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

required for the normal development of whisker representation in the mouse brain.
    Interestingly, if a select group of whiskers is given additional tactile stimulation during the first five days of life, their associated cortical barrels grow larger than those associated with less used whiskers. Hence, extra stimulation of specific whiskers leads to a larger portion of the brain devoted to processing their tactile information. Together, these experiments demonstrate the critical importance of sensory experience during brain development. Lack of stimulation during a critical period can lead to stunted growth of the corresponding brain region and a loss of function, while extra stimulation can facilitate growth.
     
     
    An interesting finding with practical implications from the past few years is that stimulation in one sensory form, such as touch, often leads to improvements in other areas, such as learning and memory, and these effects can last into adulthood. For instance, when mice are placed in enriched environments filled with interesting objects to scramble over and make contact with, their cortical whisker barrels grow significantly larger than those of mice from control groups living in sparser conditions without increased opportunities for stimulation. Surprisingly, enriched mice also end up being “smarter” adults compared to their impoverished cousins even if they were exposed to the enriched environment for only a brief time. Adult mice that were raised in enriched environments for durations as short as one month as juveniles perform far better than mice reared in more sterile conditions on practically every task designed to assess learning ability in rodents.
    Additional studies have since shown a second important observation. When young mice are given a choice between an enriched environment (with toys, bedding, water, and chow) versus a typical environment consisting simply of bedding, water, and chow, the vast majority spend significantly longer periods of time in the enriched environment. This effect persists into adulthood, but becomes less significant in aged animals. These studies tell us that rodents have an innate preference for enriched surroundings where more opportunities for stimulation exist, and that this type of stimulation improves brain growth and functioning.
    Of course, any parent will tell you the same is true of us primates. Hedonic preferences have evolved in every sensory modality to nudge us toward environments and behaviors that satisfy the experience-expectant requirement for normal brain development. Ultimately, the sequence of behaviors that emerge in human infants, toddlers, and throughout childhood is a guide to what experiences the brain needs to fine-tune itself and function effectively in the particular ecological niche in which its owner resides. To this end, one might ask why certain forms of touch sensation are pleasurable and clearly preferred over others, and how these kinds of experiences help wire the brain.

The Evolution of the Rocking Chair
    The kinds of experiences sought out by newborns, toddlers, and children are not simply a random collection of idiosyncratic tendencies that vary from individual to individual. Rather, the preferences for certain forms of stimulation (such as motion), and the periods during development when they emerge, are fairly consistent across individuals and cultures. This is exactly what one would expect if these patterns of stimulation are critical for the development of brain systems involved in touch, and these systems, in turn, have provided selective advantages during the evolution of our species.
    While early touch sensation is required for normal somatosensory development, it is unclear how long the critical period lasts in humans. Some recent studies have shown that the brain systems involved in touch continue to change as a result of experience well into adulthood. This makes intuitive sense, since adult primates can obviously learn