A Mean Field Model for Orientation Tuning , Contrast Saturation and Contextual E ects in thePrimary Visual

Orientation selective cells in the primary visual cortex of monkeys and cats 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 eld, the neuronal response decreases with intersection angle. When two stimuli are presented inside and outside the classical receptive eld, the response of the cell increases with intersection angle. Both cats and monkeys show iso-orientation suppression, which was sometimes reported to be combined with cross-orientation facilitation. This property has previously been described as sensitivity to orientation contrast. We address the emergence of these eeects by a model which describes the processing of geniculocortical signals through cortical circuitry. We hypothesize that short intracortical bers mediate the classical receptive eld eeects whereas long-range collaterals evoke contextual eeects such as sensitivity to orientation contrast. We model this situation by setting up a mean-eld description of two neighboring cortical hypercolumns, which may process a non-overlapping center and a (nonclas-sical) surround stimulus. Both hypercolumns interact via idealized long-range connections. For an isolated model hypercolumn we nd, that either contrast saturation or contrast-invariant orientation tuning emerges, depending on the strength of the lateral excitation. There is no parameter regime, however, where both phenomena emerge simultaneously. In the regime, where contrast saturation is found, the model also correctly reproduces suppression due to a second, cross-oriented grid within the classical receptive eld. If two model hypercolumns are mutually coupled by long-range connections which are iso-orientation speciic, nonclassical surround stimuli 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 speciic than those targeting inhibitory neurons.