Touch is often called the “mother of the senses” because it is believed to have been the first sensory ability to have evolved—the sense upon which all others have been based. Well before the appearance of primates and even before mammals, touch existed in the earliest unicellular organisms, such as the predecessors of the modern coliform bacteria
E. coli
. This phylogenetic observation has not been lost on developmental biologists, who point out that the skin is the first organ to develop embryologically in complex, multicellular organisms, and it is their first portal of communication. Touch, the sensation that is most intimately associated with the skin, is the earliest sense to develop in all species of mammals, birds, and fish. Before a primate embryo is even an inch long from head to rump—long before it has eyes and ears—it will respond to gentle stroking of its lips and outer nose by extending its trunk away from the source of stimulation.
In the human embryo, tactile reactivity begins at about six weeks of gestation. As with all primates, a human embryo first responds to touch around its lips and nose, and gradually, with further development, begins to sense touch on other parts of its body. By nine weeks of gestation it will respond to touch of its fingers and hands, and by twelve weeks it will curl its toes if the soles of its feet are stroked. This well-known head-to-toe progression occurs in all primates and results from two important forces involving brain development.
First, lower brain-stem regions develop and begin to function well before neocortical areas of the brain come online.This growth process limits the sensory and behavioral capabilities of the newborn to those supported by the brain regions that are developed and connected enough to show some degree of functionality. The reflexive turn of the embryo at six weeks of gestation is controlled predominantly by brain-stem circuits without any of the information being passed on to “higher” neocortical sites. In other words, the embryo will react when its mouth or nose is stroked at six weeks of gestation, but it will have no conscious perception of the experience. For touch to be perceived, the embryo will have to wait an additional six weeks for those higher sites to develop and begin to exhibit even the most basic physiological functions.
The second constraint that influences the head-to-toe development of touch sensation is found in the way bodily sensations are mapped in the brain and how this organization changes with experience. Anyone who has taken an introductory psychology course has seen the grotesquely shaped homunculus with its exaggerated lips, nose, hands, and genitalia. The relative size of each body part is drawn to be proportional in size to the amount of cortex devoted to processing information from that area. The point is, not all portions of the body have equal representation in the brain—it is not a democracy. To understand how brain maps of the sensory world develop and how this process 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 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 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 associative learning (see chapter 3).