consciousness research


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Bernard J. Baars, James Newman, J.G. Taylor
Neuronal mechanisms of consciousness:
A Relational Global Workspace framework.

(Pages 269-278 in S. Hameroff, A. Kaszniak, J. Laukes, Toward a Science of Consciousness II: The second Tucson discussions and debates. Cambridge, MA: MIT Press, 1998.)
"This paper explores a remarkable convergence of ideas and evidence, previously presented in separate places by its authors. That convergence has now become so persuasive that we believe we are working within substantially the same broad framework. Taylor’s mathematical papers on neuronal systems involved in consciousness dovetail well with work by Newman and Baars on the thalamocortical system, suggesting a brain mechanism much like the global workspace architecture developed by Baars (see references below). This architecture is relational, in the sense that it continuously mediates the interaction of input with memory. While our approaches overlap in a number of ways, each of us tends to focus on different areas of detail. What is most striking, and we believe significant, is the extent of consensus, which we believe to be consistent with other contemporary approaches by Weiskrantz, Gray, Crick and Koch, Edelman, Gazzaniga, Newell and colleagues, Posner, Baddeley, and a number of others. We suggest that cognitive neuroscience is moving toward a shared understanding of consciousness in the brain."
"The brain stem-thalamocortical axis supports the state, but not the detailed contents of consciousness, which are produced by cortex." [Full Text]

John G. Taylor
Constructing the Relational Mind
PSYCHE, 4(10), June 1998
"The "relational mind" approach to the inner content of consciousness is developed in terms of various control structures and processing strategies and their possible neurobiological identifications in brain sites. This leads naturally to a division of consciousness into a passive and an active part. A global control structure for the "single strand" aspect of consciousness is proposed as the thalamo-nucleus reticularis thalami-cortex coupled system, which is related to experimental data on the electrical stimulation of awareness. Local control, in terms of excitatory transfer from pre-processing sites to posterior working memory regions, is supported by data on subliminal perception timing and disambiguation of poorly defined percepts. The inner content of consciousness is understood as arising from the resulting relational features between inputs and stored pre-processing and episodic memories. Strong analogies are drawn between emergent properties of the model and suggested properties of "raw feels", supporting the thesis that working memories are the initial sites for the emergence of phenomenal awareness, and the frontal lobes for its further adumbration in terms of higher cognitive processing, including the creation of self." [Full Text]

Baars BJ, Franklin S.
How conscious experience and working memory interact.
Trends Cogn Sci 2003 Apr;7(4):166-172
"Active components of classical working memory are conscious, but traditional theory does not account for this fact. Global Workspace theory suggests that consciousness is needed to recruit unconscious specialized networks that carry out detailed working memory functions. The IDA model provides a fine-grained analysis of this process, specifically of two classical working-memory tasks, verbal rehearsal and the utilization of a visual image. In the process, new light is shed on the interactions between conscious and unconscious aspects of working memory." [Abstract]

Baars BJ.
Tutorial commentary: surprisingly small subcortical structures are needed for the state of waking consciousness, while cortical projection areas seem to provide perceptual contents of consciousness.
Conscious Cogn 1995 Jun;4(2):159-62
"The evidence can therefore be summarized as follows: (1) RF, nRt, and ILN activity seem to be necessary but not sufficient for conscious experience. (2) Stimulus representation in primary sensory projection areas also seems to be necessary but not sufficient for conscious perceptual experience (Weiskrantz, 1980). The simplest hypothesis is that both components are necessary and sufficient to support conscious perceptual experience." [Abstract] [RF = reticular formation; nRt = nucleus reticularis thalami; ILN = thalamic intralaminar nuclei]

Cho SB, Baars BJ, Newman J.
A Neural Global Workspace Model for Conscious Attention.
Neural Netw 1997 Oct 1;10(7):1195-1206
"Considerable progress is being made in interdisciplinary efforts to develop a general theory of the neural correlates of consciousness. Developments of Baars' Global Workspace theory over the past decade are examples of this progress. Integrating experimental data and models from cognitive psychology, AI and neuroscience, we present a neurocognitive model in which consciousness is defined as a global integration and dissemination system - nested in a large-scale, distributed array of specialized bioprocessors - which controls the allocation of the processing resources of the central nervous system. It is posited that this global control is effected via cortical 'gating' of a strategic thalamic nucleus. The basic circuitry of this neural system is reasonably well understood, and can be modeled, to a first approximation, employing neural network principles." [Abstract]

Bernard J. Baars
Global Workspace Theory,
A Rigorous Scientific Theory of Consciousness.

Journal of Consciousness Studies, 4, No. 4, 1997, pp. 292-309
"An array of evidence is beginning to reveal the role of consciousness in the nervous system, at least in outline. Conscious experience seems to create access to many independent knowledge sources in the brain, most of them quite unconscious. Humans seem to have a larger repertoire of uses for consciousness — including language and long-term planning, self-monitoring and self-reflection, inner speech, metaphor, symbolic representation of experience and deliberate use of imagery. When it comes to sensory consciousness, however, the brain shows little difference between humans and many other mammals." [PDF]

Baars BJ.
How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity?
Ciba Found Symp 1993;174:282-90; discussion 291-303
"Much of the nervous system can be viewed as a massively parallel, distributed system of highly specialized but unconscious processors. Conscious experience on the other hand is traditionally viewed as a serial stream that integrates different sources of information but is limited to only one internally consistent content at any given moment. Global Workspace theory suggests that conscious experience emerges from a nervous system in which multiple input processors compete for access to a broadcasting capability; the winning processor can disseminate its information globally throughout the brain. Global workspace architectures have been widely employed in computer systems to integrate separate modules when they must work together to solve a novel problem or to control a coherent new response. The theory articulates a series of increasingly complex models, able to account for more and more evidence about conscious functioning, from perceptual consciousness to conscious problem-solving, voluntary control of action, and directed attention. Global Workspace theory is consistent with, but not reducible to, other theories of limited-capacity mechanisms. Global workspace architectures must show competition for input to a neural global workspace and global distribution of its output. Brain structures that are demonstrably required for normal conscious experience can carry out these two functions. The theory makes testable predictions, especially for newly emerging, high-speed brain imaging technology." [Abstract]

Baars BJ.
The conscious access hypothesis: origins and recent evidence.
Trends Cogn Sci 2002 Jan 1;6(1):47-52
"Consciousness might help to mobilize and integrate brain functions that are otherwise separate and independent. Evidence for this 'conscious access hypothesis' was described almost two decades ago, in a framework called global workspace theory. The theory had little impact at first, for three reasons: because consciousness was controversial; the evidence, though extensive, was indirect; and integrative theory was unfashionable. Recent neuroimaging evidence appears broadly to support the hypothesis, which has implications for perception, learning, working memory, voluntary control, attention and self systems in the brain." [Abstract] [PDF]

Dehaene S, Naccache L.
Towards a cognitive neuroscience of consciousness: basic evidence and a workspace framework.
Cognition 2001 Apr;79(1-2):1-37
"This introductory chapter attempts to clarify the philosophical, empirical, and theoretical bases on which a cognitive neuroscience approach to consciousness can be founded. We isolate three major empirical observations that any theory of consciousness should incorporate, namely (1) a considerable amount of processing is possible without consciousness, (2) attention is a prerequisite of consciousness, and (3) consciousness is required for some specific cognitive tasks, including those that require durable information maintenance, novel combinations of operations, or the spontaneous generation of intentional behavior. We then propose a theoretical framework that synthesizes those facts: the hypothesis of a global neuronal workspace. This framework postulates that, at any given time, many modular cerebral networks are active in parallel and process information in an unconscious manner. An information becomes conscious, however, if the neural population that represents it is mobilized by top-down attentional amplification into a brain-scale state of coherent activity that involves many neurons distributed throughout the brain. The long-distance connectivity of these 'workspace neurons' can, when they are active for a minimal duration, make the information available to a variety of processes including perceptual categorization, long-term memorization, evaluation, and intentional action. We postulate that this global availability of information through the workspace is what we subjectively experience as a conscious state. A complete theory of consciousness should explain why some cognitive and cerebral representations can be permanently or temporarily inaccessible to consciousness, what is the range of possible conscious contents, how they map onto specific cerebral circuits, and whether a generic neuronal mechanism underlies all of them. We confront the workspace model with those issues and identify novel experimental predictions. Neurophysiological, anatomical, and brain-imaging data strongly argue for a major role of prefrontal cortex, anterior cingulate, and the areas that connect to them, in creating the postulated brain-scale workspace." [Abstract] [PDF]

Stanislas Dehaene, Michel Kerszberg, and Jean-Pierre Changeux
A neuronal model of a global workspace in effortful cognitive tasks
PNAS 95: 14529-14534, 1998.
"A minimal hypothesis is proposed concerning the brain processes underlying effortful tasks. It distinguishes two main computational spaces: a unique global workspace composed of distributed and heavily interconnected neurons with long-range axons, and a set of specialized and modular perceptual, motor, memory, evaluative, and attentional processors. Workspace neurons are mobilized in effortful tasks for which the specialized processors do not suffice. They selectively mobilize or suppress, through descending connections, the contribution of specific processor neurons. In the course of task performance, workspace neurons become spontaneously coactivated, forming discrete though variable spatio-temporal patterns subject to modulation by vigilance signals and to selection by reward signals. A computer simulation of the Stroop task shows workspace activation to increase during acquisition of a novel task, effortful execution, and after errors. We outline predictions for spatio-temporal activation patterns during brain imaging, particularly about the contribution of dorsolateral prefrontal cortex and anterior cingulate to the workspace." [Full Text]

Damasio AR.
Investigating the biology of consciousness.
Philos Trans R Soc Lond B Biol Sci 1998 Nov 29;353(1377):1879-82
"The fact that consciousness is a private, first-person phenomenon makes it more difficult to study than other cognitive phenomena that, although being equally private, also have characteristic behavioural signatures. Nonetheless, by combining cognitive and neurobiological methods, it is possible to approach consciousness, to describe its cognitive nature, its behavioural correlates, its possible evolutionary origin and functional role; last but not least, it is possible to investigate its neuroanatomical and neurophysiological underpinnings. In this brief essay I distinguish between two kinds of consciousness: core consciousness and extended consciousness. Core consciousness corresponds to the transient process that is incessantly generated relative to any object with which an organism interacts, and during which a transient core self and transient sense of knowing are automatically generated. Core consciousness requires neither language nor working memory, and needs only a brief short-term memory. Extended consciousness is a more complex process. It depends on the gradual build-up of an autobiographical self, a set of conceptual memories pertaining to both past and anticipated experiences of an individual, and it requires conventional memory. Extended consciousness is enhanced by language." [Abstract] [Full Text]

Hans C. Lou, Bruce Luber, Michael Crupain, Julian P. Keenan, Markus Nowak, Troels W. Kjaer, Harold A. Sackeim, and Sarah H. Lisanby
Parietal cortex and representation of the mental Self
PNAS published April 19, 2004, 10.1073/pnas.0400049101
"For a coherent and meaningful life, conscious self-representation is mandatory. Such explicit "autonoetic consciousness" is thought to emerge by retrieval of memory of personally experienced events ("episodic memory"). During episodic retrieval, functional imaging studies consistently show differential activity in medial prefrontal and medial parietal cortices. With positron-emission tomography, we here show that these medial regions are functionally connected and interact with lateral regions that are activated according to the degree of self-reference. During retrieval of previous judgments of Oneself, Best Friend, and the Danish Queen, activation increased in the left lateral temporal cortex and decreased in the right inferior parietal region with decreasing self-reference. Functionally, the former region was preferentially connected to medial prefrontal cortex, the latter to medial parietal. The medial parietal region may, then, be conceived of as a nodal structure in self-representation, functionally connected to both the right parietal and the medial prefrontal cortices. To determine whether medial parietal cortex in this network is essential for episodic memory retrieval with self-representation, we used transcranial magnetic stimulation over the region to transiently disturb neuronal circuitry. There was a decrease in the efficiency of retrieval of previous judgment of mental Self compared with retrieval of judgment of Other with transcranial magnetic stimulation at a latency of 160 ms, confirming the hypothesis. This network is strikingly similar to the network of the resting conscious state, suggesting that self-monitoring is a core function in resting consciousness." [Abstract]

