Can we tell where our bodies end and the world begins? Evidence for precise three dimensional internal body models

Abstract

Distinguishing our body from the external world is crucial for our sense of self, environmental interaction, and sensory processing. Yet the accuracy with which we perceive this boundary remains underexplored. Here, we developed a novel psychophysical protocol to directly assess how accurately individuals perceive their body boundaries. Participants were asked to determine whether the midpoint between two tactile stimuli applied on the skin was inside or outside their perceived body boundary. 3D scans of the tested regions were used to determine objective anatomical boundaries, allowing psychometric functions to be fitted. Results revealed remarkable overall accuracy, on average within millimeters, in localizing body boundaries across multiple body regions. However, accuracy was not uniform: while palm boundaries were localized with near-perfect precision, stimuli straddling the wrist boundaries were frequently misjudged as extending beyond their true anatomical limit, revealing a systematic perceptual bias. Although perceptual judgments adapted to changes in posture, accuracy declined when the detailed local 3D structure was disregarded, indicating that proprioceptive cues are combined with detailed local body models. Finally, participants whose anatomy deviated from the average tended to align their responses with a typical body model rather than their unique physiology, suggesting that top-down processes influence boundary judgments. Together, our findings suggest that body boundary representation combines detailed three-dimensional body models with proprioceptive feedback into an integrated perceptual model of the anatomical body.