Eigenvectors of the inertia tensor and perceiving the orientations of limbs and objects

We report several experiments directed at the ability of humans to perceive the spatial orientation of occluded objects, to position an occluded limb relative to targets or directions in the environment, and to match the spatial orientations of occluded contralateral limbs. Results suggest that each of these abilities is tied to the inertial eigenvectors of each object or limb, which correspond to the object's or limb's principal axes of rotational inertia. Discussion focuses on the dynamic nature of proprioception, the importance of physical invariants for perception, and the relation of invariants to hypothesized frames of reference for proprioception and motor control. It is suggested that the detection of invariants revealed through movement is a major mechanism in kinesthetic perception involving intact limbs, neuropathic or anesthetized limbs, prosthetic devices, and hand-held tools and implements. The inertia tensor is identified as one such invariant.