A model for orientation tuning and contextual effects of orientation selective receptive fields

Abstract

We investigate a mean-field 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 amplifiers based on local recurrent excitation and inhibition. We first 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 effects found experimentally may be a consequence of the non-linear properties of single neurons rather than being an effect of different gains for inhibitory and excitatory cells [13]. We then extend this model to cover non-classical receptive fields and contextual effects. 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 different orientation preference are connected across hypercolumns. These results hint at deficiencies of this simple approach and suggest that contextual effects are mediated by populations of neurons, which are not take part of the local gain control.