Inside Out: Functional Flexibility in Assigning the Body's inside-outside

Abstract

Tactile surface perception is often assumed to reflect a fixed boundary between body and world, with the skin providing a stable oriented surface that separates inside from outside. We show that this assumption fails in functionally specific ways. Using moving tactile stimuli on the hand, participants perceived stimulus direction on the fingertips as reversed when hand posture changed - reporting motion as if it originated from the opposite side of the skin, while responses on the palm were less consistent and less sensitive to posture. Congenitally blind participants exhibited stable, individualized surface assignments on the fingertips, demonstrating that vision is not required to construct oriented tactile surfaces. However, visual experience modulated posture effects: sighted individuals showed posture-dependent, skin-orientation assignments driven by motion direction, whereas congenitally blind individuals showed posture-invariant assignments determined by functional use. These findings indicate that tactile surface perception is not globally fixed but dynamically inferred according to functional demands; rather than enforcing a purely geometric body-world boundary, the brain flexibly assigns where the surface is and which side we occupy.