Modulation of a patterned stimulus elicits a visual evoked response (VER) that reflects the combined activity of many neural mechanisms. Underlying mechanisms with qualitatively different spatial features my have indistinguishable responses to the highly symmetric reversing checkerboard stimulus. However, additonal information about the spatial organization of mechanisms contributing to the VER can be obtained by using stimuli with a more complex spatial structure. For example, stimuli based on alternation between pairs of stochastic black and white textures may be constructed in which luminance, contrast, and higher-order local structure can vary independently. To explain the dependence of the measure VER on the parameters of the stimulus, it is necessary to postulate two qualitatively different mechanisms. One mechanism appears to be sensitive only to contrast changes across individual edges. A second mechanism must rely on a more complex local process, in whichedge-edge interactions arae important, and must be of formal nonlinear order at least four.