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- M. Stetter, P. Adorjan,
H. Bartsch, and K. Obermayer. Modelling Contrast Adaptation and Contextual
Effects in Primary Visual Cortex.
.
In Proceedings of the ICONIP 98 Conference, Kitakyushu, volume 2,
pages 669-672, 1998.
(FTP Gzipped PostScript, 4 pages, 55 kb)
Orientation selective cortical neurons show contrast-invariant
orientation tuning curves, contrast saturation and contrast adaptation.
Additionally, their responses are modulated by oriented stimuli outside the
classical receptive field. We characterize the extent, to which these effects
can be caused by a recurrent network of simple neurons, and which properties
are probably due to plasticity of geniculocortical synapses. For this we
compare the predictions of a mean-field-model based on cortical hypercolumns
with those of a model that incorporates spiking neurons with detailed
time-resolved synaptic dynamics. We find that in a mean-field model
contrast-invariant orientation tuning and contextual effects can be generated
by local and long-range intracortical circuitry respectively. However, they
are incompatible with contrast-saturation and contrast-adaptation. Using a
recurrent neural network of excitatory spiking neurons with adapting
synapses, we then show that within that model framework (i) contrast
saturation can be explained by fast adaptation of the lateral or the afferent
synapses, (ii) phase advance with increasing contrast is due to fast
adaptation of the afferent synapses, and (iii) adaptation of the synaptic
transmitter release probability of the geniculocortical synapses is a
necessary and sufficient mechanism for contrast adaptation. In summary, we
suggest that recurrent lateral connections play a key role in orientation
processing, while contrast processing is mainly a feed-forward
phenomenon.
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