A computer-controlled instrument that creates complex two-deminsional patterns on a CRT monitor is described. These patterns are used to elicit visual evoked resposnes. Patterns are produced on a raster that is rotatable about its center. It is possible to assign to arbitrary regions in the raster any of four independent one-dimensional spatial temporal functions. For each spatial-temporal function, the experimenter can select an arbitray spatial profile, the spatial frequency of the profile, the starting phase of the profile, the temporal function, and the depth of modulation.