Debra A. Gusnard, Erbil Akbudak, Gordon L. Shulman, and Marcus E. Raichle
Medial prefrontal cortex and self-referential mental activity: Relation to a default mode of brain function
PNAS 98: 4259-4264; published online before print as 10.1073/pnas.071043098
"Medial prefrontal cortex (MPFC) is among those brain regions having the highest baseline metabolic activity at rest and one that exhibits decreases from this baseline across a wide variety of goal-directed behaviors in functional imaging studies. This high metabolic rate and this behavior suggest the existence of an organized mode of default brain function, elements of which may be either attenuated or enhanced. Extant data suggest that these MPFC regions may contribute to the neural instantiation of aspects of the multifaceted "self." We explore this important concept by targeting and manipulating elements of MPFC default state activity. In this functional magnetic resonance imaging (fMRI) study, subjects made two judgments, one self-referential, the other not, in response to affectively normed pictures: pleasant vs. unpleasant (an internally cued condition, ICC) and indoors vs. outdoors (an externally cued condition, ECC). The ICC was preferentially associated with activity increases along the dorsal MPFC. These increases were accompanied by decreases in both active task conditions in ventral MPFC. These results support the view that dorsal and ventral MPFC are differentially influenced by attentiondemanding tasks and explicitly self-referential tasks. The presence of self-referential mental activity appears to be associated with increases from the baseline in dorsal MPFC. Reductions in ventral MPFC occurred consistent with the fact that attention-demanding tasks attenuate emotional processing. We posit that both self-referential mental activity and emotional processing represent elements of the default state as represented by activity in MPFC. We suggest that a useful way to explore the neurobiology of the self is to explore the nature of default state activity." [Full Text]

Zysset S, Huber O, Samson A, Ferstl EC, von Cramon DY.
Functional specialization within the anterior medial prefrontal cortex: a functional magnetic resonance imaging study with human subjects.
Neurosci Lett. 2003 Jan 2;335(3):183-6.
"This study investigated the functional neuroanatomy of the anterior medial prefrontal cortex (aMPFC). Previous studies have shown that the aMPFC is involved in evaluative judgment and self-referential processes. Specifically, different sections of the aMPFC are differentially influenced by attention demanding processes. Whereas the dorsal section is supposed to be involved in self-referential processes, the ventral section is assumed to be attenuated during attention demanding processes. The present study investigates the involvement of the dorsal and ventral aMPFC in evaluative judgment by using functional magnetic resonance imaging with spin-echo echo-planar-imaging. Processes involved in evaluative judgment are attention-demanding, self-referential and activate regions in the dorsal and ventral section of the aMPFC. Attention demanding tasks do not necessarily lead to an attenuation of the ventral section of the aMPFC, a region mainly involved in emotional and affective processing." [Abstract]

Reinders AA, Nijenhuis ER, Paans AM, Korf J, Willemsen AT, den Boer JA.
One brain, two selves.
Neuroimage. 2003 Dec;20(4):2119-25.
"Having a sense of self is an explicit and high-level functional specialization of the human brain. The anatomical localization of self-awareness and the brain mechanisms involved in consciousness were investigated by functional neuroimaging different emotional mental states of core consciousness in patients with Multiple Personality Disorder (i.e., Dissociative Identity Disorder (DID)). We demonstrate specific changes in localized brain activity consistent with their ability to generate at least two distinct mental states of self-awareness, each with its own access to autobiographical trauma-related memory. Our findings reveal the existence of different regional cerebral blood flow patterns for different senses of self. We present evidence for the medial prefrontal cortex (MPFC) and the posterior associative cortices to have an integral role in conscious experience." [Abstract]

Sterling C. Johnson, Leslie C. Baxter, Lana S. Wilder, James G. Pipe, Joseph E. Heiserman, and George P. Prigatano
Neural correlates of self-reflection

Brain 125: 1808-1814, 2002.
"The capacity to reflect on one’s sense of self is an important component of self-awareness. In this paper, we investigate some of the neurocognitive processes underlying reflection on the self using functional MRI. Eleven healthy volunteers were scanned with echoplanar imaging using the blood oxygen level-dependent contrast method. The task consisted of aurally delivered statements requiring a yes–no decision. In the experimental condition, participants responded to a variety of statements requiring knowledge of and reflection on their own abilities, traits and attitudes (e.g. ‘I forget important things’, ‘I’m a good friend’, ‘I have a quick temper’). In the control condition, participants responded to statements requiring a basic level of semantic knowledge (e.g. ‘Ten seconds is more than a minute’, ‘You need water to live’). The latter condition was intended to control for auditory comprehension, attentional demands, decision-making, the motoric response, and any common retrieval processes. Individual analyses revealed consistent anterior medial prefrontal and posterior cingulate activation for all participants. The overall activity for the group, using a random-effects model, occurred in anterior medial prefrontal cortex (t = 13.0, corrected P = 0.05; x, y, z, 0, 54, 8, respectively) and the posterior cingulate (t = 14.7, P = 0.02; x, y, z, –2, –62, 32, respectively; 967 voxel extent). These data are consistent with lesion studies of impaired awareness, and suggest that the medial prefrontal and posterior cingulate cortex are part of a neural system subserving self-reflective thought." [Abstract]

Wicker B, Ruby P, Royet JP, Fonlupt P.
A relation between rest and the self in the brain?
Brain Res Brain Res Rev. 2003 Oct;43(2):224-30.
"Neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are widely used to identify the cerebral correlates of cognitive tasks. The resting state presents the advantage to serve as a reference in all experiments but is also an ill-defined mental state because it may vary both from one subject to another and within the same subject. The most challenging question concerns the areas whose activity (revealed by PET or fMRI imaging) is greater in rest state than in an active condition. The present work reports the result of a meta-analysis including five previously published studies. The five different tasks involved are the following: attribution of intention, judgement of stimulus pleasantness, discrimination of spatial attributes, judgement of other peoples' belief and perception of gaze. For each study, the general linear model was used to assess statistical difference and a contrast resting state minus other conditions was calculated. The intersection of the five contrasts was used to search for the variation jointly observed across the different experiments. This lead to a reduced number of clusters: one cluster in the lower/anterior part of the cingulate gyrus and four clusters located in the medial/superior frontal gyrus, along the superior frontal sulcus. We discuss the location of these areas with respect to the location of activations induced by different tasks: externally focused attention, memory, general reasoning, theory of mind and self-referential tasks. We observed that medial prefrontal cortex exhibits a lower activity when the subject's attention is focused towards the external world than when the subject has to additionally refer to some internal states. By contrast, this activity is greater during resting state than during both externally directed and internally directed attention. Thus, we hypothesize that during rest, the subject is in a state where he refers only to his own self." [Abstract]

Vogeley K, Fink GR.
Neural correlates of the first-person-perspective.
Trends Cogn Sci 2003 Jan;7(1):38-42
"Human self-consciousness depends on the metarepresentation of mental and bodily states as one's own mental and bodily states. First-person-perspective taking is not sufficient, but necessary for human self-consciousness. To assign a first-person-perspective is to center one's own multimodal experiential space upon one's own body, thus operating in an egocentric reference frame. The brain regions involved in assigning first-person-perspective comprise medial prefrontal, medial parietal and lateral temporoparietal cortex. These empirical findings complement recent neurobiologically oriented theories of self-consciousness which focus on the relation between the subject and his/her environment by supplying a neural basis for its key components." [Abstract]

Frith C.
Attention to action and awareness of other minds.
Conscious Cogn. 2002 Dec;11(4):481-7.
"We have only limited awareness of the system by which we control our actions and this limited awareness does not seem to be concerned with the control of action. Awareness of choosing one action rather than another comes after the choice has been made, while awareness of initiating an action occurs before the movement has begun. These temporal differences bind together in consciousness the intention to act and the consequences of the action. This creates our sense of agency. Activity in the anterior cingulate cortex and medial prefrontal cortex is associated with awareness of our own actions and also occurs when we think about the actions of others. I propose that the mechanism underlying awareness of how our own intentions lead to actions can also be used to represent the intentions that underlie the actions of others. This common system enables us to communicate mental states and thereby share our experiences." [Abstract]

Frith CD, Frith U.
Interacting minds--a biological basis.
Science. 1999 Nov 26;286(5445):1692-5.
"The ability to "mentalize," that is to understand and manipulate other people's behavior in terms of their mental states, is a major ingredient in successful social interactions. A rudimentary form of this ability may be seen in great apes, but in humans it is developed to a high level. Specific impairments of mentalizing in both developmental and acquired disorders suggest that this ability depends on a dedicated and circumscribed brain system. Functional imaging studies implicate medial prefrontal cortex and posterior superior temporal sulcus (STS) as components of this system. Clues to the specific function of these components in mentalizing come from single cell recording studies: STS is concerned with representing the actions of others through the detection of biological motion; medial prefrontal regions are concerned with explicit representation of states of the self. These observations suggest that the ability to mentalize has evolved from a system for representing actions." [Abstract]

Kelley WM, Macrae CN, Wyland CL, Caglar S, Inati S, Heatherton TF.
Finding the self? An event-related fMRI study.
J Cogn Neurosci. 2002 Jul 1;14(5):785-94.
"Researchers have long debated whether knowledge about the self is unique in terms of its functional anatomic representation within the human brain. In the context of memory function, knowledge about the self is typically remembered better than other types of semantic information. But why does this memorial effect emerge? Extending previous research on this topic (see Craik et al., 1999), the present study used event-related functional magnetic resonance imaging to investigate potential neural substrates of self-referential processing. Participants were imaged while making judgments about trait adjectives under three experimental conditions (self-relevance, other-relevance, or case judgment). Relevance judgments, when compared to case judgments, were accompanied by activation of the left inferior frontal cortex and the anterior cingulate. A separate region of the medial prefrontal cortex was selectively engaged during self-referential processing. Collectively, these findings suggest that self-referential processing is functionally dissociable from other forms of semantic processing within the human brain." [Abstract]

Johnson SC, Baxter LC, Wilder LS, Pipe JG, Heiserman JE, Prigatano GP.
Neural correlates of self-reflection.
Brain. 2002 Aug;125(Pt 8):1808-14.
"The capacity to reflect on one's sense of self is an important component of self-awareness. In this paper, we investigate some of the neurocognitive processes underlying reflection on the self using functional MRI. Eleven healthy volunteers were scanned with echoplanar imaging using the blood oxygen level-dependent contrast method. The task consisted of aurally delivered statements requiring a yes-no decision. In the experimental condition, participants responded to a variety of statements requiring knowledge of and reflection on their own abilities, traits and attitudes (e.g. 'I forget important things', 'I'm a good friend', 'I have a quick temper'). In the control condition, participants responded to statements requiring a basic level of semantic knowledge (e.g. 'Ten seconds is more than a minute', 'You need water to live'). The latter condition was intended to control for auditory comprehension, attentional demands, decision-making, the motoric response, and any common retrieval processes. Individual analyses revealed consistent anterior medial prefrontal and posterior cingulate activation for all participants. The overall activity for the group, using a random-effects model, occurred in anterior medial prefrontal cortex (t = 13.0, corrected P = 0.05; x, y, z, 0, 54, 8, respectively) and the posterior cingulate (t = 14.7, P = 0.02; x, y, z, -2, -62, 32, respectively; 967 voxel extent). These data are consistent with lesion studies of impaired awareness, and suggest that the medial prefrontal and posterior cingulate cortex are part of a neural system subserving self-reflective thought." [Abstract]

