PROCESSING SPEED IN THE MOTION-INDUCTION EFFECT

The motion-induction effect, where an illusory motion is perceived wit hin a bar when it is shown next to a spot presented slightly earlier, was studied with respect to the idea that it is based on differential processing speeds between the two ends of the bar. First, by using jus t a bar with a luminance gradient, the existence of a motion illusion (gradient motion) within such a bar was demonstrated, presumably due t o the different processing speeds of differential luminances. When suc h a bar was used in the motion-induction effect, it was shown to modul ate, for short delays, the strength of the effect up or down, accordin g to the direction of the gradient with respect to the position of the spot. When the same bar was used in the double-motion-induction effec t (split priming), in which motion is usually away from the later spot , it totally determined the perceived direction of illusory motion, in dependently of gradient direction with respect to the later spot or th e time between the two spots. These results demonstrate, on the one ha nd, that differential local processing speed is a likely mechanism to underlie the motion-induction effect. On the other hand, they also sug gest the involvement of other more global (and perhaps top-down) proce sses.