Rotational Invariants and Dynamic Touch

Skilled users of a long-cane are quite fluid in registering the layout of surfaces that surround them. What they need to know, of course, concerns properties with consequences for behavior—whether the ground is sufficiently flat and solid to be negotiated safely, whether discontinuities in that ground surface are gaps that can be stepped over or brinks that must be descended. At first blush, this skill may seem like a remarkable cognitive achievement, one in which the unsighted individual has managed to associate tactile impressions with some basic spatial elements, perhaps plugging them into modified variants of the inferential algorithms presumed to serve vision. Such a characterization, however, ignores the fundamental informativeness of the haptic perceptual system in its own right. The skillful use of a long-cane is possible because the properties registered through touch are lawful and reliable. Wielding the cane and using it to strike and probe surfaces deforms the tissues of the hand, arm, and body. Despite moment to moment, incidental variation in the impressions on the skin, we argue that there must be reliable structure in the deforming tissue that informs about not only the probed surfaces but about the probe itself. The commonality between perceiving probes and perceiving by means of probes—more generally, between perceiving objects and perceiving by means of objects—reinforces the anchoring of what may seem like a highly specialized, extraordinary skill in the very ordinary operation of the basic haptic perceptual system. Our goal in the present chapter is two-fold. First, we hope to highlight the integral contribution of haptics to the abilities not only of the blind but also of the sighted in getting around in and knowing about their surroundings. Second, we will make a case for understanding those abilities as fundamentally perceptual rather than inferential, an understanding that rests on an appreciation of the physical underpinnings of wielding hand-held objects.