Some Illusions Are More Inconsistent Than Others

Abstract

Illusions are characterized by inconsistencies. For instance, in the motion aftereffect, we see motion without an equivalent change in position. We used a simple pencil-and-paper experiment to determine whether illusions that influence an object’s apparent size give rise to equivalent changes in apparent positions along the object’s outline. We found different results for two equally strong size illusions. The Ebbinghaus illusion affected perceived positions in a way that was consistent with its influence on perceived size, but a modified diagonal illusion did not affect perceived positions. This difference between the illusions might explain why there are so many conflicting reports about the effects of size illusions on the maximum grip aperture during reach-to-grasp movements.

Keywords: Sander illusion; Titchener illusion; grasping; inconsistency; perception/action; size; two visual systems; visual illusions.

Figure 1.

Determining two illusions’ effects on perceived positions and sizes. (a) Ebbinghaus illusion with the first set of instructions. (b) The Ebbinghaus illusion affected judgments based on positions to a similar extent as those of size (diameter). (c) The modified diagonal illusion (right line seems longer due to the surrounding shape and additional white dots) with the second set of instructions. (d) The modified diagonal illusion only affected judgments of size (length). Text in (a) and (c) is not to scale. Error bars in (b) and (d) indicate 95% confidence intervals.