Every so often I cat-sit a pair of large Maine Coon cats with luxuriant, flowing fur. These cats like to greet me by rubbing up against whatever body part they can reach. After several visits, I started giving in to temptation and stuck my face out so the cats could brush their silky bulk against it.
Petting a friendly, fluffy cat might seem like one of life's simplest pleasures. But the satisfaction we get from this experience and all others that feature touching and being touched is a complex blend of many elements, some physical and some emotional.
What feels good to touch and what makes us cringe depends on texture, several types of nerve cells on our bodies and our brain's read of the situation. The experience varies from person to person, and from one body part to another. In fact, an increasing body of research on touch suggests that the joy we get from feeling fur along our fingers versus our faces might even be different kinds of pleasure.
What feels good?
Researchers have long been curious about what properties make material appealing. An 1895 article published in the American Journal of Psychiatry had people rub 51 different cloths and report which type of materials felt pleasant to touch. And a 1937 study suggested that people find smooth and soft fabrics "relaxing."
More recently in a 2012 study, 123 design students were asked to discuss the kinds of fabrics they liked to touch. The students disliked fabrics that were scratchy, rough or itchy; 41 percent mentioned wool. Coarse, bumpy or cold fabrics were also not popular.
The positive properties that cropped up most often were soft, smooth and warm (some of the runners-up were silky, fuzzy and fluffy). Silk, cotton, and fleece got the highest rankings. The students frequently used emotion-laden words to explain their partialities, such as cozy, cuddly, safe and comforting.
"Cognitive touch properties become more meaningful and memorable when they are linked with affective properties, which combine to satisfy physiological and psychological needs," the researchers wrote.
Our own personality may also affect how much we like soft things. In one study, researchers in France asked several hundred people to take a personality test and evaluate different fabrics. People who liked soft-feeling fabrics tended to score high in "affect intensity"—the likelihood to experience positive and negative feelings strongly.
It's possible that people who have high affect intensity and aren't as emotionally stable are more motivated to seek out stimuli that match their need for stability and security, the researchers suggested. And softness may reasonably be associated with feelings of comfort and protection, they said.
In fact, soft fabrics might be comforting enough to deliver a good night's sleep. A researcher in Korea found that women tossed and turned less in their sleep and reported having slept better when they slept with soft bedding. They also had less adrenaline and noradrenaline—hormones that increase heart rate and blood pressure—in their urine.
How we touch
Touch doesn't just depend on what we are touching, but also on how we touch it. In general, people find that having an object rubbed slowly and lightly against them feels better than being quickly buffed by it.
And although soft and smooth surfaces generally feel more pleasant than rough and sticky ones, sometimes texture lies in the eyes—or rather, the finger—of the beholder. One texture-ranking study found that people's perception was influenced by how moist their fingertips were.
People with moist fingertips thought that rough and sticky materials were more pleasant than did people with drier fingertips. Rough and sticky materials allow a sheen of moisture to accumulate on their surfaces. This decreases friction and lets the finger glide along more smoothly, causing moist-fingered people to find these materials more pleasant. In contrast, moisture made genuinely smooth materials feel sticky and therefore less pleasant.
Despite these differences, the participants could agree that very rough or sticky items (like latex, cling film, or wax) were overall less pleasant to touch than soft materials such as silk, velvet, or paper.
Feeling with the hand or the heart?
Scientists refer to the hide on our palms, fingertips and the soles of our feet as smooth or glabrous skin; the rest is called hairy skin. Glabrous skin is extremely sensitive to the kind of touch that helps us explore and define the world around us. But it also lacks a special kind of nerve fiber that scientists believe may be necessary for the pleasurable, rewarding aspect of touch.
This second kind of touch, affective touch, describes our emotional response to tactile stimulation. "Slow, gentle stimulation of hairy skin is likely to occur during close affiliative interactions…such as those between a parent and offspring, between siblings, between trusted associates, and not least between mates," wrote the authors of one review. "Such affective touch may constitute a distinct domain of touch, characterized not by its sensory-discriminative functions, but by its social context and accompanying subjective component."
Within nerve cells the long, wiry axons that transmit impulses vary in how much fatty lining they have. This covering is called the myelin sheath, and the thicker it is the faster an axon can conduct nerve impulses. The nerve fibers necessary for discriminative touch, called A-fibers, have thicker sheaths than those used for affective touch, which are called C-fibers.
All of these different types of fibers aren't pinged in isolation. Grabbing a carton of milk from the refrigerator conveys both mechanical (smooth) and thermal (cool) information. And being caressed or stubbing your toe activates both A-fibers and C-fibers.
The C-fibers used for affective touch were first identified in mammals in 1939. But it was assumed that humans did not have this seemingly primitive tactile system. Affective touch fibers weren't found in humans until 1988, when their signatures were discovered in recordings of the electrical impulses in facial nerves.
Pleasure and pain
To feel pain, which is both sensation and emotional experience, we use fibers with and without the thick myelin sheath. Together, they give us the experience researchers call "first and second pain." First pain is fast; it's what makes you snatch your hand back from a hot kettle. C-fibers transmit information and are responsible for second pain, the throbbing ache that builds as you realize you've burned yourself.
Something similar may be going on for pleasure, says Francis McGlone, a neuroscientist at the University of Liverpool: there are two touch systems parallel to the two pain systems; first touch is served by A-fibers that let us quickly identify the physical properties of whatever we are feeling, such as wetness, hardness or roughness. The second touch is conveyed by a type of C-fibers that are sensitive to slow, gentle stroking touches, which emulate a caress. These fibers may be the reason that patients touched by a nurse the day before an operation are less stressed, or that being stroked lowers blood pressure and increases pain tolerance.
"Touch may be viewed as a biologically necessary form of stimulation, not just a sentimental and romantic human indulgence," McGlone and colleagues wrote in a review of affective touch research. "It could be argued that the need for touch does not diminish throughout life, that in order to flourish as an adult one needs physical contact with others."
Touching or being touched
But what about those cats I loved to run my hands over? For many people, the feeling of soft fur under their palms and fingertips is irresistible, as the fabric and fingertip exploration studies found.
In contrast, other studies have shown that people generally find gentle rubbing more pleasant not on their palms but on hairy skin.
In fact, when researchers took PET scans of the brains of people being gently stroked on the forearm or palm, they saw that these different kinds of pleasant touches stimulate different areas of the brain. Have one's palms rubbed triggers a burst of activity in the somatosensory cortex, which receives input related to touch. Being stroked on the forearm, however, primarily activated the insular cortex, which is involved in processing emotion.
It's possible that the pleasure we feel in running our hands over a soft cat or bolt of velvet might be informed by affective touch, suggest
Rochelle Ackerley of the University of Gothenburg in Sweden and her colleagues. Our brain may be able to use touch input from speedy A-fibers on the palm and link it to the pleasant perception learned from previous experiences of gentle touch on C-fiber-innervated areas.
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