IN THIS CHAPTER ON THE SENSE OF TOUCH, we focus on the hand because of its importance for this modality, in particular its role in the appreciation of object properties and in performance of skilled motor tasks. The human hand is one of evolution’s great creations. The fine manipulative capacity provided by our fingers is possible because of their fine sensory capacity; if we lose tactile sensation in our fingers, we lose manual dexterity.
The softness and compliance of the glabrous skin play a major role in the sense of touch. When an object contacts the hand, the skin conforms to its contours, forming a mirror image of the object’s surface. The resultant displacement and indentation of the skin stretches the tissue, thereby stimulating the sensory endings of mechanoreceptors at or near the region of contact.
These receptors are highly sensitive and are continually active as we manipulate objects and explore the world with our hands. They provide information to the brain about the object’s position in the hand, its shape and surface texture, the amount of force applied at the contact points, and how these features change over time when the hand or the object moves. The fingertips are among the most densely innervated parts of the body, providing extensive and redundant somatosensory information about objects manipulated by the hand.
Moreover, the anatomical structure of the hand, with its multiple joints and apposable digits, enables humans to shape the hand in ways that mirror an object’s overall shape, providing a hand-centered proprioceptive representation of the external world. This ability to internalize the shape of objects allows us to create tools that extend the abilities of our hands alone.
When we become skilled in the use of a tool, such as a scalpel or a pair of scissors, we feel conditions at the working surface of the tool as though our fingers were there because two groups of touch receptors monitor the vibrations and forces produced by those distant conditions. When we scan our fingers across a surface, we feel its form and texture because another group of mechanoreceptors has high spatial and temporal acuity. A blind person uses this capacity to read Braille at a hundred words per minute. When we grip and manipulate an object, we do so delicately, with only as much force as needed, because specific mechanoreceptors continually monitor slip and adjust our grip appropriately.
We are also able to recognize objects placed in the hand from touch alone. When we are handed a baseball, we recognize it instantly without having to look at it because of its shape, size, weight, density, and texture. We do not have to think about the information provided by each finger to deduce that the object must be a baseball; the information flows to memory and instantly matches previously stored representations of baseballs. Even if ...