preprints available

[Klaus Obermayer]

Dear Connectionists,

attached please find abstracts and preprint locations of two manuscripts on
the analysis of optical recording data and on visual cortex modelling.

Comments are welcome!

Cheers

Klaus

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Prof. Dr. Klaus Obermayer         phone:  49-30-314-73442
FR2-1, NI, Informatik                     49-30-314-73120
Technische Universitaet Berlin    fax:    49-30-314-73121
Franklinstrasse 28/29             e-mail: oby at cs.tu-berlin.de
10587 Berlin, Germany             http://ni.cs.tu-berlin.de/

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Principal component analysis and blind separation of sources for optical
imaging of intrinsic signals

M. Stetter^1, I. Schiessl^1, T. Otto^1, F. Sengpiel^2, M. Huebener^2,
T. Bonhoeffer^2, and K. Obermayer^1

^1 Fachbereich Informatik, Technische Universitaet Berlin
^2 Max-Planck-Institute for Neurobiology, Martinsried

The analysis of data sets from optical imaging of intrinsic signals requires
the separation of signals, which accurately reflect stimulated neuronal
activity (mapping signal), from signals related to background activity. Here
we show that blind separation of sources by Extended Spatial Decorrelation
(ESD) is a powerful method for the extraction of the mapping signal from the
total recorded signal. ESD is based on the assumptions, (i) that each signal
component varies smoothly across space and (ii) that every component has zero
cross-correlation functions with the other components. In contrast to the
standard analysis of optical imaging data, the proposed method (i) is
applicable to non-orthogonal stimulus-conditions, (ii) can remove the global
signal, blood-vessel patterns and movement artifacts, (iii) works without ad
hoc assumptions about the data structure in the frequency domain, and (iv)
provides a confidence measure for the signals (Z-score). We first demonstrate
on orientation maps from cat and ferret visual cortex, that Principal
Component Analysis (PCA), which acts as a preprocessing step to ESD, can
already remove global signals from image stacks, as long as data stacks for
at least two -- not necessarily orthogonal -- stimulus conditions are
available. We then show that the full ESD analysis can further reduce global
signal components and -- finally -- concentrate the mapping signal within a
single component both for differential image stacks and for image stacks
recorded during presentation of a single stimulus.

in: NeuroImage, in press

available at: http://ni.cs.tu-berlin.de/publications/


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A mean field model for orientation tuning, contrast saturation and
contextual effects in the primary visual cortex

M. Stetter, H. Bartsch, and K. Obermayer

Fachbereich Informatik, Technische Universitaet Berlin

Orientation selective cells in the primary visual cortex of monkeys and cats
are often characterized by an orientation-tuning width that is invariant
under stimulus contrast. At the same time their contrast response function
saturates or even super-saturates for high values of contrast. When two bar
stimuli are presented within their classical receptive field, the neuronal
response decreases with intersection angle. When two stimuli are presented
inside and outside the classical receptive field, the response of the cell
increases with intersection angle. Both cats and monkeys show iso-orientation
suppression, which was sometimes reported to be combined with
cross-orientation facilitation. This property has previously been described
as sensitivity to orientation contrast.

We address the emergence of these effects by a model which describes the
processing of geniculocortical signals through cortical circuitry. We
hypothesize that short intracortical fibers mediate the classical receptive
field effects whereas long-range collaterals evoke contextual effects such
as sensitivity to orientation contrast. We model this situation by setting
up a mean-field description of two neighboring cortical hypercolumns, which
may process a non-overlapping center and a (nonclassical) surround stimulus.
Both hypercolumns interact via idealized long-range connections. For an
isolated model hypercolumn we find, that either contrast saturation or
contrast-invariant orientation tuning emerges, depending on the strength of
the lateral excitation. There is no parameter regime, however, where both
phenomena emerge simultaneously. In the regime, where contrast saturation
is found, the model also correctly reproduces suppression due to a second,
cross-oriented grid within the classical receptive field. If two model
hypercolumns are mutually coupled by long-range connections which are
iso-orientation specific, nonclassical surround stimuli show either
suppression or facilitation for all surround orientations. Sensitivity to
orientation contrast is not observed. This property requires
excitatory-to-excitatory long-range couplings that are less orientation
specific than those targeting inhibitory neurons.

in Biological Cybernetics, in press

available at: http://ni.cs.tu-berlin.de/publications/