THEORY OF ORIENTATION TUNING IN VISUAL-CORTEX

The role of intrinsic cortical connections in processing sensory input and in generating behavioral output is poorly understood. We have exa mined this issue in the context of the tuning of neuronal responses in cortex to the orientation of a visual stimulus. We analytically study a simple network model that incorporates both orientation-selective i nput from the lateral geniculate nucleus and orientation-specific cort ical interactions. Depending on the model parameters, the network exhi bits orientation selectivity that originates from within the cortex, b y a symmetry-breaking mechanism. In this case, the width of the orient ation tuning can be sharp even if the lateral geniculate nucleus input s are only weakly anisotropic. By using our model, several experimenta l consequences of this cortical mechanism of orientation tuning are de rived. The tuning width is relatively independent of the contrast and angular anisotropy of the visual stimulus. The transient population re sponse to changing of the stimulus orientation exhibits a slow ''virtu al rotation.'' Neuronal cross-correlations exhibit long time tails, th e sign of which depends on the preferred orientations of the cells and the stimulus orientation.