Free The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music by Gene Wallenstein Page A

influences behavior, we should first think about how touch sensation works.

Sensations
    Physical sensations arise from the stimulation of a variety of receptors distributed throughout the body. Although we often think of touch as tactile sensation, it actually has four forms: discriminative touch, which is used to identify the shape, size, and texture of objects and their movement across the skin; proprioception, which gives us a sense of body position and movement through space; nociception, which signals that tissue has been damaged or irritated and is perceived as pain or itch; and temperature, which indicates relative warmth and coolness. Each of these sensory modalities is regulated by a distinct set of receptors and brain circuits that comprise the somatosensory or touch system.
    When we talk about touch, we are typically referring to the most colloquial use of the term, which implies tactile sensations. By the time my son is old enough to grab his first teething ring, his sense of touch will be far more developed than any of his other senses. As he squeezes the ring, touch and temperature receptors embedded in his skin become activated and originate electrochemical signals that journey up his spinal cord to his brain-stem. From there, the signals travel to his thalamus and finally his somatosensory cortex, where the activation results in the conscious perception of the tactile and temperature qualities of the ring.
    But all of this is still months away because Kai is a fetus, and his somatosensory cortex is still in its earliest stages of development. Eventually this region of his brain will become a highly ordered map of his skin surface. But this process will depend on his ability to stimulate these early circuits. A very active area of research over the past twenty years has been aimed at understanding how early life experiences shape the way these topographical maps form in the brain.
    We’ve known for some time that brain map development is highly sensitive to experience, particularly during the periods of synaptogenesis and synaptic pruning. Psychologist William Greenough has popularized the idea that brain maps are shaped by two predominating forces during development (and probably even in adulthood)—those that are experience-expectant and those that are experience-dependent. Experience-expectant interactions are those forms of stimulation that all humans must experience to ensure normal brain development. Experience-expectant stimuli fine-tune the brain during major growth periods, and in a very real sense pick up where the primate genome, limited in its capacity to precisely specify each developmental detail, leaves off. In contrast, experience-dependent interactions involve experiences that are unique to individuals—for example, information about their personal identity, familial structure, and the particular social mores that exist in their community.
    During synaptogenesis and the prolonged period of synaptic pruning, the brain is an experience-expectant organ par excellence. Mammalian genetic code provides only enough information to build a beginner mapping between body surface and each somatosensory brain region; the rest of the job depends on stimulation. Mice, for instance, use their whiskers to sense objects and each other much like primates use their hands. A mouse’s cerebral cortex contains a very detailed somatosensory map of its whisker region—topographically organized into rows that correspond to the way its whiskers are organized on its face. Each whisker has a small cortical region shaped like a barrel (hence they are referred to as cortical barrels) that forms in the first few days after birth. If, however, a whisker follicle is removed during this period, the corresponding cortical barrel fails to develop. Instead, adjacent cortical barrels that correspond to adjacent whiskers encroach and take over the space that was once devoted to the plucked whisker. Thus, whisker sensation is