By David J. Linden
Viking. 261 pp. $28.95
Of all the gifts that a parent can give a child, one of the most important is a simple, loving touch. Babies who are deprived of human touch, such as those who spend their early years in understaffed orphanages, display profound abnormalities. Their growth is stunted, and they’re slow to gain cognitive and motor skills. They display repetitive, self-soothing behaviors, such as rocking endlessly back and forth. They are more likely to develop obesity, diabetes and heart disease, or suffer from mental illnesses ranging from depression to psychosis. But the smallest of interventions — as little as 20 minutes of gentle physical contact a day — can help touch-deprived infants avoid the worst outcomes.
The implication, neuroscientist David J. Linden says, is clear. “Touch is not optional for human development,” he writes in his accessible and engaging new book, “Touch.” Linden studies learning and memory, not touch, but was inspired to write the book because he became “an unabashed fanboy of touch research.” His enthusiastic tour of an oft-overlooked sense is a reminder of the central role that touch plays in our lives. “From consumer choice to sexual intercourse,” he writes, “from tool use to chronic pain to the process of healing, the genes, cells, and neural circuits involved in the sense of touch have been crucial to creating our unique human experience.”
Linden begins with the biological basics, explaining that our skin is studded with sensors that allow us to experience an exquisite range of touch sensations. Four types of “mechanoreceptors” are specialized for detecting certain kinds of mechanical stimuli, such as vibration and pressure, while free nerve endings respond to temperature, pain and certain chemicals. All these tactile sensations are transmitted from the skin to the spinal cord and brain, where the streams of information from different nerves are integrated and processed, largely in regions known as the primary and secondary somatosensory cortices.
But the story of touch doesn’t end there. The somatosensory cortices also communicate with the brain’s emotional-processing areas, which help imbue a tactile sensation with meaning. Context matters. As Linden explains, “For most of us, the feeling of a finger tracing our lips is delightful and arousing in a romantic setting with a lover but decidedly unerotic when it takes place in the doctor’s office.”
Touch and emotion are so intertwined that tactile information can cease to make sense when severed from its emotional meaning. This is particularly true in the case of pain. Patients who have damage in the posterior insula or the dorsal anterior cingulate cortex — brain regions involved in processing the emotional aspects of pain — can identify painful stimuli, accurately reporting, for instance, that they’re being pricked by pins on their palms. But they don’t find the sensations unpleasant and might even smile calmly as they’re jabbed. “Pain simply has no emotional resonance for them,” Linden writes.
On the other hand, patients who have damage to the somatosensory cortices, where the basic properties of a painful stimulus are processed, may react emotionally when they’re injured but be unable to identify what kind of pain they’re experiencing — burning, sharp, dull — or where on their bodies it hurts.
Our tactile behavior is shaped by our biology and our emotions, but it’s also influenced by our culture, as psychologist Sidney Jourard discovered when he watched people conversing in coffee shops scattered across the globe. Americans, he found, were not particularly demonstrative, though the Brits were even less so. “Jourard found that couples in San Juan, Puerto Rico, touched an average of 180 times per hour, compared with 110 times per hour in Paris, 2 times per hour in Gainesville, Florida, and 0 times per hour in London,” Linden recounts.
Though Linden can occasionally get a bit bogged down in biological minutiae, he’s an able guide to the world of touch, with a true gift for simplifying the complex. (The illustrations and diagrams throughout the book help, too.) The book is packed with cocktail-party trivia — scientists believe that touch is the first sense to develop in utero; some people with schizophrenia can tickle themselves — and satisfying explanations of everyday tactile experiences.
Why, for instance, do we describe chili peppers as “hot” and mint as “cool”? As Linden explains, the nerve endings that detect heat contain a receptor called TRPV1. This particular receptor is activated by high temperatures, but, as it turns out, it’s also activated by capsaicin, a compound in chili peppers. Likewise, TRPM8, a receptor that responds to freezing temperatures, also responds to menthol. “The answer to our puzzle is that the metaphor is not in the culture, or even in the brain region,” Linden writes. “The metaphor is encoded within the sensor molecules in the nerve endings of the skin.”
Linden also tackles bigger, more consequential questions, such as how touch shapes our lives and social interactions. Touch is crucial not only to our early development but also to our adult relationships — and those of our primate cousins. Many species of monkeys and apes spend enormous chunks of time grooming one another. They groom their mates, their offspring and their friends, and a young male may attempt to win over a dominant male by picking through his skin and hair. All this touching seems to solidify the social bonds between the animals; scientists have found that chimpanzees and macaque monkeys “are more likely to respond to a distress call (thereby putting themselves in danger) when the call was recorded from an animal with which they recently groomed,” Linden explains.
Social touch can build trust in humans, too. Consider a 2010 study of interpersonal touch in the National Basketball Association. Researchers found that teams that celebrated successful plays with hugs, high-fives, fist bumps and other kinds of touch early in the season subsequently displayed more cooperative, selfless behavior on the court and were more successful as the season progressed.
Scientists have documented these kinds of effects across many different domains. “People who are gently touched by a server in a restaurant tend to leave larger tips,” Linden writes. “Doctors who touch their patients are rated as more caring, and their patients have reduced stress-hormone levels and better medical outcomes. Even people with clipboards at the mall are more likely to get you to sign their petitions or take their surveys if they touch your arm lightly.”
Touch may not have the same glamour as sight or taste, but, Linden argues, it’s “a crucial form of social glue” and “a central aspect of our human experience.” “Touch,” the book, will make you think more deeply about every itch, scrape and caress.