receptive field phenomena & short - range connectivity in V 1

Neural mechanisms of extraclassical receptive field phenomena in V1 are commonly assumed to result from long-range lateral connections and/or extras-triate feedback. We address two such phenomena: surround suppression and contrast dependent receptive field size. We present rigorous computational support for the hypothesis that the phenomena largely result from local short-range (< 0.5 mm) cortical connections and LGN input. Surround suppression in our simulations results from (A) direct cortical inhibition or (B) suppression of recurrent cortical excitation, or (C) action of both these mechanisms simultaneously. Mechanisms B and C are substantially more prevalent than A. We observe an average growth in the range of spatial summation of excitatory and inhibitory synaptic inputs for low contrast. However, we find this is neither sufficient nor necessary to explain contrast dependent receptive field size, which usually involves additional changes in the relative gain of these inputs.