The dissociation between perception and action in the Ebbinghaus illusion:Nonillusory effects of pictorial cues on grasp

According to a recently proposed distinction [1] between vision for percept ion and vision for action, visually guided movements should be largely immu ne to the perceptually compelling changes in size produced by pictorial ill usions. Tests of this prediction that use the Ebbinghaus illusion have reve aled only small effects of the illusion on grasp scaling as compared to its effect on perception [2-4], Nevertheless, some have argued that the small effect on grasp implies that there is a single representation of size for b oth perception and action [5], Recent findings, however, suggest that the 2 -D pictorial elements, such as those comprising illusory backgrounds, can s ometimes be treated as obstacles and thereby influence the programming of g rasp [6], The arrangement of the 2-D elements commonly used in previous stu dies examining the Ebbinghaus illusion could therefore give rise to an effe ct on grasp scaling that is independent of its effect on perceptual judgeme nts, even though the two effects are in the same direction. We present evid ence demonstrating that when the gap between the target and the illusion-ma king elements in the Ebbinghaus illusion is equidistant across different pe rceptual conditions (Figure 1a), the apparent effect of the illusion on gra sp scaling is eliminated.