A mean-field model for orientation tuning, contrast saturation, and contextual effects in the primary visual cortex

Orientation-selective cells in the primary visual cortex of monkeys and cat s are often characterized by an orientation-tuning width that is invariant under stimulus contrast. At the same time their contrast response function saturates or even super-saturates for high values of contrast. When two bar stimuli are presented within their classical receptive field, the neuronal response decreases with the intersection angle. When two stimuli are prese nted inside and outside the classical receptive field, the response of the cell increases with the intersection angle. Both cats and monkeys show iso- orientation suppression, which has sometimes been reported to be combined w ith cross-orientation facilitation. This property has previously been descr ibed as sensitivity to orientation contrast. We address the emergence of th ese effects with a model that describes the processing of geniculocortical signals through cortical circuitry. We hypothesize that short intracortical fibers mediate the classical receptive field effects, whereas long-range c ollaterals evoke contextual effects such as sensitivity to orientation cont rast. We model this situation by setting up a mean-field description of two neighboring cortical hypercolumns, which can process a nonoverlapping cent er and a (nonclassical) surround stimulus. Both hypercolumns interact via i dealized long-range connections. For an isolated model hypercolumn, we find that either contrast saturation or contrast-invariant orientation tuning e merges, depending on the strength of the lateral excitation. There is no pa rameter regime, however, where both phenomena emerge simultaneously. In the regime where contrast saturation is found, the model also correctly reprod uces suppression due to a second, crossoriented grid within the classical r eceptive field. If two model hypercolumns are mutually coupled by long-rang e connections that are iso-orientation specific, nonclassical surround stim uli show either suppression or facilitation for all surround orientations. Sensitivity to orientation contrast is not observed. This property requires excitatory-to-excitatory long-range couplings that are less orientation sp ecific than those targeting inhibitory neurons.