Supplementary Information , accompanying : Topography of contextual modulations resulting from short-range interactions in primary visual cortex

We wish to address here how inhibitory interactions between two neurons, as seen in the suppressive effect of flanking stimuli, is consistent with the excitatory effective physiological interconnections between them as manifested by significant positive and narrow cross correlations (Fig. 3, main paper). Here we present a physiologically plausible model of local circuitry that could account for these apparently contradictory observations. Consider an experimental configuration as in Fig. 3 of the main paper, recording simultaneously from a test neuron and a flank neuron in neighbouring, orthogonal cortical columns. The model circuit is shown in Fig S1a. T and F represent the test and flank neurons respectively, separated by an orientation singularity (dashed line). Two distinct populations of neurons, C F and CT, provide local common input to F and T: C F represents other neurons in the same column as F, while C T