Figure 5. Effect of color. For each observer, and at each of the two display durations, illusory motion was maximized for the grayscale RAP at left in (a) by holding black and white constant while varying the two gray values. Yellow and blue were then added to the light and dark gray values, respectively, while holding their luminances approximately constant, as shown at right in (a). Adding color caused an increase in matched speed for one of two observers in a display of duration 0.125 sec (b), and for two of three observers in a display of duration 1 sec (c). To collect the data shown in these graphs, the dark gray (or blue) level was held constant and the light gray (or yellow) level was varied in luminance as shown by the abscissa. Experimental procedures were similar to those of the experiment described in Fig. 2.