Maguire EA, Mummery CJ.
Differential modulation of a common memory retrieval network revealed by positron emission tomography.
Hippocampus. 1999;9(1):54-61.
"Functional neuroimaging is uniquely placed to examine the dynamic nature of normal human memory, the distributed brain networks that support it, and how they are modulated. Memory has traditionally been classified into context-specific memories personally experienced ("episodic memory") and impersonal non-context-specific memories ("semantic memory"). However, we suggest that another useful distinction is whether events are personally relevant or not. Typically the factors of personal relevance and temporal context are confounded, and it is as yet not clear the precise influence of either on how memories are stored or retrieved. Here we focus on the retrieval of real-world memories unconfounding personal relevance and temporal context during positron emission tomography (PET) scanning. Memories differed along two dimensions: They were personally relevant (or not) and had temporal specificity (or not). Recollection of each of the resultant four memory subtypes-autobiographical events, public events, autobiographical facts, and general knowledge-was associated with activation of a common network of brain regions. Within this system, however, enhanced activity was observed for retrieval of personally relevant, time-specific memories in left hippocampus, medial prefrontal cortex, and left temporal pole. Bilateral temporoparietal junctions were activated preferentially for personal memories, regardless of time specificity. Finally, left parahippocampal gyrus, left anterolateral temporal cortex, and posterior cingulate cortex were involved in memory retrieval irrespective of person or time. Our findings suggest that specializations in memory retrieval result from associations between subsets of regions within a common network. We believe that these findings throw new light on an old debate surrounding episodic and declarative theories of memory and the precise involvement of the hippocampus." [Abstract]

Bernard J. Baars
Understanding Subjectivity: Global Workspace Theory and the Resurrection of the Observing Self
Journal of Consciousness Studies, 3, No. 3, 1996, pp. 211-16
"Why is the problem of subjectivity so hard, as David Chalmers claims? This essay suggests that it becomes hard when we adopt an implausible, perfectionistic standard. In the last two decades the standard has come to be 'observer empathy' -- the ability to know what it's like to be a bat or another human. That makes understanding consciousness difficult indeed. Far more practical criteria are used every day in medicine and scientific studies of consciousness, and indeed traditional philosophy from Kant to James took a much more relaxed view of subjectivity. Once we adopt these more workable standards, subjectivity is suddenly revealed to involve a familiar concept, namely 'the self as observer' of conscious experiences. Contrary to some, this sense of self is conceptually coherent and well-supported by hard evidence. For example, the 'left-hemisphere interpreter' in split-brain patients behaves as one such self. Given a modest and practical approach, we can expect to make progress toward understanding subjectivity." [Full Text]

Adam Zeman
Brain 124: 1263-1289, 2001.
"Consciousness is topical, for reasons including its renewed respectability among psychologists, rapid progress in the neuroscience of perception, memory and action, advances in artificial intelligence and dissatisfaction with the dualistic separation of mind and body. Consciousness is an ambiguous term. It can refer to (i) the waking state; (ii) experience; and (iii) the possession of any mental state. Self-consciousness is equally ambiguous, with senses including (i) proneness to embarrassment in social settings; (ii) the ability to detect our own sensations and recall our recent actions; (iii) self-recognition; (iv) the awareness of awareness; and (v) self-knowledge in the broadest sense. The understanding of states of consciousness has been transformed by the delineation of their electrical correlates, of structures in brainstem and diencephalon which regulate the sleep–wake cycle, and of these structures' cellular physiology and regional pharmacology. Clinical studies have defined pathologies of wakefulness: coma, the persistent vegetative state, the `locked-in' syndrome, akinetic mutism and brain death. Interest in the neural basis of perceptual awareness has focused on vision. Increasingly detailed neuronal correlates of real and illusory visual experience are being defined. Experiments exploiting circumstances in which visual experience changes while external stimulation is held constant are tightening the experimental link between consciousness and its neural correlates. Work on unconscious neural processes provides a complementary approach. `Unperceived' stimuli have detectable effects on neural events and subsequent action in a range of circumstances: blindsight provides the classical example. Other areas of cognitive neuroscience also promise experimental insights into consciousness, in particular the distinctions between implicit and explicit memory and deliberate and automatic action. Overarching scientific theories of consciousness include neurobiological accounts which specify anatomical or physiological mechanisms for awareness, theories focusing on the role played by conscious processes in information processing and theories envisaging the functions of consciousness in a social context. Whether scientific observation and theory will yield a complete account of consciousness remains a live issue. Physicalism, functionalism, property dualism and dual aspect theories attempt to do justice to three central, but controversial, intuitions about experience: that it is a robust phenomenon which calls for explanation, that it is intimately related to the activity of the brain and that it has an important influence on behaviour." [Full Text]

Dennett D.
Are we explaining consciousness yet?
Cognition 2001 Apr;79(1-2):221-37
"Theorists are converging from quite different quarters on a version of the global neuronal workspace model of consciousness, but there are residual confusions to be dissolved. In particular, theorists must resist the temptation to see global accessibility as the cause of consciousness (as if consciousness were some other, further condition); rather, it is consciousness. A useful metaphor for keeping this elusive idea in focus is that consciousness is rather like fame in the brain. It is not a privileged medium of representation, or an added property some states have; it is the very mutual accessibility that gives some informational states the powers that come with a subject's consciousness of that information. Like fame, consciousness is not a momentary condition, or a purely dispositional state, but rather a matter of actual influence over time. Theorists who take on the task of accounting for the aftermath that is critical for consciousness often appear to be leaving out the Subject of consciousness, when in fact they are providing an analysis of the Subject, a necessary component in any serious theory of consciousness." [Abstract] [Full Text]

Publications by Daniel Dennett

Searle JR.
How to study consciousness scientifically.
Philos Trans R Soc Lond B Biol Sci 1998 Nov 29;353(1377):1935-42
"The neurosciences have advanced to the point that we can now treat consciousness as a scientific problem like any other. The problem is to explain how brain processes cause consciousness and how consciousness is realized in the brain. Progress is impeded by a number of philosophical mistakes, and the aim of this paper is to remove nine of those mistakes: (i) consciousness cannot be defined; (ii) consciousness is subjective but science is objective; (iii) brain processes cannot explain consciousness; (iv) the problem of 'qualia' should be set aside; (v) consciousness is epiphenomenal; (vi) consciousness has no evolutionary function; (vii) a causal account of consciousness is necessarily dualistic; (viii) science is reductionistic, so a scientific account of consciousness would show it reducible to something else; and (ix) an account of consciousness must be an information processing account." [Full Text]

Searle, John R.
Annu. Rev. Neurosci. 2000 23: 557-578
"Until recently, most neuroscientists did not regard consciousness as a suitable topic for scientific investigation. This reluctance was based on certain philosophical mistakes, primarily the mistake of supposing that the subjectivity of consciousness made it beyond the reach of an objective science. Once we see that consciousness is a biological phenomenon like any other, then it can be investigated neurobiologically. Consciousness is entirely caused by neurobiological processes and is realized in brain structures. The essential trait of consciousness that we need to explain is unified qualitative subjectivity. Consciousness thus differs from other biological phenomena in that it has a subjective or first-person ontology, but this subjective ontology does not prevent us from having an epistemically objective science of consciousness. We need to overcome the philosophical tradition that treats the mental and the physical as two distinct metaphysical realms. Two common approaches to consciousness are those that adopt the building block model, according to which any conscious field is made of its various parts, and the unified field model, according to which we should try to explain the unified character of subjective states of consciousness. These two approaches are discussed and reasons are given for preferring the unified field theory to the building block model. Some relevant research on consciousness involves the subjects of blindsight, the split-brain experiments, binocular rivalry, and gestalt switching." [Abstract]

David J. Chalmers
On the Search for the Neural Correlate of Consciousness
Toward a Science of Consciousness II: The Second Tucson Discussions and Debates (S. Hameroff, A. Kaszniak, and A.Scott, eds), published with MIT Press in 1998
"Once one recognizes the central role that pre-experimental assumptions play in the search for the NCC, one realizes that there are some limitations on just what we can expect this search to tell us. Still, whether or not the NCC is the Holy Grail, I hope that I have said enough to make it clear that the quest for it is likely to enhance our understanding considerably. And I hope to have convinced you that there are important ways in which philosophy and neuroscience can come together to help clarify some of the deep problems involved in the study of consciousness." [Full Text]

David J. Chalmers
What is a Neural Correlate of Consciousness?
Neural Correlates of Consciousness: Empirical and Conceptual Questions (T. Metzinger, ed), published with MIT Press in 2000
"The search for neural correlates of consciousness (or NCCs) is arguably the cornerstone in the recent resurgence of the science of consciousness. The search poses many difficult empirical problems, but it seems to be tractable in principle, and some ingenious studies in recent years have led to considerable progress. A number of proposals have been put forward concerning the nature and location of neural correlates of consciousness." [Full Text]

Online papers on consciousness, part 3: Science of consciousness
Compiled by David Chalmers

Consciousness and the Brain
Annotated Biography

Compiled by Ralph D. Ellis and Natika Newton

Selected Bibliography
1970 - 2003
Compiled by Thomas Metzinger

Science And Consciousness Review

Crick F, Koch C.
Consciousness and Neuroscience
Cerebral Cortex, 8:97-107, 1998
"The explanation of consciousness is one of the major unsolved problems of modern science. After several thousand years of speculation, it would be very gratifying to find an answer to it." [Full Text]

Perner J, Dienes Z.
Developmental aspects of consciousness: How much theory of mind do you need to be consciously aware?
Conscious Cogn 2003 Mar;12(1):63-82
"When do children become consciously aware of events in the world? Five possible strategies are considered for their usefulness in determining the age in question. Three of these strategies ask when children show signs of engaging in activities for which conscious awareness seems necessary in adults (verbal communication, executive control, explicit memory), and two of the strategies consider when children have the ability to have the minimal form of higher-order thought necessary for access consciousness and phenomenal consciousness, respectively. The tentative answer to the guiding question is that children become consciously aware between 12 and 15 months (+/-3 months)." [Abstract]

Cooney JW, Gazzaniga MS.
Neurological disorders and the structure of human consciousness.
Trends Cogn Sci 2003 Apr;7(4):161-165
"Recent studies that identify distinct neural correlates of perceptual awareness offer a promising step towards improved understanding of the neurological underpinnings of conscious experience. Such studies indicate that perceptual awareness is modular in nature, with neural correlates of awareness consisting of the specialized structures involved in perceptual processing. However, the integrative, multimodal nature of conscious experience appears to require a functional architecture that overcomes this modular segregation of function. We propose a model in which experience emerges from the dynamic interactions of specialized component processes via a distributed neural network. Such a model offers a mechanism to explain several empirical observations of the neural correlates of perceptual awareness, cognitive function, and symptoms of neurological damage." [Abstract]

