Hands-on adaptation: Bodily stimuli increase size adaptation aftereffect

The size adaptation aftereffect is a perceptual phenomenon in which a stimulus is perceived as smaller (or larger) after exposure to a larger (or smaller) stimulus. Given that size perception of body parts is computed with the highest accuracy for biological reasons, it is currently uncertain whether these are differently susceptible to illusory size misperceptions, such as those induced by adaptation paradigms. We induced the Uznadze illusion (i.e., a size-contrast adaptation aftereffect) to investigate its effect over stimuli depicting body parts (Hands) or non-body stimuli (i.e., abstract shapes). In three experiments, pairs of Hands or Non-Hands were presented in separate sessions. After repeated exposure to two stimuli with different sizes, one larger and one smaller, participants judged the size of two new stimuli. In Experiments 1 and 2, we found a stronger effect for Hands than Non-Hands. In Experiment 3, we enhanced the similarity between Hand and Non-Hand stimuli, and we confirmed a stronger adaptation for Hands, but only when participants performed the task with Non-Hand stimuli in the first session. These results indicate that visual hand stimuli would be more susceptible to size adaptation, suggesting that the identity and meaning attributed to the stimulus can influence the perceptual aftereffect.