The role of the inertia tensor in kinesthesis.

We report 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 orientation 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. It is suggested that the mechanisms supporting the perception of intact limbs, neuropathic or anesthetized limbs, prosthetic devices, and hand-held tools and implements via kinesthesis may be one and the same--the detection of movement-produced physical invariants such as the inertia tensor. While the research reported presently has focused on the perception of intact limbs and hand-held objects, future research should be directed at possible generalizations of this work to a variety of clinical populations, including those involving peripheral neuropathies and prosthetic devices.