Beyond five: How the experience of a supernumerary sixth finger alters action perception.

When interacting with the world, one's bodily features are estimated in relation to the objects we handle. Perceived changes to one's hand size and shape alter the perception of the actions that can be performed with that hand. Here, we investigated whether experiencing the illusion of a sixth finger on the hand alters individuals' perception of their action abilities and movement boundaries. After inducing the sensation of having a sixth finger, healthy adult participants estimated the smallest aperture their hand could fit through and the largest object they felt capable of grasping. Our results show that the illusion of a sixth finger expanded both the smallest aperture that participants believed their hand could pass through and the largest object they felt capable of grasping. Importantly, these effects occurred even in participants who did not explicitly report feeling a sixth finger, indicating that the representation of the extra digit operates at both implicit and explicit levels. These findings demonstrate that hand representation and the perceived action capabilities and boundaries it defines can be rapidly and flexibly updated in response to the illusion of an additional finger, highlighting the plasticity of the human body schema. (PsycInfo Database Record (c) 2026 APA, all rights reserved).