Another possibility is that the sensation of motion is actually a primary reinforcer itself and is inherently pleasurable independent of any associations it may have with other forms of stimulation. In this scenario, the sensation of motion is desirable and soothing because it is critical for primates to experience these types of stimulation for normal development. The fact that both motion and touch sensation are the earliest sensory capacities to emerge and are keenly dependent on stimulation during development suggest that newborns, toddlers, and children may seek out experiences of motion as a primary reinforcer that promotes the continued growth and maturation of these systems. Children, of course, are unaware of this relationship—they needn’t be consciously aware of why they enjoy certain forms of movement for this growth and maturation to take place. They only need to crave the experience enough so that they seek it out. This process is reminiscent of the relationship between our fondness for sweets and their ability to produce energy. Evolutionary mechanisms do not require that primates understand the biochemical reactions involved in converting sugars to ATP for them to benefit from the relationship. They only have to seek out sugar (for whatever reason) to give those individuals in a population who crave these high-energy food sources a selective advantage over their competing hominids.The end result is a contemporary population of sweet-toothed primates with an extraordinary means for fulfilling their biologically driven desires.
How Movement and Touch Promote Brain Development
The early emergence of touch and motion sensation is critical to the normal development of other parts of the nervous system. Because these sensory capacities have such an early onset, they organize other sensory and motor systems. Biologists have been moved by these findings to begin thinking of development as a cumulative process rather than a simple schedule of programmed events. For instance, children who have delayed vestibular system maturation tend to reach motor milestones such as crawling, sitting up, and walking more slowly than normal children, sometimes taking twice as long to pass these hurdles. Each of these behaviors depends on a sense of balance, and hence vestibular function becomes a critical force in organizing motor behavior.
Children born with deficits in touch and vestibular function also frequently have emotional and cognitive disturbances that often involve learning and memory, attention, visual-motor integration, language, and autism, to name just a partial list. Neuroscientists sensitive to this new view of development are now beginning to understand how early touch and vestibular experiences organize and jump-start the growth of so many other processes.
Although there is enormous variability in the way newborns respond to being touched, there is remarkable consistency from baby to baby in terms of their bodies’physiological response. Gentle touching or vestibular stimulation such as rocking initially produces a state of biological arousal that resembles the classic stress response. Increased brain-stem activation triggers a series of chemical cascades involving all the usual suspects—increases in cortisol, ACTH, and other stress hormones. These effects reverse, however, with continued stimulation. During prolonged touch or slow, repetitive movements, the brainstem systems that were once activated become inhibited, and slowly this process down-regulates the body’s stress system. The stress hormones that were at first elevated begin to fall with continued stimulation and actually decrease below normal baseline levels (measured while the baby is at rest and not being touched or moved).
Studies now show that down-regulation of the stress system continues long after the touching or rocking stops. The effects of touch and motion are enduring, measurable hours after stimulation has ceased, and it is this property that seems to have such a significant impact on the development of other systems. Stress hormones such as cortisol have been found to have deleterious effects on synaptogenesis and synaptic pruning in laboratory animals, and brain limbic regions seem particularly susceptible. Rodents and primates that have been subjected to elevated cortisol levels show significantly reduced brain volume in several limbic regions including the hippocampus when compared to those that are not stressed. Moreover, animals that are given these experimental manipulations repeatedly for periods as short as two weeks exhibit a range of behavioral problems as juveniles that often persist into adulthood, including deficits in learning ability, memory, attention, sensory-motor integration, solving tasks that require flexibility and adaptability, and regulating their emotions. This suggests that the pleasure of touch and motion contributes to normal brain growth by regulating stress hormone levels during development.
These findings have led some researchers to ask if there are protective benefits for developing animals (including humans) exposed to stimulation and experiences that lower stress hormone levels. The answer, of course, is yes. For instance, gentle daily massage of laboratory mice for a period as short as two weeks during their first three months of life results in faster maturation of several cortical brain regions (hippocampus, somatosensory cortex, cerebellum) when compared to controls not given the stimulation.These mice are also less fearful and prefer novelty more than controls that are not given the extra stimulation.
The implications of studies like this have not been lost on neonatal care units. Preterm babies are now routinely swaddled in an effort to improve their rate of maturation and ensure that the prerequisite tactile sensations needed for proper brain development are experienced as much as possible. Some hospitals have even gone a step farther and offer swaddling in conjunction with programmed movement stimulation that occurs on specially constructed infant waterbeds. Although there have not been many controlled evaluations of these treatments, those that have been conducted have found that swaddling and movement therapy significantly increase the rate at which preterm babies reach critical developmental milestones when compared to preemies given the usual care. These benefits include reported increases in a number of behavioral measures such as learning to crawl and walk earlier, increases in formula intake and weight gain, improved responsiveness to touch, better muscle tone, increased visual acuity, recognition memory, and attention, among many others.
Older babies also benefit from touch and motion stimulation. In one experiment babies aged three to sixteen months were given scheduled vestibular stimulation by being placed on their parents’ laps and rotated in a swivel chair for ten minutes each day over a period of two weeks. The babies, of course, loved the experiment—giggling and jumping during the rotation. Interestingly, the babies who were spun reached key developmental landmarks such as crawling and walking more quickly than their nonspun counterparts. This effect was even demonstrated in identical twins. The twin who received the motion stimulation began walking four months earlier than his brother.
