The two-dimensional spatial structure of nonlinear subunits in complex cell receptive fields

We have used a modified two-dimensional (2D), Gaussian-white-noise spatial stimulus to examine receptive field (RF) properties in 83 complex cells in cat striate cortex. We determined the 2D first-order receptive fields along with 2D second-order Wiener-like kernels (WLK), which depict the distribution of nonlinear spatial interactions. We have also measured 2D frequency domain response profiles for 31 of these complex cells. Spectral response profiles consisted of one or two smooth, single-peaked responsive regions, often elliptically shaped with a long axis pointing towards the origin. First-order spatial RFs consisted of overlapping, single-peaked smoothly decaying responsive regions to both bright and dark stimuli. For approximately two-thirds of the cells examined, second-order spatial WLKs consisted of a central, elongated subregions of augmented response flanked by one or two elongated subregions of suppressed response. This highly structured arrangement was found to exist at many locations across the cells RF and is thus termed a subunit. The spatial structure of the subunits was found to be consistent with or responsible for peak spatial frequency and orientation selectivity. Subunits may be responsible for complex cell\u27s phase insensitivity. No structural organization was found in the remaining one-third of the cells examined