Critical color differences determined with a visual search task.

Response times were measured for a visual search task in which the observer was required to find a target that differed from distracting stimuli only in color. In the first experiment the search time was measured as a function of display density for both small and large color differences. With small color differences response time increased with display density, indicating a serial search, but with large color differences response time was constant, indicating a parallel search. In the second experiment the color difference required for parallel search was measured in eight different directions from the distracter chromaticity. These color differences were much larger than threshold color differences and were not well represented by the ellipse used to describe the threshold contour around a point in color space.