Figure 3. Spatial consequences of isotropic lateral interaction zone in V1.

(a) A simple geometry of V1 is assumed with cortical hypercolumns arranged in a hexagonal mosaic. The receptive fields of the computational elements within the hypercolumns scale up linearly with eccentricity. Each hypercolumn is assumed to have lateral (long-range horizontal) connections within an isotropic neighborhood of hypercolumns on the cortex (lateral interaction zone). The radius of the neighborhood in cortical distance, or equivalently, the number of hypercolumns, is independent of eccentricity. (b) The extent of the lateral interaction zone is projected to visual space for three reference hypercolumns at eccentricities 2°, 4° and 6°. The radius of the zones is 6 hypercolumns as suggested by several studies (Online Methods). (c) Half the end-to-end distances of the interaction zones along the radial axis (the line joining the receptive field center of the hypercolumn to the fovea) are plotted against the eccentricity of the corresponding reference hypercolumn. The dotted line is the prediction of Bouma’s law (Fig. 1b; Bouma, 1970). (d) The radial distance from the receptive field center of a reference hypercolumn to the outer extremity (d_out) and to the inner extremity (d_in) of the interaction zone is plotted against the eccentricity of the reference hypercolumn. That d_out is always greater than d_in (for non-zero eccentricities), explains the inward-outward asymmetry.