thesis aimed to investigate hemispheric asymmetries related to motor dominance, to evaluate the relative contribution of the contralateral and ipsilateral M1 during unilateral reaching preparation and finally to identify the neural correlates underlying the formation of a predictive internal model enabling to adapt movements to new environments. To this end electroencephalography (EEG), transcranial magnetic stimulation (TMS), simultaneous TMS-EEG were employed during a simple motor and a highly standardised robot-mediated task. The first study used TMS-EEG to examine differences in cortical excitability related to motor dominance by applying TMS over the dominant and non-dominant M1 at rest and during contraction. No hemispheric asymmetries related to hand dominance were found. The second study assessed the temporal dynamics of bi-hemispheric motor cortical excitability during right arm reaching preparation. TMS was applied either to the ipsilateral or contralateral M1 during different times of movement preparation. Significant bilateral M1 activation during unilateral reaching preparation was observed, with no significant differences between the contralateral and ipsilateral M1. Unimanual reaching preparation was associated with significant interactions of excitatory and inhibitory processes in both motor cortices. The third study investigated the neural correlates of motor adaptation. EEG was recorded during a robot-mediated adaptation task involving right arm reaching movements and cortical excitability was assessed by applying TMS over the contralateral M1 and simultaneously recording TMS responses with EEG before and after motor adaptation. It was found that an error-related negativity (ERN) over fronto-central regions correlated with performance improvements during adaptation, suggesting that this neural activity reflects the formation of a predictive internal model. Motor adaptation underlay significant modulations in cortical excitability (i.e. neuroplasticity) in sensorimotor regions. Finally, it was shown that native cortical excitability was linked to motor learning improvements during motor adaptation and explained the variability in motor learning across individuals. These experiments demonstrated that even unimanual motor control relies on interactions between excitatory and inhibitory mechanisms not only in the contralateral M1 but in a wider range of brain regions, shown by a bi-hemispheric activity during movement preparation, the formation of a predictive model in fronto-central regions during motor adaptation and neuroplastic changes in sensorimotor regions underlying motor adaptation during unimanual reaching.

In this paper, we outline some important milestones in the history of the term "plasticity" in reference to the nervous system. Credit is given to William James for first adopting the term to denote changes in nervous paths associated with the establishment of habits; to Eugenio Tanzi for first identifying the articulations between neurons, not yet called synapses, as possible sites of neural plasticity; to Ernesto Lugaro for first linking neural plasticity with synaptic plasticity; and to Cajal for complementing Tanzi's hypothesis with his own hypothesis of plasticity as the result of the formation of new connections between cortical neurons. Cajal's early use of the word plasticity is demonstrated, and his subsequent avoidance of the term is tentatively accounted for by the fact that other authors extended it to mean neuronal reactions partly pathological and no doubt quite different from those putatively associated with normal learning. Evidence is furnished that in the first two decades of the twentieth century the theory was generally accepted that learning is based on a reduced resistance at exercized synapses, and that neural processes become associated by coactivation. Subsequently the theory fell in disgrace when Lashley's ideas about mass action and functional equipotentiality of the cortex tended to outmode models of the brain based on orthodox neural circuitry. The synaptic plasticity theory of learning was rehabilitated in the late 1940s when Konorski and particularly Hebb argued successfully that there was no better alternative way to think about the modifiability of the brain by experience and practice. Hebb's influential hypothesis about the mechanism of adult learning contained elements strikingly similar to the early speculations of James, Tanzi and Cajal, but Hebb did not acknowledge specifically these roots of his thinking about the brain, though he was fully aware that he had resurrected old ideas wrongly neglected for a long time. Lately the concept of neural plasticity has been complicated by attributing considerably different meanings to it. A scholarly paper by Paillard is used to show how an analysis in depth can clarify some confusion engendered by an unrestricted use of the concept and term of neural plasticity.