Selective perception by dynamic touch

Perceiving the length of a rod by dynamic touch is tied to the inertia tensor I-ij, a quantification of its resistance to rotational acceleration. Perception of the portion extending in front of the grasp has previously been ascribed to decomposing one component of I-ij by attention. The tensorial nature of dynamic touch suggests that this ability must be anchored wholly in the tenser. Three experiments show that perceived partial length is a function of two components of the tensor, one tied primarily to magnitude and the other tied primarily to direction, whereas perceived whole length is a function of a magnitude component alone. Dynamic touch is characterized in terms of a haptic perceptual instrument that softly assembles to exploit I-ij differently depending on the intention, producing 1:1 maps that are appropriately scaled for each intention.