Taylor JG.
The central role of the parietal lobes in consciousness.
Conscious Cogn 2001 Sep;10(3):379-417
"There are now various approaches to understand where and how in the brain consciousness arises from neural activity, none of which is universally accepted. Difficulties among these approaches are reviewed, and a missing ingredient is proposed here to help adjudicate between them, that of "perspectivalness." In addition to a suitable temporal duration and information content of the relevant bound brain activity, this extra component is posited as being a further important ingredient for the creation of consciousness from neural activity. It guides the development of what is termed the "Central Representation," which is supposed to be present in all mammals and extended in humans to support self-consciousness as well as phenomenal consciousness. Experimental evidence and a theoretical framework for the existence of the central representation are presented, which relates the extra component to specific buffer working memory sites in the inferior parietal lobes, acting as attentional coordinators on the spatial maps making up the central representation. The article closes with a discussion of various open questions." [Abstract]

J.G. Taylor
Paying Attention to Consciousness
Trends in Cognitive Sciences Vol. 6 No.5 May 2002
"Despite being much studied by cognitive neuroscience, consciousness has resisted attempts to understand it. Recent neuroscientific papers on the problem have surprisingly neglected attention as a guide to consciousness. A new neural mechanism is proposed here, guided by a control approach to attention, which identifies the source of consciousness, especially that of the ownership of experience." [PDF]

J.G. Taylor
From Matter To Mind
Journal of Consciousness Studies 2002 Apr;9(4):3-22
"The relation between mind and matter is considered in terms of recent ideas from both phenomenology and brain science. Phenomenology is used to give clues to help bridge the brain–mind gap by providing constraints on any underlying neural architecture suggested from brain science. A tentative reduction of mind to matter is suggested and used to explain various features of phenomenological experience and of ownership of conscious experience. The crucial mechanism is the extended duration of the corollary discharge of attention movement, with its gating of activity for related content. Aspects of experience considered in terms of the model are the discontinuous nature of consciousness, immunity to error through misidentification, and the state of ‘pure’ consciousness as experienced through meditation. Corollary discharge of attention movement is proposed as the key idea bringing together basic features of meditation, consciousness and neuroscience, and helping to bridge the gap between mind and matter." [Abstract]

Journal of Consciousness Studies

Rosenthal DM.
How many kinds of consciousness?
Conscious Cogn 2002 Dec;11(4):653-65
"Ned Block's influential distinction between phenomenal and access consciousness has become a staple of current discussions of consciousness. It is not often noted, however, that his distinction tacitly embodies unargued theoretical assumptions that favor some theoretical treatments at the expense of others. This is equally so for his less widely discussed distinction between phenomenal consciousness and what he calls reflexive consciousness. I argue that the distinction between phenomenal and access consciousness, as Block draws it, is untenable. Though mental states that have qualitative character plainly differ from those with no mental qualities, a mental state's being conscious is the same property for both kinds of mental state. For one thing, as Block describes access consciousness, that notion does not pick out any property that we intuitively count as a mental state's being conscious. But the deeper problem is that Block's notion of phenomenal consciousness, or phenomenality, is ambiguous as between two very different mental properties. The failure to distinguish these results in the begging of important theoretical questions. Once the two kinds of phenomenality have been distinguished, the way is clear to explain qualitative consciousness by appeal to a model such as the higher-order-thought hypothesis." [Abstract]

Faw B.
Pre-frontal executive committee for perception, working memory, attention, long-term memory, motor control, and thinking: A tutorial review.
Conscious Cogn 2003 Mar;12(1):83-139
"As an explicit organizing metaphor, memory aid, and conceptual framework, the prefrontal cortex may be viewed as a five-member 'Executive Committee,' as the prefrontal-control extensions of five sub-and-posterior-cortical systems: (1) the 'Perceiver' (dominant-right-hemisphere ventral-lateral prefrontal cortex-VL/PERC-PFC) is the frontal extension of the ventral perceptual stream (the VL/PERC system) which represents the world and self in object coordinates; (2) the 'Verbalizer' (dominant-left-hemisphere ventral-lateral prefrontal cortex system-VL/VERB-PFC) is the frontal extension of the language stream (the VL/VERB system) which represents the world and self in language coordinates; (3) the 'Motivator' (ventral/medial-orbital pre-frontal cortex-VMO-PFC) is the frontal cortical extension of a subcortical extended-amygdala stream (the VMO system) which represents the world and self in motivational/emotional coordinates; (4) the 'Attender' (dorsal-medial/anterior cingulate-DM/AC-PFC) is the frontal cortical extension of a subcortical extended-hippocampal stream (the DM/AC system) which represents the world and self in spatiotemporal coordinates and directs attention to internal and external events; and (5) the 'Coordinator' (the dorsolateral prefrontal cortex-DL-PFC) is the frontal extension of the dorsal perceptual stream (the DL system) which represents the world and self in body- and eye-coordinates and controls willed action and working memory. This tutorial review examines the interacting roles of these five systems in perception, working memory, attention, long-term memory, motor control, and thinking." [Abstract]

M. Steriade
Impact of Network Activities on Neuronal Properties in Corticothalamic Systems
J Neurophysiol 86: 1-39, 2001.
"Data from in vivo and in vitro experiments are discussed to emphasize that synaptic activities in neocortex and thalamus have a decisive impact on intrinsic neuronal properties in intact-brain preparations under anesthesia and even more so during natural states of vigilance. Thus the firing patterns of cortical neuronal types are not inflexible but may change with the level of membrane potential and during periods rich in synaptic activity. The incidences of some cortical cell classes (defined by their responses to depolarizing current pulses) are different in isolated cortical slabs in vivo or in slices maintained in vitro compared with the intact cortex of naturally awake animals. Network activities, which include the actions of generalized modulatory systems, have a profound influence on the membrane potential, apparent input resistance, and backpropagation of action potentials. The analysis of various oscillatory types leads to the conclusion that in the intact brain, there are no "pure" rhythms, generated in simple circuits, but complex wave sequences (consisting of different, low- and fast-frequency oscillations) that result from synaptic interactions in corticocortical and corticothalamic neuronal loops under the control of activating systems arising in the brain stem core or forebrain structures. As an illustration, it is shown that the neocortex governs the synchronization of network or intrinsically generated oscillations in the thalamus. The rhythmic recurrence of spike bursts and spike trains fired by thalamic and cortical neurons during states of decreased vigilance may lead to plasticity processes in neocortical neurons. If these phenomena, which may contribute to the consolidation of memory traces, are not constrained by inhibitory processes, they induce seizures in which the neocortex initiates the paroxysms and controls their thalamic reflection. The results indicate that intact-brain preparations are necessary to investigate global brain functions such as behavioral states of vigilance and paroxysmal activities." [Full Text]

Steriade M.
Corticothalamic resonance, states of vigilance and mentation.
Neuroscience 2000;101(2):243-76
"During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus. The coherent activity in corticothalamic networks, under the control of brainstem and forebrain modulatory systems, requires investigations in intact-brain animals. During behavioral states associated with brain disconnection from the external world, the large-scale synchronization of low-frequency oscillations is accompanied by the inhibition of synaptic transmission through thalamocortical neurons. Despite the coherent oscillatory activity, on the functional side there is dissociation between the thalamus and neocortex during slow-wave sleep. While dorsal thalamic neurons undergo inhibitory processes due to the prolonged spike-bursts of thalamic reticular neurons, the cortex displays, periodically, a rich spontaneous activity and preserves the capacity to process internally generated signals that dominate the state of sleep. In vivo experiments using simultaneous intracellular recordings from thalamic and cortical neurons show that short-term plasticity processes occur after prolonged and rhythmic spike-bursts fired by thalamic and cortical neurons during slow-wave sleep oscillations. This may serve to support resonant phenomena and reorganize corticothalamic circuitry, determine which synaptic modifications, formed during the waking state, are to be consolidated and generate a peculiar kind of dreaming mentation. In contrast to the long-range coherent oscillations that occur at low frequencies during slow-wave sleep, the sustained fast oscillations that characterize alert states are synchronized over restricted territories and are associated with discrete and differentiated patterns of conscious events." [Abstract]

Niedermeyer E.
Electrophysiology of the frontal lobe.
Clin Electroencephalogr 2003 Jan;34(1):5-12
"The electrophysiology of the frontal lobe appears to be unimpressive when the view is limited to the routine EEG recording of a healthy waking adult. There is usually low voltage fast activity, which becomes more pronounced when recorded with depth leads. Three special EEG patterns of marginal to slightly abnormal character are discussed: a) rhythmical midfrontal 6-7/sec activity of juveniles, b) rhythmical midfrontal sharp 4-6/sec activity of infancy and early childhood with arousal from sleep, and c) frontal intermittent rhythmical delta activity (FIRDA) in waking adults with frontopolar maximum, possibly related to thought processes under abnormal conditions. With extension of the frequency range, ultraslow (DC-like) as well as fast beta (gamma, 40-80/sec) and ultrafast activity (80-1000/sec) are found particularly over the frontal lobes. Ultraslow baseline shifts are arousal-related and mixed with overlying ultrafast waves. Attention control and the "working memory" involve chiefly the dorsolateral prefrontal cortex, investigated with P300 responses and likely to show ultrafast spectra. Perception-related 40-80/sec gamma activity has been thought to be associated with the entrance into consciousness. Initiation and design of motor activity spreads from prefrontal to the frontomotor cortex, associated with powerful event-related potentials: contingent negative variation (CNV) and "Bereitschafts potential" ("readiness potential," RP). Neuroscientific research of the highest frontal lobe functions has become a very active domain of neuroimaging. With the use of the extended frequency range, EEG and also evoked potential studies could add further information with acquisition in real time. Ultrafast frequency ranges presented in computerized frequency analysis and mapping might show impressive correlates of highest frontal lobe functions." [Abstract]

McAlonan K, Brown VJ.
The thalamic reticular nucleus: more than a sensory nucleus?
Neuroscientist 2002 Aug;8(4):302-5
"Sensory information is routed to the cortex via the thalamus, but despite this sensory bombardment, animals must attend selectively to stimuli that signal danger or opportunity. Sensory input must be filtered, allowing only behaviorally relevant information to capture limited attentional resources. Located between the thalamus and cortex is a thin lamina of neurons called the thalamic reticular nucleus (Rt). The thalamic reticular nucleus projects exclusively to thalamus, thus forming an essential component of the circuitry mediating sensory transmission. This article presents evidence supporting a role for Rt beyond the mere relay of sensory information. Rather than operating as a component of the sensory relay, the authors suggest that Rt represents an inhibitory interface or "attentional gate," which regulates the flow of information between the thalamus and cortex. Recent findings have also implicated Rt in higher cognitive functions, including learning, memory, and spatial cognition. Drawing from recent insights into the dynamic nature of the thalamic relay in awake, behaving animals, the authors present a speculative account of how Rt might regulate thalamocortical transmission and ultimately the contents of consciousness." [Abstract]

Llinas R, Ribary U, Contreras D, Pedroarena C.
The neuronal basis for consciousness.
Philos Trans R Soc Lond B Biol Sci 1998 Nov 29;353(1377):1841-9
"Attempting to understand how the brain, as a whole, might be organized seems, for the first time, to be a serious topic of inquiry. One aspect of its neuronal organization that seems particularly central to global function is the rich thalamocortical interconnectivity, and most particularly the reciprocal nature of the thalamocortical neuronal loop function. Moreover, the interaction between the specific and non-specific thalamic loops suggests that rather than a gate into the brain, the thalamus represents a hub from which any site in the cortex can communicate with any other such site or sites. The goal of this paper is to explore the basic assumption that large-scale, temporal coincidence of specific and non-specific thalamic activity generates the functional states that characterize human cognition." [Abstract] [Full Text]

