Special issue on "Cell Assemblies"

Wed Jul 2 09:36:37 EDT 2003

Dear all,

the following collection of papers appeared recently as a special issue
on "Cell Assemblies" at "Theory in Biosciences".

Preprint versions of the papers are available under

http://www.neuroinformatik.rub.de/thbio/publications/specialissue/cellassemblies.html

Final versions should be available from the authors.

Best wishes,
Thomas

Thomas Wennekers, Friedrich T. Sommer, and Ad Aertsen

Editorial: Cell Assemblies

Theory in Biosciences 122 (2003) 1-4.

Thomas Wennekers and Nihat Ay

Spatial and Temporal Stochastic Interaction in Neuronal Assemblies

Theory in Biosciences 122 (2003) 5-18.

The observation of various types of spatio-temporal correlations
in spike patterns of multiple cortical neurons has shifted attention
from rate coding paradigms to computational processes based on
the precise timing of spikes in neuronal ensembles. In the present
work we develop the notion of "spatial" and "temporal interaction"
which provides measures for statistical dependences in coupled
stochastic processes like multiple unit spike trains. We show that
the classical Willshaw network and Abeles' synfire chain model both
reveal a moderate spatial interaction, but only the synfire chain
model reveals a positive temporal interaction, too. Systems that
maximize temporal interaction are shown to be almost deterministic
globally, but posses almost unpredictable firing behavior on the
single unit level.

Anders Lansner, Erik Fransén, and Anders Sandberg

Cell assembly dynamics in detailed and abstract
attractor models of cortical associative memory

Theory in Biosciences 122 (2003) 19-36.

During the last few decades we have seen a
convergence among ideas and hypotheses regarding functional
principles underlying human memory. Hebb's now more than fifty
years old conjecture concerning synaptic plasticity and cell
assemblies, formalized mathematically as attractor neural
networks, has remained among the most viable and productive
theoretical frameworks. It suggests plausible explanations for
Gestalt aspects of active memory like perceptual completion,
reconstruction and rivalry.

We review the biological plausibility of these theories and
discuss some critical issues concerning their associative
memory functionality in the light of simulation studies of
models with palimpsest memory properties. The focus is on
memory properties and dynamics of networks modularized in
terms of cortical minicolumns and hypercolumns. Biophysical
compartmental models demonstrate attractor dynamics that
support cell assembly operations with fast convergence and low
firing rates. Using a scaling model we obtain reasonable
relative connection densities and amplitudes. An abstract
attractor network model reproduces systems level psychological
phenomena seen in human memory experiments as the Sternberg
and von Restorff effects.

We conclude that there is today considerable substance in
Hebb's theory of cell assemblies and its attractor network
formulations, and that they have contributed to increasing our
understanding of cortical associative memory function.
The criticism raised with regard to biological and
psychological plausibility as well as low storage capacity,
slow retrieval etc has largely been disproved. Rather, this
paradigm has gained further support from new experimental data
as well as computational modeling.

Andreas Knoblauch and Günther Palm

Synchronization of Neuronal Assemblies in
Reciprocally Connected Cortical Areas

Theory in Biosciences 122 (2003) 37-54.

To investigate scene segmentation in the visual system we
present a model of two reciprocally connected visual areas
comprising spiking neurons. The peripheral area P is modeled
similar to the primary visual cortex, while the central
area C is modeled as an associative memory representing
stimulus objects according to Hebbian learning. Without
feedback from area C, spikes corresponding to stimulus
representations in P are synchronized only locally (slow
state). Feedback from C can induce fast oscillations and
an increase of synchronization ranges (fast state).
Presenting a superposition of several stimulus objects,
scene segmentation happens on a time scale of hundreds of
milliseconds by alternating epochs of the slow and fast
state, where neurons representing the same object are
simultaneously in the fast state. We relate our simulation
results to various phenomena observed in neurophysiological
experiments, such as stimulus-dependent synchronization of
fast oscillations, synchronization on different time scales,
ongoing activity, and attention-dependent neural activity.

Friedrich T. Sommer and Thomas Wennekers

Models of distributed associative memory networks in the brain

Theory in Biosciences 122 (2003) 55-69.

Although experimental evidence for distributed cell assemblies
is growing, theories of cell assemblies are still marginalized
in theoretical neuroscience. We argue that this has to do with
shortcomings of the currently best understood assembly theories,
the ones based on formal associative memory models. These only
insufficiently reflect anatomical and physiological properties
of nervous tissue and their functionality is too restricted to
provide a framework for cognitive modeling. We describe cell
assembly models that integrate more neurobiological constraints
and review results from simulations of a simple nonlocal
associative network formed by a reciprocal topographic
projection. Impacts of nonlocal associative projections in the
brain are discussed with respect to the functionality they can
explain.

Hualou Liang and Hongbin Wang

Top-Down Anticipatory Control in Prefrontal Cortex

Theory in Biosciences 122 (2003) 70-86.

The prefrontal cortex has been implicated in a wide variety
of executive functions, many involving some form of
anticipatory attention. Anticipatory attention involves
the pre-selection of specific sensory circuits to allow
fast and efficient stimulus processing and a subsequently
fast and accurate response. It is generally agreed that the
prefrontal cortex plays a critical role in anticipatory
attention by exerting a facilitatory "top-down" bias on
sensory pathways. In this paper we review recent results
indicating that synchronized activity in prefrontal cortex,
during anticipation of visual stimulus, can predict features
of early visual stimulus processing and behavioral response.
Although the mechanisms involved in anticipatory attention
are still largely unknown, we argue that the synchronized
oscillation in prefrontal cortex is a plausible candidate
during sustained visual anticipation. We further propose a
learning hypothesis that explains how this top-down anticipatory
control in prefrontal cortex is learned based on accumulated
prior experience by adopting a Temporal Difference learning
algorithm.

Friedemann Pulvermüller

Sequence detectors as a basis of grammar in the brain

Theory in Biosciences 122 (2003) 87-104.

Grammar processing may build upon serial-order mechanisms
known from non-human species. A circuit similar to that
underlying direction-sensitive movement detection in arthropods
and vertebrates may become selective for sequences of words,
thus yielding grammatical sequence detectors in the human
brain. Sensitivity to the order of neuronal events arises from
unequal connection strengths between two input units and a
third element, the sequence detector. This mechanism, which
critically depends on the dynamics of the input units, can
operate at the single neuron level and may be relevant at the
level of neuronal ensembles as well. Due to the repeated
occurrence of sequences, for example word strings, the
sequence-sensitive elements become more firmly established
and, by substitution of elements between strings, a process
called auto-associative substitution learning (AASL) is
triggered. AASL links the neuronal counterparts of the
string elements involved in the substitution process to the
sequence detector, thereby providing a brain basis of what can
be described linguistically as the generalization of rules of
grammar. A network of sequence detectors may constitute
grammar circuits in the human cortex on which a separate set
of mechanisms establishing temporary binding and recursion
can operate.

____________________________________________________________________________

Jr.Prof.Dr.Thomas Wennekers
Theoretical Neuroscience Group
Institute for Neuroinformatics
Ruhr-Universitaet Bochum
Universitaetsstrasse 150
ND 04/589a
44780 Bochum

Phone: +49-234-3224231
Fax:   +49-234-3214209
Priv.: +49-160-6123416

Email: Thomas.Wennekers at neuroinformatik.rub.de
____________________________________________________________________________