Visuomotor Control Laboratory @ NASA Ames

We describe a novel technique in which color table animation in conjunction with a single base image can be used to generate a broad class of motion stimuli. We have applied this technique to the generation of drifting sine-wave gratings (and by extension, sine-wave plaids). For each drifting grating, sine and cosine spatial phase components are first reduced to 1bit/pixel by using a digital halftoning technique. The resulting pairs of 1-bit images are then loaded into pairs of bit planes of the display memory. To animate the patterns, the display hardware's color lookup table is modified on a frame-by-frame basis. Because the contrasts and temporal frequencies of the various components are mutually independent, a large number of two-dimensional stimulus motions can be produced from a single image file. We also analyze the effects on the stimulus of a variety of artifacts: the spatial artifacts produced by the halftoning process, the spurious motion signals produced by the interaction of these spatial artifacts and the temporal animation, the artifacts produced by intensity quantization (and inaccurate gamma correction), and, finally, the important but rarely considered artifact of nonlinear spatial summation. We propose a physical model to account for the observed failures of pixel independence that provides an excellent fit to our measurements.