Perception and Embodiment for Motion-Scaled Virtual Hands

Virtual reality (VR) provides a flexible platform for investigating human perception of augmented bodily abilities. While motion scaling of virtual limbs has been explored in previous studies, psychophysical detection thresholds and embodiment have rarely been examined together, and direct comparisons between motion enlargement and reduction remain limited. In this study, we systematically manipulated the motion gain of a virtual hand in an immersive VR environment and evaluated both detection thresholds for deviations from unity gain and embodiment, including the sense of ownership and agency. Fifteen participants took part in the experiment. The results showed that the 50% detection thresholds for motion gain were 1.18 for motion expansion and 0.86 for motion shrinkage. These detection thresholds were approximately symmetric about the unity gain. In contrast, embodiment ratings showed an asymmetric decline, with ownership and agency decreasing more steeply for motion shrinkage than for motion expansion. Perceptual detection and subjective embodiment therefore exhibited both consistency and divergence: the onset of significant degradation in ownership and agency occurred near the perceptual detection range, whereas beyond these thresholds, embodiment declined asymmetrically. These findings provide quantitative guidance for designing motion-scaled body augmentation in VR, highlighting the importance of considering not only detectability but also the direction-dependent robustness of embodiment.