Jones EG.
Thalamic circuitry and thalamocortical synchrony.
Philos Trans R Soc Lond B Biol Sci 2002 Dec;357(1428):1659-73
"The corticothalamic system has an important role in synchronizing the activities of thalamic and cortical neurons. Numerically, its synapses dominate the inputs to relay cells and to the gamma-amino butyric acid (GABA)ergic cells of the reticular nucleus (RTN). The capacity of relay neurons to operate in different voltage-dependent functional modes determines that the inputs from the cortex have the capacity directly to excite the relay cells, or indirectly to inhibit them via the RTN, serving to synchronize high- or low-frequency oscillatory activity respectively in the thalamocorticothalamic network. Differences in the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subunit composition of receptors at synapses formed by branches of the same corticothalamic axon in the RTN and dorsal thalamus are an important element in the capacity of the cortex to synchronize low-frequency oscillations in the network. Interactions of focused corticothalamic axons arising from layer VI cortical cells and diffuse corticothalamic axons arising from layer V cortical cells, with the specifically projecting core relay cells and diffusely projecting matrix cells of the dorsal thalamus, form a substrate for synchronization of widespread populations of cortical and thalamic cells during high-frequency oscillations that underlie discrete conscious events." [Abstract]

Balkin TJ, Braun AR, Wesensten NJ, Jeffries K, Varga M, Baldwin P, Belenky G, Herscovitch P.
The process of awakening: a PET study of regional brain activity patterns mediating the re-establishment of alertness and consciousness.
Brain 2002 Oct;125(Pt 10):2308-19
"Awakening from sleep entails rapid re-establishment of consciousness followed by the relatively slow (20-30 min later) re-establishment of alertness--a temporal dissociation that facilitates specification of the physiological underpinnings of each of these facets of the awakening process. H(2)(15)O PET was used to assess changes in regional cerebral blood flow (rCBF) upon awakening from stage 2 sleep. Cerebral blood flow (CBF) was most rapidly re-established in centrencephalic regions (e.g. brainstem and thalamus), suggesting that the reactivation of these regions underlies the re-establishment of conscious awareness. Across the ensuing 15 min of wakefulness, further increases in CBF were evident primarily in anterior cortical regions, suggesting that the dissipation of sleep inertia effects (post-awakening performance and alertness deficits) is effected by reactivation of these regions. Concomitant shifts in correlation patterns of regional brain activity across the post-awakening period [in particular, a waning negative correlation between prefrontal cortex and mesencephalic reticular formation (RF) activity, and a waxing positive correlation between prefrontal cortex and ventromedial caudate nucleus (CAUD) activity] suggest that the post-awakening reversal of sleep inertia effects may be mediated by more than mere reactivation--it may also involve the functional reorganization of brain activity. Conversely, stable post-awakening correlations--such as those found between the anterior cingulate cortex (ACC) and most other brain regions--may denote the pattern of functional connectivity that underlies consciousness itself." [Abstract]

Steriade M.
The corticothalamic system in sleep.
Front Biosci 2003 May 1;8:D878-99
"The transition from wakefulness to NREM sleep is associated with typical signs of brain electrical activity, characterized by prolonged periods of hyperpolarization and increased membrane conductance in thalamocortical (TC) neurons, with the consequence that incoming messages are inhibited and the cerebral cortex is deprived of signals from the outside world. There are three major oscillations during NREM sleep. Spindles are generated within the thalamus, due to thalamic reticular (RE) neurons that impose rhythmic inhibitory sequences onto TC neurons, but the widespread synchronization of this rhythm is governed by corticothalamic projections. There are two types of delta activity: clock-like waves generated in TC neurons by the interplay between two hyperpolarization-activated inward currents; and cortical waves that survive extensive thalamectomy. The hallmark of NREM sleep activity is the slow oscillation, generated intracortically, which has the virtue of grouping the other types of sleep activities, thus leading to a coalescence of different rhythms that can only be observed in intact-brain animals and humans. Far from being epiphenomena, with no functional role, NREM sleep oscillations, particularly spindles and their experimental model augmenting responses, produce synaptic plasticity in target cortical neurons and resonant activity in corticothalamic loops, as in "memory" processes. Upon brain arousal, spindles are blocked by inhibition of RE neurons, the spindles' pacemakers; clock-like delta rhythm is obliterated by depolarization of TC neurons; and the cortically generated slow oscillation is abolished by selective erasure of its hyperpolarizing components. Fast (beta and gamma) oscillations are roduced by the depolarizing effects of mesopontine cholinergic neurons acting on TC neurons and nucleus basalis neurons acting on cortical neurons." [Abstract]

Alkire MT, Haier RJ, Fallon JH.
Toward a unified theory of narcosis: brain imaging evidence for a thalamocortical switch as the neurophysiologic basis of anesthetic-induced unconsciousness.
Conscious Cogn 2000 Sep;9(3):370-86
"A unifying theory of general anesthetic-induced unconsciousness must explain the common mechanism through which various anesthetic agents produce unconsciousness. Functional-brain-imaging data obtained from 11 volunteers during general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used volatile agents. These findings are discussed in relation to findings from sleep neurophysiology and the implications of this work for consciousness research. It is hypothesized that the essential common neurophysiologic mechanism underlying anesthetic-induced unconsciousness is, as with sleep-induced unconsciousness, a hyperpolarization block of thalamocortical neurons. A model of anesthetic-induced unconsciousness is introduced to explain how the plethora of effects anesthetics have on cellular functioning ultimately all converge on a single neuroanatomic/neurophysiologic system, thus providing for a unitary physiologic theory of narcosis related to consciousness." [Abstract]

Laureys S, Antoine S, Boly M, Elincx S, Faymonville ME, Berre J, Sadzot B, Ferring M, De Tiege X, van Bogaert P, Hansen I, Damas P, Mavroudakis N, Lambermont B, Del Fiore G, Aerts J, Degueldre C, Phillips C, Franck G, Vincent JL, Lamy M, Luxen A, Moonen G, Goldman S, Maquet P.
Brain function in the vegetative state.
Acta Neurol Belg 2002 Dec;102(4):177-85
"Positron emission tomography (PET) techniques represent a useful tool to better understand the residual brain function in vegetative state patients. It has been shown that overall cerebral metabolic rates for glucose are massively reduced in this condition. However, the recovery of consciousness from vegetative state is not always associated with substantial changes in global metabolism. This finding led us to hypothesize that some vegetative patients are unconscious not just because of a global loss of neuronal function, but rather due to an altered activity in some critical brain regions and to the abolished functional connections between them. We used voxel-based Statistical Parametric Mapping (SPM) approaches to characterize the functional neuroanatomy of the vegetative state. The most dysfunctional brain regions were bilateral frontal and parieto-temporal associative cortices. Despite the metabolic impairment, external stimulation still induced a significant neuronal activation (i.e., change in blood flow) in vegetative patients as shown by both auditory click stimuli and noxious somatosensory stimuli. However, this activation was limited to primary cortices and dissociated from higher-order associative cortices, thought to be necessary for conscious perception. Finally, we demonstrated that vegetative patients have impaired functional connections between distant cortical areas and between the thalami and the cortex and, more importantly, that recovery of consciousness is paralleled by a restoration of this cortico-thalamo-cortical interaction." [Abstract] [PDF]

Kostopoulos GK.
Involvement of the thalamocortical system in epileptic loss of consciousness.
Epilepsia 2001;42 Suppl 3:13-9
"Experiments on putative neuronal mechanisms underlying absence seizures as well as clinical observations are critically reviewed for their ability to explain apparent "loss of consciousness." It is argued that the initial defect in absences lies with corticothalamic (CT) neuronal mechanisms responsible for selective attention and/or planning for action, rather than with those establishing either the states or the contents of consciousness. Normally, rich thalamocortical (TC)-CT feedback loops regulate the flow of information to the cortex and help its neurons to organize themselves in discrete assemblies, which through high-frequency (>30 Hz) oscillations bind those distributed processes of the brain that are considered important, so that we are able to focus on what is needed from moment to moment and be aware of this fact. This ability is transiently lost in absence seizures, because large numbers of CT loops are recruited for seconds in much stronger, low-frequency ( approximately 3 Hz) oscillations of EPSP/IPSP sequences, which underlie electroencephalographic (EEG) spike-and-wave discharges (SWDs). These oscillations probably result from a transformation of the normal EEG rhythm of sleep spindles on an abnormal increase of cortical excitability that results in strong activation of inhibitory neurons in the cortex and in nucleus reticularis thalami. The strong general enhancement of CT feedback during SWDs may disallow the discrete feedback, which normally selects specific TC circuits for conscious perception and/or motor reaction. Such a mechanism of SWD generation allows variability in the extent to which different TC sectors are engaged in the SWD activity and thus explains the variable ability of some patients to respond during an absence, depending on the sensory modality examined." [Abstract]

Lee KH, Meador KJ, Park YD, King DW, Murro AM, Pillai JJ, Kaminski RJ.
Pathophysiology of altered consciousness during seizures: Subtraction SPECT study.
Neurology 2002 Sep 24;59(6):841-6
"BACKGROUND: The mechanisms underlying altered consciousness during seizures are poorly understood. Previous clinicopathologic studies suggest a role for the thalamus and upper brainstem in consciousness mechanisms. OBJECTIVE: To examine blood flow changes associated with altered consciousness during seizures. METHODS: Seventy-one patients with epilepsy who underwent video-EEG monitoring and ictal/interictal SPECT were studied. Patients were divided into three groups depending on their conscious state during seizures: 1) complete impairment of consciousness (CI), 2) no impairment of consciousness (NI), or 3) uncertain impairment of consciousness (UI). The distribution of blood flow changes during these seizures was assessed by subtraction (ictal - interictal) SPECT co-registered to MRI. Conscious state was assessed in relation to secondary ictal hyperperfusion in subcortical regions (i.e., thalamus and upper brainstem). RESULTS: Impairment of consciousness showed a strong association with secondary hyperperfusion in the thalamic/upper brainstem region (p = 0.01), occurring in 92% (45/49) of CI, 69% (9/13) of UI, and 11% (1/9) of NI. CONCLUSIONS: These findings are consistent with a role for the thalamus and upper brainstem in consciousness mechanisms. The authors suggest that the spread of epileptic discharges or a trans-synaptic activation (diaschisis) of these structures is an important mechanism in the alteration of consciousness during seizures. Variance in the results may be due to differences in timing of radioisotope injection, sensitivity of the subtraction SPECT technique, and the ability to clinically assess the conscious state." [Abstract]

Schiff ND, Plum F.
The role of arousal and "gating" systems in the neurology of impaired consciousness.
J Clin Neurophysiol. 2000 Sep;17(5):438-52.
"A brief taxonomy of neurologic disorders resulting in global impairments of consciousness is presented. Particular emphasis is placed on focal injuries of subcortical structures that may produce disorders that are otherwise associated to large bilateral cortical injuries. A distinction between subcortical arousal and "gating" systems is developed. Both clinical and experimental studies are reviewed in the context of these disorders and their possible underlying mechanisms." [Abstract]

