Grouping and “objecthood” effects in the Ebbinghaus illusion

We examined the effect of grouping processes on the Ebbinghaus illusion by manipulating objecthood: the degree to which an object is perceived as a cohesive entity. We presented observers with squares as targets and inducers and manipulated the degree of their objecthood: a) squares were composed of different parts (i.e., corners or sides). Corners usually produce more cohesive squares than sides because the former combine closure, symmetry and collinearity, whereas the latter lacks collinearity; b) gap size between 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 the magnitude of the illusion decreased with increasing gap size. Also, overestimation of target size was greater when the target was composed of its sides compared with its corners. These results suggest that objecthood and mid-level processes play a role in the Ebbinghaus illusion.