Fig. 1

Methods for assessing the Ebbinghaus illusion. a Traditional method, where participants are asked whether the two stimuli have central circles that are the same size or not, and/or to judge which stimulus has the largest central circle. In this example, the central circles are identical in size. b Two-alternative-forced-choice method as described by Morgan et al. [29]. Participants are asked which of two sequentially presented comparison stimuli (the standard or test) has a central circle that is most similar in size to that presented in the reference. In this example, the central circle in the standard is 5% smaller than in the reference and the test is 4% larger than the standard