Role of the inertial eigenvectors in proprioception near the limits of arm adduction range of motion

Common everyday activities such as reaching for a cup, combing one's hair, or positioning a tool involve the accurate registration of the limb's orientation relative to the body and the environment. Past work has demonstrated that the ability of humans to perceive the spatial orientation of an occluded arm is tied to the arm's inertial eigenvectors, invariant mechanical parameters corresponding to a limb's axes of rotational symmetry. In a recent study participants pointed to visible targets while the arms' inertial eigenvectors were displaced by asymmetrically weighted splints. The lack of an effect of those splints on pointing error led them to conclude that inertial eigenvectors are not the primary invariants used for proprioception. However, their data suggested an effect of the eigenvectors, but only for conditions in which pointing did not require placing the arm near the limits of shoulder adduction range of motion. In the present study we found a significant effect of displacing the eigenvector for all tested arm angles, regardless of degree of shoulder adduction. Overall, the results supported the hypothesis that proprioception is based on sensitivity to the limb's inertial eigenvectors, with no reduction of the eigenvector effect when adducting the arm so as to reach far across the body midline.