Vertes RP.
Analysis of projections from the medial prefrontal cortex to the thalamus in the rat, with emphasis on nucleus reuniens.
J Comp Neurol. 2002 Jan 7;442(2):163-87.
"The medial prefrontal cortex (mPFC) is involved in high-order cognitive processes, including, but not limited to, decision making, goal directed behavior, and working memory. Although previous reports have included descriptions of mPFC projections to the thalamus in overall examinations of mPFC projections throughout the brain, no previous study has comprehensively examined mPFC projections to the thalamus. The present report compares and contrasts projections from the four divisions of the mPFC, i.e., the infralimbic, prelimbic, anterior cingulate and medial agranular cortices, to the thalamus in the rat by using the anterograde anatomic tracer Phaseolus vulgaris-leucoagglutinin. We showed that (1) the infralimbic, prelimbic, anterior cingulate cortices distribute heavily and selectively to midline/medial structures of the thalamus, including the paratenial, paraventricular, interanteromedial, anteromedial, intermediodorsal, mediodorsal, reuniens, and the central medial nuclei; (2) the medial agranular cortex distributes strongly to the rostral intralaminar nuclei (central lateral, paracentral, central medial nuclei) as well as to the ventromedial and ventrolateral nuclei of thalamus; and (3) all four divisions of the mPFC project densely to the nucleus reuniens (RE) of the thalamus. The nucleus reuniens is the major source of thalamic afferents to the hippocampal formation. There are essentially no direct projections from the mPFC to the hippocampus. The present demonstration of pronounced mPFC projections to RE suggests that the nucleus reuniens is a critical relay in the transfer of information from the medial prefrontal cortex to the hippocampus. Our further demonstration of strong mPFC projections to several additional thalamic nuclei, particularly to the mediodorsal nucleus, suggests that these thalamic nuclei, like RE, represent important output stations (or gateways) for the actions of mPFC on diverse subcortical and cortical structures of the brain." [Abstract]

Scannell, J.W., Burns, G.A.P.C., Hilgetag, C.C., O'Neil, M.A., Young, M.P.
The Connectional Organization of the Cortico-thalamic System of the Cat
Cereb. Cortex 1999 9: 277-299
"Data on connections between the areas of the cerebral cortex and nuclei of the thalamus are too complicated to analyse with naked intuition. Indeed, the complexity of connection data is one of the major challenges facing neuroanatomy. Recently, systematic methods have been developed and applied to the analysis of the connectivity in the cerebral cortex. These approaches have shed light on the gross organization of the cortical network, have made it possible to test systematically theories of cortical organization, and have guided new electrophysiological studies. This paper extends the approach to investigate the organization of the entire corticothalamic network. An extensive collation of connection tracing studies revealed ~1500 extrinsic connections between the cortical areas and thalamic nuclei of the cat cerebral hemisphere. Around 850 connections linked 53 cortical areas with each other, and around 650 connections linked the cortical areas with 42 thalamic nuclei. Non-metric multidimensional scaling, optimal set analysis and non-parametric cluster analysis were used to study global connectivity and the `place' of individual structures within the overall scheme. Thalamic nuclei and cortical areas were in intimate connectional association. Connectivity defined four major thalamocortical systems. These included three broadly hierarchical sensory or sensory/motor systems (visual and auditory systems and a single system containing both somatosensory and motor structures). The highest stations of these sensory/motor systems were associated with a fourth processing system composed of prefrontal, cingulate, insular and parahippocampal cortex and associated thalamic nuclei (the `fronto-limbic system'). The association between fronto-limbic and somato-motor systems was particularly close." [Full Text]

Ergenzinger ER, Glasier MM, Hahm JO, Pons TP.
Cortically induced thalamic plasticity in the primate somatosensory system.
Nat Neurosci 1998 Jul;1(3):226-9
"The influence of cortical feedback on receptive field organization in the thalamus was assessed in the primate somatosensory system. Chronic and acute suppression of neuronal activity in primary somatosensory cortex resulted in a striking enlargement of receptive fields in the ventroposterior thalamus. This finding demonstrates a dramatic 'top-down' influence of cortex on receptive field size in the somatosensory thalamus. In addition, this result has important implications for studies of adult neuronal plasticity because it indicates that changes in 'higher-order' areas of the brain can trigger extensive changes in the receptive field characteristics of neurons located earlier in the processing pathway." [Abstract]

Guillery RW.
Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review.
J Anat 1995 Dec;187 ( Pt 3):583-92
"Two distinct types of thalamic nucleus are proposed on the basis of the afferent fibres that they receive from ascending pathways and from the cerebral cortex. 'First order nuclei' receive primary afferent fibres, definable on the basis of their origin and their intrathalamic synaptic relationships, from ascending pathways. These nuclei receive corticothalamic afferents from pyramidal cells in cortical layer 6, which also send branches to the thalamic reticular nucleus and appear to have a modulatory function. 'Higher order nuclei' receive most or all of their 'primary afferents' from pyramidal cells in cortical layer 5. These resemble the ascending primary afferents in the first order nuclei in terms of fine structure, synaptic relationships and in lacking a branch to the thalamic reticular nucleus. The higher order nuclei also receive modulatory afferents from layer 6. It is proposed that the higher-order nuclei are largely concerned with transmitting information about the output of one cortical area to another cortical area, and that they are likely to play a key role in corticocortical communication and higher cortical functions." [Abstract]

Crosson B.
Subcortical mechanisms in language: -semantic mechanisms and the thalamus.
Brain Cogn 1999 Jul;40(2):414-38
"Four previously published cases of dominant thalamic lesion in which the author has participated are reviewed to gain a better understanding of thalamic participation in lexical-semantic functions. Naming deficits in two cases support Nadeau and Crosson's (1997) hypothesis of a selective engagement mechanism involving the frontal lobes, inferior thalamic peduncle, nucleus reticularis, and other thalamic nuclei, possibly the centromedian nucleus. This mechanism selectively engages those cortical areas required to perform a cognitive task, while maintaining other areas in a state of relative disengagement. Deficits in selective engagement disproportionately affect lexical retrieval based on semantic input, as opposed to lexical and sublexical processes, because the former is more dependent upon this attentional system. The concept of selective engagement is also useful in understanding thalamic participation in working memory, as supported by data from one recent functional neuroimaging study. Other processes also may be compromised in more posterior thalamic lesions which damage the pulvinar but not other components of this selective engagement system. A third case with aphasia after a more superior and posterior thalamic lesion also had oral reading errors similar to those in neglect dyslexia. The pattern of deficits suggested a visual processing problem in the early stages of reading. The fourth case had a category-specific naming deficit after posterior thalamic lesion. Taken together, the latter two cases indicate that the nature of language functions in more posterior regions of the dominant thalamus depends upon the cortical connectivity of the thalamic region. Together, findings from the four cases suggest that thalamic nuclei and systems are involved in multiple processes which directly or indirectly support cortical language functions." [Abstract]

Fernandez de Molina y Canas A.
[Thalamo-cortical system and consciousness]
An R Acad Nac Med (Madr) 2000;117(4):855-69; discussion 869-81
"After reviewing the concept of the specific and non specific thalamo-cortical systems, the connectivity of the relay and intralaminar nuclei is analyzed as well as the recent data concerning the chemical identity of thalamic neurones, the concept and distribution of "matrix" and "core" neurones and its functional role. The intrinsic electrical properties of thalamic neurones, its mode of discharge--depending of the membrane potential level--and its functional significance in the context of the brain's global activity are discussed. Of special interest are the studies on the effects of lesion of the relay and intralaminar nuclei as well as its repercussion in the interpretation of the sensory perception. After intralaminar nuclei lesion the individual is not aware of the information conveyed through the specific channels. It follows a discussion on the importance of the temporal and spatial mapping in the elaboration of perception and cognition. Due to the intrinsic electrical properties and the connectivity of thalamic neurones two groups of corticothalamic loops are generated, which resonate at a frequency of 40 Hz. The specific thalamo-cortical loops give the content of cognition and the non specific loop, the temporal binding required for the unity of the cognitive experience. Consciousness is then, a product of the resonant thalamo-cortical activity, and the dialogue between the thalamus and cortex, the process that generates subjectivity, the unique experience we all recognized as the existence of the "self"." [Abstract]

Jones EG.
Viewpoint: the core and matrix of thalamic organization.
Neuroscience 1998 Jul;85(2):331-45
"The integration of the whole cerebral cortex and thalamus during forebrain activities that underlie different states of consciousness, requires pathways for the dispersion of thalamic activity across many cortical areas. Past theories have relied on the intralaminar nuclei as the sources of diffuse thalamocortical projections that could facilitate spread of activity across the cortex. A case is made for the presence of a matrix of superficially-projecting cells, not confined to the intralaminar nuclei but extending throughout the whole thalamus. These cells are distinguished by immunoreactivity for the calcium-binding protein, D28K calbindin, are found in all thalamic nuclei of primates and have increased numbers in some nuclei. They project to superficial layers of the cerebral cortex over relatively wide areas, unconstrained by architectonic boundaries. They generally receive subcortical inputs that lack the topographic order and physiological precision of the principal sensory pathways. Superimposed upon the matrix in certain nuclei only, is a core of cells distinguished by immunoreactivity for another calcium-binding protein, parvalbumin, These project in highly ordered fashion to middle layers of the cortex in an area-specific manner. They are innervated by subcortical inputs that are topographically precise and have readily identifiable physiological properties. The parvalbumin cells form the basis for sensory and other inputs that are to be used as a basis for perception. The calbindin cells, especially when recruited by corticothalamic connections, can form a basis for the engagement of multiple cortical areas and thalamic nuclei that is essential for the binding of multiple aspects of sensory experience into a single framework of consciousness." [Abstract]

Jones EG.
The thalamic matrix and thalamocortical synchrony.
Trends Neurosci 2001 Oct;24(10):595-601
"High-frequency synchronous activity of neurons in the cerebral cortex and thalamus is a concomitant of discrete conscious events. In the primate thalamus, a newly identified population of neurons provides a basis for this synchronization. A matrix of calbindin-immunoreactive neurons extends throughout the thalamus and projects to superficial layers of cortex over wide areas, unconstrained by boundaries between areas. In some nuclei, a core of parvalbumin-immunoreactive neurons is superimposed upon the matrix. Core neurons project in a topographically ordered fashion to middle layers of the cortex in an area-specific manner. Matrix neurons, recruited by corticothalamic connections, can disperse activity across cortical areas and thalamic nuclei. Their superficial terminations can synchronize specific and nonspecific elements of the thalamocortical network in coherent activity that underlies cognitive events." [Abstract]

Pare D, Llinas R.
Conscious and pre-conscious processes as seen from the standpoint of sleep-waking cycle neurophysiology.
Neuropsychologia 1995 Sep;33(9):1155-68
"The literature on state-dependent fluctuations in thalamocortical activities indicates that in electrophysiological terms, waking and paradoxical sleep are fundamentally identical states, with the provision that the handling of sensory information is altered in REM sleep. The central paradox of REM sleep, namely the apparent lack of cognitive responsiveness to sensory stimulation in spite of increased thalamocortical responsiveness to sensory stimuli, will lead us to hypothesize that the processing of sensory inputs in REM sleep is similar to that underlying preconscious processing of sensory inputs in the waking state. This will lead to a general discussion of the role of fast (approximately equal to 40 Hz) thalamocortical oscillations and temporal binding in sensory processing and conscious experience." [Abstract]

