Nonvisible Perception of Segments of a Hand-Held Object and the Attitude Spinor

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Abstract
When a rod is grasped at a place intermediate between its ends, the nonvisible perception of length by wielding can be directed to either side of the hand. The selective perception of the lengths of rod segments 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 attitude spinor connecting the rod's reference frame to the hand's. That hypothesis was tested in 3 experiments m which 8-10 subjects participated; asymmetry in the rod's mass distribution relative to the hand was induced either by the addition of a metal ring to one end or by grasping the rod at a place other than its midpoint. Planes of wielding, style of wielding, and object size were varied across the experiments. The results conformed to expectation: For a given asymmetric rod configuration, perceived length for attending to one direction from the hand (e g., above or left) differed from perceived length for attending to the other direction (below or right); for a given segment of an asymmetric rod, perception of its length did not differ as a function of us direction from the hand. In each experiment, variance in perceived partial length was accommodated by the rod's major eigenvalue and the spinor rotation angle, with rotation sense dictated by the direction of attention.