Perceiving aperture size by striking.

This study examined the ability to perceive surface separation on the basis of mechanical stimulation resulting from striking the surfaces' interiors with a hand-held rod. The variables manipulated were aperture size, angular displacement theta, distance b of the point of contact with the surfaces from the axis of rotation, hand-rod mass m, location of the hand-rod's center of mass a, and moment of inertia Io of the hand-rod. Given a particular rod and an aperture at a given distance, lambda = sin(theta/2)[1 - (2a/b) + (ma2/Io)] was invariant over explorations. Perception of aperture size was specific to lambda; it predicted successfully the interdependent effects of theta, b, and the mechanical properties of the implement. The results of 7 experiments were discussed in terms of the specificity of perception to information and the general ability to perceive distant things by means of body appendages.