Humans are programmed from birth to experience certain forms of touch and motion as intrinsically pleasurable. The roots of these hedonic preferences can be found in babies who are pacified by cradling and rocking; toddlers and children who self-stimulate their touch and vestibular systems using age-specific behaviors such as bouncing, rocking, and self-hugging; on through adolescents and young adults who have a need for speed. Like other hedonic preferences such as our desire for sweets, the pleasure we find in touch and motion satisfies critical developmental requirements for normal brain and behavioral maturation.The problem, however, is that technology has radically outpaced evolutionary pressures in the past two hundred years, leading to new and potentially harmful methods of satisfying this biological imperative. In chapter 11 we will consider how our need for touch and motion often couples with other hedonic preferences that together foster maladaptive behaviors such as addiction and thrill-seeking.
Chapter 5
In Praise of Odors
I will be arriving in Paris tomorrow evening. Don’t wash.
—
Napoleon to Josephine
All good kumrads you can tell by their altruistic smell.
—
E. E. Cummings
No other sense is so intimately bound to memory and emotion as smell. To this day, the mere hint of something sulfuric takes me back to a steamy August birthday and the gift of a Junior Scientist Chemistry Set from my mother. The blurb on the back of the box was encouraging: “Perform over 1,500 experiments and procedures in the gaseous phases of matter, chemical models, solutions, acids, bases, electrochemistry, organic chemistry and more.” For the next few weeks I couldn’t stop playing with this thing. Mom was thrilled and, I’m sure, convinced that one day I would be the next Louis Pasteur. But parents often forget that eight-year-old boys are not terribly interested in analyzing the covalent bond properties of solvents or learning how to neutralize an acid; they tend to like things that make loud noises, blow up, or best of all, some combination of the two.
Scouring the list of experiments one evening, I found a promising entry called “Outrageous Ooze,” which guaranteed an “explosive miniature volcanic reaction with real lava flow.” Mom was busy cooking and cleaning in preparation for a dinner party at our house that night, and Dad was out driving across the state and back to pick up some fancy German chocolate ice cream for dessert. I was given advance warning to be on my best behavior, and yes, my friend Hector could come over as long as we played in the back room.
Hector was the best lab assistant I ever had; he was always eager to see what happened if we mixed this and that, and had a natural talent for combining compounds that we were warned against mixing. A rule for toy manufacturers: The phrase “Warning—never combine Chemical A and Chemical B” is usually translated by eight-year-olds into “Attention—please do this immediately.” Just before the first guests began arriving I ran in the kitchen and asked my mother for some vinegar, baking soda, and dishwashing detergent. She hesitated for a moment, but then the doorbell rang and she didn’t have time to protest.
Back in the lab, Hector and I mixed the ingredients carefully. We stood back and waited for the Vesuvial display, only to be disappointed by the slow trickle that emanated pathetically from the jar, so we consulted the next paragraph, which instructed us to “incorporate the following mixture to produce hydrogen sulfide gas for extra realism.” I began heating the sulfuric mix before Hector finished reading the sentence, and suddenly we were treated to a thick display of smoke, and a horrible stench of rotten eggs began to fill the room. Even after removing the mixture from the heat, the pungent smell and smoke worsened and eventually, to my parents’ horror, spread throughout the entire house.The memory I usually associate with this smell today consists of my parents and the complete dinner party standing out in the street watching fetid-smelling smoke billow from the front and side windows of our small house.
The chemical senses of smell and taste are as phylogentically ancient as touch, and their age is given away by basic anatomy. Whereas the perceptual seat of sight and sound resides in the neocortex—a recent addition in mammals—the representations of the chemical senses are stowed away in the rather archaic limbic and paralimbic regions. This is true for humans, primates, and virtually all mammals large and small.
The cortical representations of smell and taste are located in regions of the brain long believed to be important for processing the motivational state of an animal as well as the emotional significance of external stimuli. Experiments have shown that when humans are stimulated through taste or smell, large portions of the brain that are critical for processing emotional information and memory become activated, including the amygdala, insula, cingulate cortex, and orbitofrontal cortex.
Let’s consider smell—it is the one sense that simply can’t be turned off without immediate consequences. We can close our eyes, cover our ears, shut our mouths, and refrain from touching things, but stop breathing for a moment and you quickly realize that we are all slaves to olfaction. Humans take more than 23,000 breaths each day, passing close to 450 cubic feet of air through their nose. Our nasal passages act as miniature wind tunnels powered by a respiratory vacuum that induces air molecules to enter with astonishing force. Odor molecules have a bumpy ride as they enter the nose—first heated by the frictional forces as they pass on either side of the septum and then thrust up through three complicated horizontal chambers shaped by vascular tissue. The turbulent journey ends at the roof of these interior passages as the molecules collide with a small patch of yellowish tissue on either side of the septum known as the
olfactory epithelia
. At this point, the air molecules have reached the brain.
Each cell in the olfactory epithelia—and there are hundreds of thousands of them—has receptors that are tuned to a particular odor. The shape of the odor molecule is what matters most. If an odor molecule has a shape, or a very close match, that allows it to bind to one of the many olfactory epithelia cells, it can cause that cell to send a signal in the form of an action potential on to the next stage of neural processing. The sole job of the olfactory epithelia cells is to convert chemical signals that find their way up our noses into electrical signals that the brain will understand. Although we generally think of our sense of smell as being rather limited compared to other mammals—dogs, for example—humans can perceive and distinguish differences among thousands of odors.
In her book
A Natural History of the Senses
, Diane Ackerman refers to smell as “the mute sense.” While we can detect and even perceive thousands of smells, we are woefully inept at describing them without reference to other things or, even more often, how they make us feel.This verbal shortfall may arise in part because the brain regions that register smells are only weakly and indirectly connected to those areas that support language processing. A more direct set of connections exists between areas that deal with emotions and language, and so the lexicon of smells is riddled with descriptions of how a smell makes us feel. Try to describe the smell of camphor without reference to a pine tree; or imagine explaining the smell of the ocean in the morning to someone who has never had the experience.