Lumer ED, Edelman GM, Tononi G.
Neural dynamics in a model of the thalamocortical system. I. Layers, loops and the emergence of fast synchronous rhythms.
Cereb Cortex 1997 Apr-May;7(3):207-27
"A large-scale computer model was constructed to gain insight into the structural basis for the generation of fast synchronous rhythms (20-60 Hz) in the thalamocortical system. The model consisted of 65,000 spiking neurons organized topographically to represent sectors of a primary and secondary area of mammalian visual cortex, and two associated regions of the dorsal thalamus and the thalamic reticular nucleus. Cortical neurons, both excitatory and inhibitory, were organized in supragranular layers, infraganular layers and layer IV. Reciprocal intra- and interlaminar, interareal, thalamocortical, corticothalamic and thalamoreticular connections were set up based on known anatomical constraints. Simulations of neuronal responses to visual input revealed sporadic epochs of synchronous oscillations involving all levels of the model, similar to the fast rhythms recorded in vivo. By systematically modifying physiological and structural parameters in the model, specific network properties were found to play a major role in the generation of this rhythmic activity. For example, fast synchronous rhythms could be sustained autonomously by lateral and interlaminar interactions within and among local cortical circuits. In addition, these oscillations were propagated to the thalamus and amplified by corticothalamocortical loops, including the thalamic reticular complex. Finally, synchronous oscillations were differentially affected by lesioning forward and backward interareal connections." [Abstract]

Lumer ED, Edelman GM, Tononi G.
Neural dynamics in a model of the thalamocortical system. II. The role of neural synchrony tested through perturbations of spike timing.
Cereb Cortex 1997 Apr-May;7(3):228-36
"Activity in the mammalian thalamocortical system is often accompanied by a synchronous discharge of cortical and thalamic neurons. Although many functions have been attributed to such synchronous firing, it is not known whether or how synchrony of firing per se affects thalamocortical operations. Direct experimental tests of the consequences of neuronal synchronization in vivo are hard to carry out, whereas theoretical studies based on single-neuron models cannot reveal the effects of synchrony at the system level. To overcome these limitations, we have used a perturbational approach to test the causal efficacy of synchrony per se in large-scale simulations of the thalamocortical system. The test consists of selectively disrupting firing synchrony by 'jittering' the timing of action potentials in the simulations and determining whether firing rates are modified by this perturbation. The simulations are based in detail on the known anatomy and physiology of the thalamocortical-visual system of the cat, and have been shown in a companion paper to produce episodes of fast synchronous activity at multiple levels. By carrying out the perturbation analysis, we established that neurons can have long membrane time constants (8-16 ms) and balanced synaptic activations, and yet function collectively in such a way that synchrony within a time window of 4 ms significantly affects the rates and selectivity of the responses to visual stimuli. The simulations also revealed a complex interplay, at the network level, between synchrony of firing and rate of firing. The dynamic consequences of firing synchrony were most evident when spike jittering was applied to specific polysynaptic loops involving corticocortical and corticothalamic connections. These results support the view that firing synchrony within thalamocortical and corticocortical loops plays a causal role in the cooperative and competitive neural interactions that produce pattern-selective responses in the cortex." [Abstract]

Bogen JE.
Some neurophysiologic aspects of consciousness.
Semin Neurol 1997 Jun;17(2):95-103
"An anatomico-physiologic approach to consciousness is facilitated by recognizing that the various meanings of consciousness have in common a crucial core C variously called subjectivity, awareness, consciousness-as-such, or consciousness per se. A sharp distinction is made between the property C and the contents of consciousness, partial loss of which is typical of cerebro-cortical lesions. The neuronal mechanism producing subjectivity also acts as an attention-action coordinator, hence must have specific connectivity requirements. These requirements are best met by the thalamic intralaminar nuclei (ILN). Whereas large lesions elsewhere leave C undisturbed, quite small bilateral lesions in ILN engender immediate unresponsiveness. This combination of anatomic and neurologic evidence is bolstered by a variety of physiologic evidence leading to the conclusion that further investigations of the ILN, and their interaction with lower centers as well as cerebral cortex, are most apt to yield a better understanding of consciousness." [Abstract]

Sporns O, Tononi G, Edelman GM.
Theoretical neuroanatomy and the connectivity of the cerebral cortex.
Behav Brain Res 2002 Sep 20;135(1-2):69-74
"Over recent years, a wealth of neuroanatomical information on the pattern of interconnections between segregated areas of the cerebral cortex has become available. Here, we describe a set of structural measures, based on graph theory, which can be used to analyze these anatomical patterns. We describe relationships between these structural measures and measures based on patterns of functional connectivity, i.e. patterns of correlations in neural activity. We find that networks capable of producing highly complex functional dynamics share common structural motifs. These motifs are also found in cortical connection matrices, which are characterized by the existence of densely linked groups of areas, low potential wiring length, and a high abundance of reciprocal connections and short cycles. An analysis of cortical functional connectivity demonstrates the existence of functional clusters of highly interactive areas, producing highly complex dynamics. The combined structural and functional analysis outlined in this chapter provides insight into the large-scale functional organization of distributed cortical systems." [Abstract] [PDF]

Sporns, O., Tononi, G., Edelman, G.M.
Theoretical Neuroanatomy: Relating Anatomical and Functional Connectivity in Graphs and Cortical Connection Matrices
Cereb. Cortex 2000 10: 127-141
"Neuroanatomy places critical constraints on the functional connectivity of the cerebral cortex. To analyze these constraints we have examined the relationship between structural features of networks (expressed as graphs) and the patterns of functional connectivity to which they give rise when implemented as dynamical systems. We selected among structurally varying graphs using as selective criteria a number of global information-theoretical measures that characterize functional connectivity. We selected graphs separately for increases in measures of entropy (capturing statistical independence of graph elements), integration (capturing their statistical dependence) and complexity (capturing the interplay between their functional segregation and integration). We found that dynamics with high complexity were supported by graphs whose units were organized into densely linked groups that were sparsely and reciprocally interconnected. Connection matrices based on actual neuroanatomical data describing areas and pathways of the macaque visual cortex and the cat cortex showed structural characteristics that coincided best with those of such complex graphs, revealing the presence of distinct but interconnected anatomical groupings of areas. Moreover, when implemented as dynamical systems, these cortical connection matrices generated functional connectivity with high complexity, characterized by the presence of highly coherent functional clusters. We also found that selection of graphs as they responded to input or produced output led to increases in the complexity of their dynamics. We hypothesize that adaptation to rich sensory environments and motor demands requires complex dynamics and that these dynamics are supported by neuroanatomical motifs that are characteristic of the cerebral cortex." [Full Text]

Tononi G, Edelman GM.
Consciousness and complexity.
Science 1998 Dec 4;282(5395):1846-51
"Conventional approaches to understanding consciousness are generally concerned with the contribution of specific brain areas or groups of neurons. By contrast, it is considered here what kinds of neural processes can account for key properties of conscious experience. Applying measures of neural integration and complexity, together with an analysis of extensive neurological data, leads to a testable proposal-the dynamic core hypothesis-about the properties of the neural substrate of consciousness." [Abstract]

Srinivasan, Ramesh, Russell, D. Patrick, Edelman, Gerald M., Tononi, Giulio
Increased Synchronization of Neuromagnetic Responses during Conscious Perception
J. Neurosci. 1999 19: 5435-5448 [Full Text]

Gilbert PF.
An outline of brain function.
Brain Res Cogn Brain Res 2001 Aug;12(1):61-74
"An outline of how the brain may compute is proposed. In the cerebral cortex memories are stored through long-term potentiation at synapses from layer 1 cortical inputs (representing contexts) on layer 2/3 pyramidal cells linked with the thalamus in a cortico-thalamic (CT) unit. The signals which are memorized are the layer 3 inputs from the thalamus or other cortical areas. Signals are memorized (and later recalled) at the gamma frequency. A conscious thought comprises the outputs of layer 5 cells in CT units in different cortical regions firing in synchrony through the contribution of oscillatory thalamic and cortical inputs. This cortical output influences sub-cortical areas to cause or participate in a movement. Cerebral cortical outputs may be stored in the cerebellum and generated later in a particular context by the basal ganglia and cerebellum. Thus the brain may either generate 'conscious' outputs using the cerebral cortex or 'automatic' outputs using the basal ganglia and cerebellum. When contexts are recognized by the basal ganglia it permits outputs stored in the cerebellum to commence and in this way the basal ganglia can control complex sequences of outputs or movements. Working memory involves the prefrontal cortex using similarly the basal ganglia and cerebellum. The hippocampus has a role in the storage and recall of cortical outputs by providing unique layer 1 contexts to all the CT loops in different cortical areas in a conscious thought. With further recall of the thought new layer 1 contexts may become associated with the CT loops enabling recall without the hippocampal input." [Abstract]

Cudeiro-Mazaira FJ, Rivadulla-Fernandez JC.
[The thalamus: a dynamic door to perception]
Rev Neurol 2002 Jan 16-31;34(2):121-30
"Far from behaving as a simple relay station, the thalamic circuits represent the framework on which to build a truly dynamic biological filter which can select the messages to reach the cortex according to their relevance and the behavioural state of the person. Such filtering implies the participation of many neurotransmitters which represent as many different systems. Outstanding amongst these are the groups formed by acetylcholine and nitric oxide axons arising from the brainstem, and corticofugal fibers which are sent back from the cortex to the thalamus." [Abstract]

Castro-Alamancos, Manuel A.
Role of Thalamocortical Sensory Suppression during Arousal: Focusing Sensory Inputs in Neocortex
J. Neurosci. 2002 22: 9651-9655
"The thalamus serves as a gate that regulates the flow of sensory inputs to the neocortex, and this gate is controlled by neuromodulators from the brainstem reticular formation that are released during arousal. We found recently that sensory-evoked responses are suppressed in the neocortex during arousal. This sensory suppression results from the activity-dependent depression of the thalamocortical connection caused by increased tonic firing of thalamocortical cells during arousal. In the present study, the functional consequences of thalamocortical suppression during arousal were investigated using the vibrissae system of rodents. The results show that thalamocortical suppression is associated with a strong reduction in the spread of sensory inputs through the cortex, thus reducing the size of sensory representations. In addition, when the responses of single cells to principal and adjacent whiskers are compared, the response to the adjacent whiskers was found to be strongly suppressed, much more so than that of principal whiskers. Consequently, the receptive fields of cortical neurons become more focused to the principal whisker. The results indicate that thalamocortical suppression during arousal serves to focus sensory inputs to their appropriate representations in neocortex, which may be computationally helpful for the spatial processing of sensory information."

