Wolf Singer, Consciousness Researcher - Abstracts

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Alfredo Pereira Jr (group admin)

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27 August 2007

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1: J Neurosci. 2007 Mar 14;27(11):2858-65.

Synchronization of neural activity across cortical areas correlates with
conscious perception.

Melloni L, Molina C, Pena M, Torres D, Singer W, Rodriguez E.

Laboratorio de Neurociencias, Escuela de Psicologia, Pontificia Universidad
Catolica de Chile, Vicuña Mackenna 4860, San Joaquin, 8940000 Santiago, Chile.
melloni@mpih-frankfurt.mpg.de

Subliminal stimuli can be deeply processed and activate similar brain areas as
consciously perceived stimuli. This raises the question which signatures of
neural activity critically differentiate conscious from unconscious processing.
Transient synchronization of neural activity has been proposed as a neural
correlate of conscious perception. Here we test this proposal by comparing the
electrophysiological responses related to the processing of visible and invisible
words in a delayed matching to sample task. Both perceived and nonperceived words
caused a similar increase of local (gamma) oscillations in the EEG, but only
perceived words induced a transient long-distance synchronization of gamma
oscillations across widely separated regions of the brain. After this transient
period of temporal coordination, the electrographic signatures of conscious and
unconscious processes continue to diverge. Only words reported as perceived
induced (1) enhanced theta oscillations over frontal regions during the
maintenance interval, (2) an increase of the P300 component of the event-related
potential, and (3) an increase in power and phase synchrony of gamma oscillations
before the anticipated presentation of the test word. We propose that the
critical process mediating the access to conscious perception is the early
transient global increase of phase synchrony of oscillatory activity in the gamma
frequency range.

Publication Types: Comparative Study Research Support, Non-U.S. Gov’t

PMID: 17360907 [PubMed – indexed for MEDLINE]

2: Ann N Y Acad Sci. 2001 Apr;929:123-46.

Consciousness and the binding problem.

Singer W.

Max Planck Institute for Brain Research, Frankfurt/Main, Germany.
singer@mpih-frankfurt.mpg.de

It is proposed that phenomenal awareness, the ability to be aware of one’s
sensations and feelings, emerges from the capacity of evolved brains to analyze
their own cognitive processes by iterating and reapplying on them-selves the very
same cortical operations that they use for the interpretation of signals from the
outer world. Search for the neuronal substrate of awareness therefore converges
with the search for the cognitive mechanisms through which brains analyze their
environment. The hypothesis is put forward that the mammalian brain generates
continuously highly dynamic states that, when modulated by input signals, rapidly
converge towards points of transient stability that correspond to the respective
input constellation. It is proposed that these states are characterized by the
dynamic binding of feature-specific cells into functionally coherent cell
assemblies which as a whole represent the constellation of features defining a
particular perceptual object. Arguments are presented that favor the notion that
the cognitive operations supporting awareness consist of an iteration of such
dynamic binding processes which then lead to the formation of higher-order
assemblies that correspond to the contents of conscious awareness. Experimental
data are reviewed relating to the questions of how assemblies are formed and
which signatures define the relations among the responses of distributed neurons.
It is argued that assemblies self-organize through reciprocal interactions of
neurons coupled by reentrant loops and that the signature of relatedness consists
of the transient synchronization of the discharges of the respective neurons.
Evidence is presented that these synchronization phenomena depend on the same
state variables as awareness: Both require for their manifestation activated
brain states characterized by desynchronized EEGs. It is concluded that
phenomenal awareness is amenable to neurobiological reductionism; but it is also
proposed that self-consciousness requires a different explanatory approach
because it emerges from the dialogue between different brains and hence has the
quality of a cultural construct.

Publication Types: Review

PMID: 11349422 [PubMed – indexed for MEDLINE]

3: Trends Cogn Sci. 2001 Jan 1;5(1):16-25.

Temporal binding and the neural correlates of sensory awareness.

Engel AK, Singer W.

Institute for Medicine, Research Centre Jülich, 52425, Jülich, Germany

Theories of binding have recently come into the focus of the consciousness
debate. In this review, we discuss the potential relevance of temporal binding
mechanisms for sensory awareness. Specifically, we suggest that neural synchrony
with a precision in the millisecond range may be crucial for conscious
processing, and may be involved in arousal, perceptual integration, attentional
selection and working memory. Recent evidence from both animal and human studies
demonstrates that specific changes in neuronal synchrony occur during all of
these processes and that they are distinguished by the emergence of fast
oscillations with frequencies in the gamma-range.

PMID: 11164732 [PubMed – as supplied by publisher]

4: Conscious Cogn. 1999 Jun;8(2):128-51.

