Tensorial basis to the constancy of perceived object extent over variations of dynamic touch.

Subjects wielded occluded rods, with or without attached masses, and reported the distances reachable with their distal tips. Experiments 1-3 compared wielding about the wrist, the elbow, and the shoulder. Experiments 4 and 5 compared free wielding, using the whole arm, with wielding only about the wrist. The two comparisons, respectively, were of spatial and temporal variations in the rod's rotational inertia. Perceived extent was found to be constant in both comparisons. This constancy was tied to the inertia tensor Iij defined about a point that remains a fixed distance from the object during wielding--an invariant of the spatially and temporally dependent patterning of mechanical energy impressed upon the tissues of the body. Discussion focused on the reciprocal action and perception capabilities of multisegmented limbs, the tensorial relations in the neurobiology of dynamic touch, and the strategy of understanding perceptual constancy through invariants.