The Ebbinghaus figure is more than a size illusion

The effect that circular flankers have on judgements of a central disk's size is known as the Ebbinghaus illusion. It is not clear how the small effect of the flankers on peak grip aperture in grasping should be interpreted. It could be caused by either the use of the illusory size information for grip formation, or by other mechanisms, such as obstacle avoidance. If the latter interpretation is correct, the effect of the Ebbinghaus illusion on grasping will depend on the positions of the flankers relative to the movement of the digits. If the first explanation is true, the effect on grip aperture will be correlated with the effect on perceptual judgements. To distinguish between the two interpretations we used a display consisting of a central disk surrounded by four large or small flankers. The array of flankers could be rotated by 45 . When subjects made a perceptual judgement of the size of the central disk the effect of the size of the flankers was independent of their position. When they grasped the central disk the final grip orientation showed an effect of the flankers' positions. Peak grip aperture was influenced by the flankers' size. This effect disappeared when we corrected for differences in the grip aperture when touching the object. Moreover, the effects on grip aperture were not correlated with the perceptual effect. We conclude that the flankers of the Ebbinghaus figure have an effect on grasping that cannot be explained by a change in judged size.