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

new information and improve their sensory acuity with training. The implication of this is that adult brains are still plastic (although perhaps not as malleable as those residing in younger bodies), and that the need for specific kinds of sensory experiences continues to be important for brain maturation well beyond the early formative years.
    An example of this process can be found in our love of one particularly pleasurable somatosensory experience—the feeling of being in motion. Proprioception, the sense of the position and movement of one’s body, depends on signals from the skin surface as well as from muscles and joints. These signals travel through separate circuits in the lowest portions of the brain-stem and thalamus but eventually converge in the cerebral cortex with signals from the vestibular system. The vestibular system monitors changes in head and body posture relative to the Earth’s gravitational pull and an organism’s direction of motion. Since all organisms have had to orient themselves relative to these two elements during evolution, the vestibular system is thought to be as phylogenetically old as other components of the somatosensory system.
    Vestibular functioning begins at about the same embryonic period in primates as touch sensation. Even though Kai is still three months away from being born, his proprioceptive and vestibular systems are mature enough to function and send signals that converge in his prenatal cerebral cortex. This provides his earliest sensations of motion. It is striking that the integration of these systems begins to develop at about the same time that Kai’s general activity level cranks up to new extremes. Each night as Melissa settles down for the evening, he begins his prenatal gymnastics—head, leg, and arm flexion and extensions produced with seemingly endless repetition. By this point in gestation, Kai also attempts to compensate for sudden changes in his mother’s position—when she stands up or rolls over—by rapidly extending his arms and legs, a response known as Moro’s reflex.After he is born, our son—like all newborns—will quickly succumb to the pleasures of motion. His earliest sensory satisfaction will be found in the touch and warmth he feels when being gently caressed, and the soothing comfort he’ll find in motion will run a close second.
    Babies enjoy the sensation of motion from the moment they are born. Newborns are pacified by rocking, gentle swaying movements, and by being carried around the house, while toddlers enjoy virtually all forms of repetitive motion—particularly jumping up and down for hours in those baby bouncers. Older children graduate to more sophisticated methods of satisfying their thirst for motion—carousels, tricycles followed by bicycles, skating; the list goes on and on. Humans of all ages seem to have an innate fondness for motion, but the way this desire is manifested clearly depends on age and other developmental factors, as well as on cultural norms. The inborn pleasure we take in motion can clearly have detrimental effects when satisfied in the “wrong” way, such as by speeding or other forms of thrill-seeking that endanger personal safety.
     
     
    We’re often told that newborns and infants are soothed by rocking because this motion emulates what they experienced in the womb, and that they must take comfort in this familiar feeling. This may be true; however, to date there are no compelling data that demonstrate a significant relationship between the amount of time a mother moves during gestation and her newborn’s response to rocking. Just as plausible is the idea that newborns come to associate gentle rocking with being fed. Parents understand that rocking quiets a newborn, and they very often provide gentle, repetitive movement during feeding. Since the appearance of food is a primary reinforcer, newborns may acquire a fondness for motion because they have been conditioned through a process of