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A Model for the Generation of Orientation Tuning and Contextual Effects by Intracortical Circuits
Citation key Bartsch1997a
Author Bartsch, H. and Stetter, M. and Obermayer, K.
Title of Book 4th Hungarian Vision Symposium of the Albert Szent-Gyoergyi Medical University and the Hungarian Academy of Sciences, Szeged, 30. August
Year 1997
Abstract We investigate the behaviour of a mean-field model for the primary visual cortex, which can be interpreted as a set of two mutually coupled cortical hypercolumns. Inhibitory neurons are assumed to have higher firing thresholds and higher gains than excitatory neurons. First, one isolated hypercolumn, equipped with experimentally reasonable iso-orientation-excitation and distributed inhibition, is considered. For weak intracortical coupling strengths, the model neurons agree to experiment in showing saturation of their contrast response curves but fail to exhibit contrast-independent orientation tuning widths observed in monkeys and cats. In the regime of strong lateral interactions (defined by the autonomous emergence of activation blobs above a minimum external input strength), in contrast, the system shows contrast-independent orientation tuning widths but cannot generate saturation of the contrast gain. In both cases, however, the model correctly reproduces cross-orientation suppression within the classical receptive field. Contextual effects such as iso-orientation-suppression and cross-orientation-facilitation by stimuli within the non-classical receptive fields were modeled by mutually coupling two hypercolumns each provided with individual oriented stimuli. In the regime of strong intracortical interactions, iso-orientation connections between the two hypercolumns can generate iso-orientation suppression. However, due to the restrictions imposed by the strong within-hypercolumn-connections, cross-orientation facilitation can only be observed by adding a distributed excitation between both hypercolumns. If distributed inhibition between hypercolumns is added, the system shows repulsion of the activation blobs, which may be related to the psychophysical tilt illusion. In summary, the simple system we investigated cannot predict all experimental findings with a single parameter set suggesting the consideration of multiple mutually interconnected neuron populations in future studies.
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