NONVISIBLE PERCEPTION OF SEGMENTS OF A HAND-HELD OBJECT AND THE ATTITUDE SPINOR

When a rod is grasped at a place intermediate between its ends, the no nvisible perception of length by wielding can be directed to either si de of the hand. The selective perception of the lengths of rod segment s relative to the hand is hypothesized to depend jointly on the rod's inertia tensor about a fixed rotation point and on the 2-valued attitu de spinor connecting the rod's reference frame to the hand's. That hyp othesis was tested in 3 experiments in which 8-10 subjects participate d; asymmetry in the rod's mass distribution relative to the hand was i nduced either by the addition of a metal ring to one end or by graspin g the rod at a place other than its midpoint. Planes of wielding, styl e of wielding, and object size were varied across the experiments. The results conformed to expectation: For a given asymmetric rod configur ation, perceived length for attending to one direction from the hand ( e.g., above or left) differed from perceived length for attending to t he other direction (below or right); for a given segment of an asymmet ric rob, perception of its length did not differ as a function of its direction from the hand. In each experiment, variance in perceived par tial length was accommodated by the rod's major eigenvalue and the spi nor rotation angle, with rotation sense dictated by the direction of a ttention.