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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
IN THE THEATRE OF CONSCIOUSNESS
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
Consciousness
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.
Consciousness
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 “Consciousness”
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 rh |
