The effect of “objecthood” in the Ebbinghaus illusion

In the Ebbinghaus illusion, observers overestimate the size of a target when it is surrounded by large flakers and compared with a control target surrounded by small flankers. We examined whether “objecthood” – the degree to which an object is a cohesive entity – plays a role in the illusion. We predicted that the magnitude of the illusion will decrease with decreased objecthood. To test this hypothesis, we presented observers with squares as target and flankers, and manipulated the degree of their objecthood in two ways; a) the squares were composed of different parts (i.e., corners or edges) in different trials. Corners usually produce more cohesive squares than edges because the former combine closure, symmetry and collinearity, whereas the latter lacks collinearity; b) the gap size between the object parts was varied, so that larger gaps produced less cohesive objects than smaller gaps. The participants adjusted the test target to match a control target in size. Our results show that increasing the gap size between object parts reduced the magnitude of the illusion. In addition, the magnitude of the illusion was greater when the target was composed of its edges compared with when it was composed of its corners. These results suggest that objecthood plays a role in the Ebbinghaus illusion.