EIGENVALUES OF THE INERTIA TENSOR AND EXTEROCEPTION BY THE MUSCULAR SENSE

Muscle spindles and Golgi tendon organs constitute the receptor founda tion to the ''muscle sense.'' Muscle sensitivity has long been assumed relevant to the non-visual perception of the positions and motions of the body's segments and of the properties of hand-held objects. Dynam ic touch is the label given to the particular kind of tactile exterope rception that involves a non-spatial input from muscles and tendons. W hen a hand-held object is wielded, hefted, carried and so on, the hand movements, together with the physical properties of the object, produ ce torques and angular motions that change in time with the movement. There is, however, an unchanging quantity that relates the variable to rques and angular motions, namely, the object's inertia for rotation a bout a fixed point in the wrist. Our research revealed that the non-vi sual perception of the length of a wielded object by dynamic touch is a function of muscular sensitivity to the principal moments or eigenva lues of the inertia tenser. Across four experiments, variations in obj ect length were accompanied by variations in width, spatial and materi al heterogeneity, the relation of the tensorial components to mass, an d geometric shape. Subjects had no foreknowledge of the variations in object dimensions. Perceived lengths of occluded objects were reported by adjusting a visible marker so that its position corresponded to th e position of the felt end of the object. In each experiment, perceive d length was closely related to actual length and uniquely constrained by the major and minor eigenvalues of the inertia tenser. The present results, in conjunction with previous research, suggest that the iner tia tenser provides the domains for two sets of functions realized by the ''muscular sense,'' one consisting of the principal moments of ine rtia or eigenvalues, which map on to perceived object magnitudes (e.g. length, weight), and one consisting of the principal directions or ei genvectors, which map on to perceived relations between hand and objec t (e.g. position of grasp). The significance of information-perception specificity over cognitive mechanisms is underlined and perspectives on dynamic touch and its underlying muscular sensitivity, including a general tensorial analysis, are discussed.