paper available

[Klaus Obermayer]

Dear Connectionists,

The following tech-report and paper are available online:

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A Model for the Intracortical Origin of Orientation Preference and Tuning 
in Macaque Striate Cortex

Peter Adorjan^1, Jonathan B. Levitt^2, Jennifer S. Lund^2, and Klaus Obermayer^1

^1 CS Department, Technical University of Berlin, Berlin, Germany,
^2 Institute for Ophthalmology, UCL, London, UK


We report results of numerical simulations for a model of orientation
selectivity in macaque striate cortex. In contrast to previous models,
where the initial orientation bias is generated by convergent geniculate
input to simple cells and subsequently sharpened by lateral circuits, this
approach is based on anisotropic intracortical excitatory connections
which provide both the initial orientation bias and the subsequent
amplification. Our study shows that the emerging response properties are
similar to the response properties which are observed experimentally,
hence the hypothesis of an intracortical generation of orientation bias is
a sensible alternative to the notion of an afferent bias by convergent
geniculocortical projection patterns. In contrast to models based on an
afferent orientation bias, however, the ``intracortical hypothesis''
predicts that orientation tuning gradually evolves from an initially
nonoriented response and a complete loss of orientation tuning when the
recurrent excitation is blocked, but new experiments must be designed to
unambiguously decide between both hypotheses. 

TU Berlin Technical Report, TR 98-1,
http://kon.cs.tu-berlin.de/publications/#techrep


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Development and Regeneration of the Retinotectal Map in Goldfish: 
A Computational Study

C. Weber^1, H. Ritter^2, J. Cowan^3, and K. Obermayer^1

^1 CS Department, Technical University of Berlin, Berlin, Germany,
^2 Technische Fakultaet, University of Bielefeld, Germany,
^3 Departments of Mathematics and Neurology, The University of Chicago, IL, USA


We present a simple computational model to study the interplay of activity 
dependent and intrinsic processes thought to be involved in the formation of 
topographic neural projections. Our model consists of two input layers which 
project to one target layer. The connections between layers are described by a 
set of synaptic weights. These weights develop according to three interacting 
developmental rules:

(i) an intrinsic fiber-target interaction which generates chemospecific
adhesion between afferent fibers and target cells,

(ii) an intrinsic fiber-fiber interaction which generates mutual selective
adhesion between the afferent fibers and

(iii) an activity-dependent fiber-fiber interaction which implements Hebbian
learning.

Additionally, constraints are imposed to keep synaptic weights finite.

The model is applied to a set of eleven experiments on the regeneration of the
retinotectal projection in goldfish. We find that the model is able to 
reproduce the outcome of an unprecedented range of experiments with the same 
set of model parameters, including details of the size of receptive and 
projective fields. We expect this mathematical framework to be a useful tool 
for the analysis of developmental processes in general.


Phil. Trans. Roy. Soc. Lond. B 352, 1603-1623 (1997)
http://kon.cs.tu-berlin.de/publications/#journals