preprints available

[Klaus Obermayer]

Dear Connectionists,

attached please find abstracts and preprint-locations of two short
manuscripts on modelling the development of cortical maps.

Cheers

Klaus

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Prof. 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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M. Stetter^1, E. W. Lang^2, and K. Obermayer^1

1 FB Informatik, TU Berlin
2 Physik-Department, U. Regensburg

Unspecific long-term potentiation can evoke functional segregation in
a model of area 17.

Recently it has been shown in rat hippocampus that the synapse
specificity of Hebbian long-term potentiation breaks down at short
distances below 100 mum . Using a neural network model we show that
this unspecific component of long term potentiation can be responsible
for the robust formation and maintainance of cortical organization
during activity driven development. When the model is applied to the
formation of orientation and ocular dominance in visual cortex, we find
that the addition of an unspecific component to standard Hebbian
learning - in combination with a tendency of left-eye and right-eye
driven synapses to initially group together on the postsynaptic neuron
- induces the simultaneous emergence and stabilization of ocular 
dominance and of segregated, oriented ON-/OFF-subfields. Since standard
Hebbian learning cannot account for the simultaneous stabilization of
both structures, unspecific LTP thus induces a qualitatively new
behaviour. Since unspecific LTP only acts between synapses which are
locally clustered in space, our results imply that details of the local
grouping of synapses on the dendritic arbors of postsynaptic cells can
considerably influence the formation of the cortical functional 
organization at the systems level.

to appear in: Neuroreport 1998

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


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C. Piepenbrock and K. Obermayer

FB Informatik, TU Berlin

Effects of lateral competition in the primary visual cortex on the
development of topographic projections and ocular dominance maps

We present a Hebbian model for the development of cortical maps in the
striate cortex that includes a parameter which represents the degree of
lateral competition for activity between neurons. It has two well known
models as limiting cases: for weak competition we obtain a Correlation
Based Learning (CBL) model and for strong lateral competition we recover
the Self Organizing Map (SOM). Receptive fields develop through very
different mechanisms in these models: CBL is driven by the second order 
statistics of the input stimuli, while SOM maps stimulus features. We
study the influence of lateral interactions for intermediate changes in
competition. Increasing the competition for binocular localized stimuli
we find a transition from an unorganized map to a topographic projection
and subsequently to a topographic map with ocular dominance columns.

to appear in: In Computational Neuroscience: Trends in Research 1998, 1998.

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