The influence of resizing direction and body representation conflict on perceived hand size following virtual reality hand illusions.

Body illusions demonstrate how body representation flexibly updates when visual body cues are manipulated to differ from the physical state of the limb. However, implicit hand representations in healthy cognition are already distorted relative to the true physical structure of the hand, with hand width typically overestimated and finger length typically underestimated. This study examined whether changes in body perception depended on the similarity between visual hand cues and these distorted implicit representations. Healthy participants (n=61) experienced four VR hand illusions: Matched Hands (i.e. wider hand appearance relative to true hand width, matching the implicit representation), Mismatched Hands (i.e. narrower hand appearance relative to true hand width, mis-matching the implicit representation), Matched Fingers (i.e. shorter finger length relative to true finger lengths, matching the implicit representation) and Mismatched Fingers (i.e. longer finger length relative to true finger lengths, mis-matching the implicit representation). We assessed changes in conscious body perception, direct and indirect body representation measures, and motor control. We predicted larger perceptual changes when visual cues aligned with distorted implicit representations. All four illusions altered perceived hand size (but not motor control); however, the effects were stronger when visual hand cues were enlarged than shrunken. Although resizing direction accounted for most of the differences between illusions, we found some evidence for an additional influence of matching; however, contrary to predictions, the mismatched conditions elicited stronger changes. Effects also varied across measurement types and were more pronounced for the fingers than hands. Altogether, these findings suggest that body representation plasticity is shaped by how visual hand cues differ from the physical hand, how visual cues differ from the implicit distorted representation, and which part of the hand is targeted. Further research is needed to disentangle these influences and determine which measures best capture changes in different body representations.