Smythies J.
The functional neuroanatomy of awareness: with a focus on the role of various anatomical systems in the control of intermodal attention.
Conscious Cogn 1997 Dec;6(4):455-81
"This review considers a number of recent theories on the neural basis of consciousness, with particular attention to the theories of Bogen, Crick, Llinas, Newman, and Changeux. These theories allot different roles to various key brain areas, in particular the reticular and intralaminar nuclei of the thalamus and the cortex. Crick's hypothesis is that awareness is a function of reverberating corticothalamic loops and that the spotlight of intramodal attention is controlled by the reticular nucleus of the thalamus. He also proposed different mechanisms for attention and intention ("will"). The current review presents a new hypothesis, based on elements from these hypotheses, including intermodal attention and olfaction and pain, which may pose problems for Crick's original theory. This work reviews the possible role in awareness and intermodal attention and intention of the cholinergic system in the basal forebrain and the tegmentum; the reticular, the intralaminar, and the dorsomedial thalamic nuclei; the raphe and locus coeruleus; the reticular formation; the ventral striatum and extended amygdala; insula cortex, and other selected cortical, areas. Both clinical and basic research data are covered. The conclusion is reached that the brain may work by largely nonlinear parallel processing and much intramodal shifts of attention may be effected by intracortical, or multiple corticothalamic mechanisms (small local "flashlights" rather than one major "searchlight"). But this is constrained by the functional anatomy of the circuits concerned and waking "awareness" is modulated by the many "nonspecific" systems (cholinergic from the basal forebrain, noradrenergic from the locus coeruleus, dopaminergic from the substantia nigra and ventral tegmentum, and serotoninergic from the raphe). But the principal agents for intermodal attention shifts, the "searchlight," may be two key nuclei of the cholinergic system in the mesencephalon. Clinical loss of consciousness results from damage to these nuclei but not from damage to the cholinergic nucleus basalis of the basal forebrain." [Abstract]

Hobson JA.
Sleep and dreaming: induction and mediation of REM sleep by cholinergic mechanisms.
Curr Opin Neurobiol 1992 Dec;2(6):759-63
"The most important recent work on the neurobiology of sleep has focused on the precise cellular and biochemical mechanisms of rapid eye movement sleep mediation. Direct and indirect evidence implicates acetylcholine-containing neurons in the peribrachial pons as critical in the triggering and maintenance of rapid eye movement sleep. Other new studies provide support for the hypothesis that the cholinergic generator system is gated during waking by serotonergic and noradrenergic influences. A growing consensus regarding the basic neurobiology has stimulated new thinking about the brain basis of consciousness during waking and dreaming." [Abstract]

Perry E, Walker M, Grace J, Perry R.
Acetylcholine in mind: a neurotransmitter correlate of consciousness?
Trends Neurosci 1999 Jun;22(6):273-80
"The cholinergic system is one of the most important modulatory neurotransmitter systems in the brain and controls activities that depend on selective attention, which are an essential component of conscious awareness. Psychopharmacological and pathological evidence supports the concept of a 'cholinergic component' of conscious awareness. Drugs that antagonize muscarinic receptors induce hallucinations and reduce the level of consciousness, while the nicotinic receptor is implicated as being involved in the mechanism of action of general (inhalational) anaesthetics. In degenerative diseases of the brain, alterations in consciousness are associated with regional deficits in the cholinergic system. In Alzheimer's disease (AD), there is a loss of explicit (more than implicit) memory and hypoactivity of cholinergic projections to the hippocampus and cortex, while the visual hallucinations experienced by subjects with Dementia with Lewy bodies (DLB) are associated with reductions in neocortical ACh-related activity. In Parkinson's disease, the additional loss of pedunculopontine cholinergic neurones, which control REM (rapid eye movement) sleep or dreaming, is likely to contribute to REM abnormalities, which also occur in DLB. Widespread basal-forebrain and rostral brainstem cholinergic pathways, which include converging projections to the thalamus, appear to be located strategically for generating and integrating conscious awareness. Alleviation of a range of cognitive and non-cognitive symptoms by drugs that modulate the cholinergic system, which are being developed for the treatment of AD and related disorders, could be caused by changes in consciousness." [Abstract]

Manuel A. Castro-Alamancos, and Maria E. Calcagnotto
High-Pass Filtering of Corticothalamic Activity by Neuromodulators Released in the Thalamus During Arousal: In Vitro and In Vivo
J Neurophysiol 85: 1489-1497, 2001.
"The thalamus is the principal relay station of sensory information to the neocortex. In return, the neocortex sends a massive feedback projection back to the thalamus. The thalamus also receives neuromodulatory inputs from the brain stem reticular formation, which is vigorously activated during arousal. We investigated the effects of two neuromodulators, acetylcholine and norepinephrine, on corticothalamic responses in vitro and in vivo. Results from rodent slices in vitro showed that acetylcholine and norepinephrine depress the efficacy of corticothalamic synapses while enhancing their frequency-dependent facilitation. This produces a stronger depression of low-frequency responses than of high-frequency responses. The effects of acetylcholine and norepinephrine were mimicked by muscarinic and alpha(2)-adrenergic receptor agonists and blocked by muscarinic and alpha-adrenergic antagonists, respectively. Stimulation of the brain stem reticular formation in vivo also strongly depressed corticothalamic responses. The suppression was very strong for low-frequency responses, which do not produce synaptic facilitation, but absent for high-frequency corticothalamic responses. As in vitro, application of muscarinic and alpha-adrenergic antagonists into the thalamus in vivo abolished the suppression of corticothalamic responses induced by stimulating the reticular formation. In conclusion, cholinergic and noradrenergic activation during arousal high-pass filters corticothalamic activity. Thus, during arousal only high-frequency inputs from the neocortex are allowed to reach the thalamus. Neuromodulators acting on corticothalamic synapses gate the flow of cortical activity to the thalamus as dictated by behavioral state." [Full Text]

Sturm W, Willmes K.
On the functional neuroanatomy of intrinsic and phasic alertness.
Neuroimage 2001 Jul;14(1 Pt 2):S76-84
"Intrinsic and phasic alertness are the most basic aspects of attention intensity probably constituting the basis for the more complex and capacity-demanding aspects of attention selectivity. Intrinsic alertness represents the cognitive control of wakefulness and arousal and is typically assessed by simple reaction time tasks without a preceding warning stimulus. Phasic alertness, in contrast, is called for in reaction time tasks in which a warning stimulus precedes the target, and it represents the ability to increase response readiness subsequent to external cueing. We report PET and fMRI data from both the literature and our own experiments to delineate the cortical and subcortical networks subserving alertness, sustained attention (as another aspect of attention intensity), and spatial orienting of attention. Irrespective of stimulus modality, there seems to exist a mostly right-hemispheric frontal, parietal, thalamic, and brain-stem network which is coactivated by alerting and orienting attentional demands. These findings corroborate both the hypothesis of a frontal modulation of brain-stem activation probably via the reticular nucleus of the thalamus and of a coactivation of the posterior attention system involved in spatial orienting by the anterior alerting network. Under conditions of phasic alertness there are additional activations of left-hemisphere frontal and parietal structures which are interpreted as basal aspects of attention selectivity rather than additional features of alerting." [Abstract]

Baars BJ.
Attention versus consciousness in the visual brain: differences in conception, phenomenology, behavior, neuroanatomy, and physiology.
J Gen Psychol 1999 Jul;126(3):224-33
"A common confound between consciousness and attention makes it difficult to think clearly about recent advances in the understanding of the visual brain. Visual consciousness involves phenomenal experience of the visual world, but visual attention is more plausibly treated as a function that selects and maintains the selection of potential conscious contents, often unconsciously. In the same sense, eye movements select conscious visual events, which are not the same as conscious visual experience. According to common sense, visual experience is consciousness, and selective processes are labeled as attention. The distinction is reflected in very different behavioral measures and in very different brain anatomy and physiology. Visual consciousness tends to be associated with the "what" stream of visual feature neurons in the ventral temporal lobe. In contrast, attentional selection and maintenance are mediated by other brain regions, ranging from superior colliculi to thalamus, prefrontal cortex, and anterior cingulate. The author applied the common-sense distinction between attention and consciousness to the theoretical positions of M. I. Posner (1992, 1994) and D. LaBerge (1997, 1998) to show how it helps to clarify the evidence. He concluded that clarity of thought is served by calling a thing by its proper name." [Abstract]

Pins D, Ffytche D.
The neural correlates of conscious vision.
Cereb Cortex 2003 May;13(5):461-74
"Conflicting accounts of the neurobiology of consciousness have emerged from previous imaging studies. Some studies suggest that visual consciousness relates to a distributed network of frontal and partietal regions while others point to localized activity within individual visual areas. While the two positions seem mutually exclusive, timing issues may help reconcile the two. Networks that appear unified in functional magnetic resonance imaging (fMRI) studies may reflect processes that are widely distributed in time. To help resolve this issue, we have investigated timing across a network correlating with consciousness in parallel fMRI and evoked potential (EP) studies of grating stimuli. At threshold, a stimulus is perceived on some occasions but not on others, dissociating sensory input and perception. We have found correlates of consciousness in the occipital lobe at 100 ms and in parietal, frontal, auditory and motor regions from 260 ms onwards. The broad temporal and spatial distribution of activity argues against a unified, distributed fronto-parietal correlate of consciousness. Instead, it suggests that correlates of consciousness are divided into primary and secondary network nodes, with early activity in the occipital lobe correlating with perception and later activity in downstream areas with secondary processes contingent on the outcome of earlier perceptual processing." [Abstract]

Lamme VA.
Why visual attention and awareness are different.
Trends Cogn Sci 2003 Jan;7(1):12-18
"Now that the study of consciousness is warmly embraced by cognitive scientists, much confusion seems to arise between the concepts of visual attention and visual awareness. Often, visual awareness is equated to what is in the focus of attention. There are, however, two sets of arguments to separate attention from awareness: a psychological/theoretical one and a neurobiological one. By combining these arguments I present definitions of visual attention and awareness that clearly distinguish between the two, yet explain why attention and awareness are so intricately related. In fact, there seems more overlap between mechanisms of memory and awareness than between those of attention and awareness." [Abstract]

Kanwisher N.
Neural events and perceptual awareness.
Cognition 2001 Apr;79(1-2):89-113
"Neural correlates of perceptual awareness, until very recently an elusive quarry, are now almost commonplace findings. This article first describes a variety of neural correlates of perceptual awareness based on fMRI, ERPs, and single-unit recordings. It is then argued that our quest should ultimately focus not on mere correlates of awareness, but rather on the neural events that are both necessary and sufficient for perceptual awareness. Indeed, preliminary evidence suggests that although many of the neural correlates already reported may be necessary for the corresponding state of awareness, it is unlikely that they are sufficient for it. The final section considers three hypotheses concerning the possible sufficiency conditions for perceptual awareness." [Abstract]

Jones BE.
Arousal systems.
Front Biosci 2003 May 1;8:S438-51
"The brain contains autochthonous neural systems that evoke waking from sleep in response to sensory stimuli, prolong or enhance arousal in response to special stimuli, and also generate and maintain wakefulness regardless of sensory stimuli during the active part of the day. Through ascending projections to the cortex, these arousal systems stimulate cortical activation, characterized by high frequency gamma and low frequency rhythmic theta activity, and through descending projections to the spinal cord, they stimulate muscle tonus along with sensory-motor responsiveness and activity. They are comprised of neuronal aggregates within the brainstem reticular formation, thalamus, posterior hypothalamus and basal forebrain, and they utilize multiple different neurotransmitters. Within the brainstem, neurons of the reticular formation, which predominantly utilize glutamate as a neurotransmitter, stimulate cortical activation by exciting the widespread projecting neurons of the nonspecific thalamo-cortical projection system, which similarly utilize glutamate, and neurons of the ventral extra-thalamic relay systems located in the posterior hypothalamus and basal forebrain, many of which also utilize glutamate. In addition, these systems have descending projections by which they can enhance or modulate muscle tonus and activity. Articulating with these are cholinergic neurons of the ponto-mesencephalic tegmentum and basal forebrain that promote cortical activation during waking and also during rapid eye movement sleep (REMS), in association therein with muscle atonia. Dopaminergic ventral mesencephalic neurons stimulate a highly motivated and positively rewarding state during waking and may also do so during REMS. In contrast, noradrenergic locus coeruleus neurons promote an aroused waking state and prevent REMS as well as slow wave sleep (SWS). Serotonergic raphe neurons promote a seemingly quiet or satiated waking state, which though exclusive of REMS, can actually be conducive to SWS. Histaminergic neurons of the posterior hypothalamus act like noradrenergic neurons in enforcing waking and are joined by neurons in the region that contain orexin, a neuropeptide recently shown to maintain waking and in absentia to be responsible for narcolepsy, or the inability to maintain wakefulness. These multiple arousal systems are grossly redundant, since no one system is absolutely necessary for the occurrence of waking; yet they are differentiated, since each plays a special role in waking and sleep. During SWS, they are submitted to an inhibitory influence arising in part at least from particular GABAergic neurons co-distributed with many neurons of the arousal systems and also concentrated within the basal forebrain and adjacent preoptic region." [Abstract]


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