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- H. Bartsch, M. Stetter, and
K. Obermayer. A Model for Orientation Tuning and Contextual Effects of
Orientation Selective Receptive Fields.
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In W. Gerstner, A. Germond, M. Hasler, and J. Nicoud, editors, Artificial
Neural Networks - ICANN 97, pages 237-242. Springer-Verlag, 1997.
(FTP Gzipped PostScript, 6 pages, 53 kb)
We investigate a meanfield model which has previously been used to
explain the response properties of orientation selective neurons in the
primary visual cortex of monkeys and cats [Ben-Yishai et al. 1995]. Two
mutually coupled orientation hypercolumns are setup as local amplifiers based
on local recurrent excitation and inhibition. We first investigate the
individual h yp ercolumns. The mo del correctly predicts contrast invariant
tuning, but analytical and numerical results show that the contrast response
functions of individual orientation columns do not saturate. We therefore
hypothesize that the cortical saturation effects found experimentally may be
a consequence of the non-linear properties of single neurons rather than
being an effect of different gains for inhibitory and excitatory cells
[Todorov et al. 1996]. We then extend this model to cover non-classical
receptive fields and con textual effects. The model correctly predicts
effective iso-orientation inhibition between hypercolumns. As long as
parameters are chosen to ensure contrast invariant orientation tuning,
however, net cross-orientation facilitation emerges only, if cells of
different orientation preference are connected across hypercolumns. These
results hint at deficiencies of this simple approach and suggest that
contextual effects are mediated by populations of neurons, which are not part
of the local gain control.
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