  Neural mechanisms of extraclassical receptive field phenomena in V1 are
commonly assumed to result from long-range lateral connections and/or
extrastriate 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.