Comment in: Conscious Cogn. 1999 Jun;8(2):123-7. Conscious Cogn. 1999 Jun;8(2):152-4. Conscious Cogn. 1999 Jun;8(2):155-8. Conscious Cogn. 1999 Jun;8(2):159-63. Conscious Cogn. 1999 Jun;8(2):260-8.

Temporal binding, binocular rivalry, and consciousness.

Engel AK, Fries P, Konig P, Brecht M, Singer W.

Max-Planck-Institut fur Hirnforschung, Deutschordenstr. 46, Frankfurt, 60528,
Germany. engel@mpih-frankfurt.mpg.de

Cognitive functions like perception, memory, language, or consciousness are based
on highly parallel and distributed information processing by the brain. One of
the major unresolved questions is how information can be integrated and how
coherent representational states can be established in the distributed neuronal
systems subserving these functions. It has been suggested that this so-called
“binding problem” may be solved in the temporal domain. The hypothesis is that
synchronization of neuronal discharges can serve for the integration of
distributed neurons into cell assemblies and that this process may underlie the
selection of perceptually and behaviorally relevant information. As we intend to
show here, this temporal binding hypothesis has implications for the search of
the neural correlate of consciousness. We review experimental results, mainly
obtained in the visual system, which support the notion of temporal binding. In
particular, we discuss recent experiments on the neural mechanisms of binocular
rivalry which suggest that appropriate synchronization among cortical neurons may
be one of the necessary conditions for the buildup of perceptual states and
awareness of sensory stimuli. Copyright 1999 Academic Press.

Publication Types: Research Support, Non-U.S. Gov’t

PMID: 10447995 [PubMed – indexed for MEDLINE]

5: Philos Trans R Soc Lond B Biol Sci. 1998 Nov 29;353(1377):1829-40.

Consciousness and the structure of neuronal representations.

Singer W.

Max Planck Institute for Brain Research, Frankfurt/Main, Germany.

The hypothesis is defended that brains expressing phenomenal awareness are
capable of generating metarepresentations of their cognitive processes, these
metarepresentations resulting from an iteration of self-similar cortical
operations. Search for the neuronal substrate of awareness therefore converges
with the search for the nature of neuronal representations. It is proposed that
evolved brains use two complementary representational strategies. One consists of
the generation of neurons responding selectively to a particular constellation of
features and is based on selective recombination of inputs in hierarchically
structured feedforward architectures. The other relies on the dynamic association
of feature-specific cells into functionally coherent cell assemblies that, as a
whole, represent the constellation of features defining a particular perceptual
object. Arguments are presented that favour the notion that the
metarepresentations supporting awareness are established in accordance with the
second strategy. Experimental data are reviewed that are compatible with the
hypothesis that evolved brains use assembly codes for the representation of
contents and that these assemblies become organized through transient
synchronization of the discharges of associated neurons. It is argued that
central states favouring the formation of assembly-based representations are
similar to those favouring awareness.

Publication Types: Review

PMID: 9854255 [PubMed – indexed for MEDLINE]

6: J Neurosci. 1996 Apr 1;16(7):2381-96.

Stimulus-dependent synchronization of neuronal responses in the visual cortex of
the awake macaque monkey.

Kreiter AK, Singer W.

Max Planck Institute for Brain Research, Frankfurt/Main, Germany.

In visual areas of the cerebral cortex, most neurons exhibit preferences for
particular features of visual stimuli, but in general, the tuning is broad. Thus,
even simple stimuli evoke responses in numerous neurons with differing but
overlapping feature preferences, and it is commonly held that a particular
feature is encoded in the pattern of graded responses of the activated population
rather than in the optimal responses of individual cells. To decipher this
population code, responses evoked by a particular stimulus need to be identified
and bound together for further joint processing and must not be confounded with
responses to other, nearby stimuli. Such selection of related responses could be
achieved by synchronizing the respective discharges at a time scale of
milliseconds, as this would selectively and jointly enhance their saliency. This
hypothesis predicts that a given set of neurons should exhibit synchronized
discharges more often when responding to a single stimulus than when activated by
different but simultaneously presented stimuli. To test this prediction,
recordings were performed with two electrodes from spatially segregated cells in
the middle temporal area (MT) of the awake behaving macaque monkey. It was found
that cells with overlapping receptive fields, but different preferences for
directions of motion, can engage in synchronous activity if they are stimulated
with a single moving bar. In contrast, if the same cells are activated with two
different bars, each moving in the direction preferred by the cells at the two
respective sites, responses show no or much fewer synchronous epochs. Control
experiments exclude that this effect is attributable to changes in response
amplitude, the mere presence of two stimuli, or the specific orientation of the
bars. The critical variable determining the strength of correlation is the extent
to which both sites are activated by a common stimulus or by two different
stimuli with different directions of motion.

PMID: 8601818 [PubMed – indexed for MEDLINE]

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