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[Klaus Obermayer]

Dear Connectionists,

below please find abstract and preprint-location of a recent paper on
modelling contrast adaptation in primary visual cortex.

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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Contrast Adaptation and Infomax in Visual Cortical Neurons

Peter Adorjan, Christian Piepenbrock, and Klaus Obermayer

CS Department, Technical University of Berlin, Berlin, Germany


In the primary visual cortex (V1) the contrast response function of many neurons
saturates at high contrast and adapts depending on the visual stimulus. We
propose that both effects--contrast saturation and adaptation--can be explained
by a fast and a slow component in the synaptic dynamics. In our model the
saturation is an effect of fast synaptic depression with a recovery time
constant of about 200 ms. Fast synaptic depression leads to a contrast response
function with a high gain for only a limited range of contrast values.
Furthermore, we propose that slow adaptation of the transmitter release
probability at the geniculocortical synapses is the underlying neural mechanism
that accounts for contrast adaptation on a time scale of about 7 sec. For the
functional role of contrast adaptation we make the hypothesis that it serves to
achieve the best visual cortical representation of the geniculate input. This
representation should maximize the mutual information between the cortical
activity and the geniculocortical input by increasing the release probability
in a low contrast environment. We derive an adaptation rule for the transmitter
release probability based on this EM infomax principle. We show that changes in
the transmitter release probability may compensate for changes in the variance
of the geniculate inputs--an essential requirement for contrast adaptation.
Also, we suggest that increasing the release probability in a low contrast
environment is beneficial for signal extraction, because neurons remain
sensitive only to an increase in the presynaptic activity if it is synchronous
and, therefore, likely to be stimulus related. Our hypotheses are tested in
numerical simulations of a network of integrate-and-fire neurons for one column
of V1 using fast synaptic depression and slow synaptic adaptation. The
simulations show that changing the synaptic release probability of the
geniculocortical synapses is a better model for contrast adaptation than the
adaptation of the synaptic weights: only in the case of changing the transmitter
release probability our model reproduces the experimental finding that the
average membrane potential (DC component) adapts much stronger than the
stimulus modulated component (F1 component). In the case of changing synaptic
weights, however, the average membrane potential (DC) as well as the stimulus
modulated component (F1 component) would adapt. Furthermore, changing the
release probability at the recurrent cortical synapses cannot account for
contrast adaptation, but could be responsible for establishing oscillatory
activity often observed in recordings from visual cortical cells.

Rev. Neurosci. 1999, in press

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