The two-dimensional spatial structure of nonlinear subunits in the receptive fields of complex cells.

We have estimated the second-order response properties of complex cells in two spatial dimensions by cross-correlating their spike trains with a binary approximation of a Gaussian white noise stimulus ensemble. Wiener-like kernels were computed and generally consisted of two or three parallel, elongated subregions alternating between augmented and suppressed response. These subunits were scattered across the receptive fields of complex cells and their axes of elongation agreed with the optimal orientation determined with drifting gratings.