Peripheral Neuropathy and Length Perception by Dynamic Touch

Dynamic touch is involved whenever an object is grasped and forces are imposed in order to move, stabilize, or carry it. Given its role in manipulating commonplace tools and utensils such as forks and hammers and coffee cups, dynamic touch can be considered critical to everyday function. A good deal of research suggests that time-invariant moments of the mass distribution of hand-held objects, extracted by the nervous system during movement, constrain per-ception of object properties by dynamic touch. Nonvisual perception of object length, for example, is easily accomplished without practice or feedback and is constrained by the object’s resistance to being moved—quantified by its inertia tensor, Ik (Fig. 1). One particularly robust finding is that the maximum principal moment of inertia 11 is the primary constraint on muscle-based perception of the length of a wielded object. The sensory disorder of peripheral neuropathy poses an interesting chal-lenge for muscle-based perception. In extreme cases of the condition, individu-als cannot feel anything touching their limbs, cannot feel anything their limbs are touching, and cannot feel the limbs themselves. Although full neuropathy is Figure 1. (a) The motions of a wielded object vary over time as do the torques used to produce the movements. (b) The inertia tensor couples the motions and torques, quantifying the resistance to being moved in different directions. (c) When the tensor is transformed relative to symmetry axes, the principal moments of inertia remain. rare, partial neuropathy (typically affecting the extremities) is a fairly common consequence of aging and diabetes. Clearly, older adults and individuals with partial neuropathies continue to engage in behaviors that require them to ma-nipulate objects. But to what extent is their everyday functioning compromised? Insight into this question was provided by an evaluation of dynamic touch in a patient with pronounced neuropathy in one arm