A Model for Orientation Tuning and Contextual Effects of Orientation Selective Receptive Fields

We investigate a mean-eld model which has previously been used to explain the response properties of orientation selective neurons in the primary visual cortex of monkeys and cats 2]. Two mutually coupled orientation hypercolumns are setup as local ampliiers based on local recurrent excitation and inhibition. We rst investigate the individual hypercolumns. The model 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 eeects found experimentally may be a consequence of the non-linear properties of single neurons rather than being an eeect of diierent gains for inhibitory and excitatory cells 13]. We then extend this model to cover non-classical receptive elds and contextual eeects. 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 diierent orientation preference are connected across hypercolumns. These results hint at deeciencies of this simple approach and suggest that contextual eeects are mediated by populations of neurons, which are not take part of the local gain control.