Perceptual judgments of relative depth from binocular disparity are systematically distorted in humans, despite in principle having
access to reliable 3D information. Interestingly, these distortions vanish at a natural grasping distance, as if perceived stereo depth is
contingent on a specific reference distance for depth-disparity scaling that corresponds to the length of our arm. Here we show that the
brain’s representation of the arm indeed powerfully modulates depth perception, and that this internal calibration can be quickly
updated. We used a classic visuomotor adaptation task in which subjects execute reaching movements with the visual feedback of their
reaching finger displaced farther in depth, as if they had a longer arm. After adaptation, 3D perception changed dramatically, and became
accurate at the “new” natural grasping distance, the updated disparity scaling reference distance.We further tested whether the rapid
adaptive changes were restricted to the visual modality or were characteristic of sensory systems in general. Remarkably, we found
an improvement in tactile discrimination consistent with a magnified internal image of the arm. This suggests that the brain
integrates sensory signals with information about arm length, and quickly adapts to an artificially updated body structure. These
adaptive processes are most likely a relic of the mechanisms needed to optimally correct for changes in size and shape of the body during ontogenesis.
