When Valentine’s Day arrives Tuesday, many people will show their love and affection with cards, chocolates and gifts, but there is another way to make your partner or family member feel good — through touch.
Physical isolation during the coronavirus pandemic led many to develop “skin hunger” and resulted in an uptick in mental health problems.
One 2021 study surveying almost 1,500 participants reported that deprivation of intimate touch from close family and partners was associated with worse feelings of anxiety and loneliness.
Lack of friendly or professional touch from friends, acquaintances or work colleagues did not have the same impact on mental health.
How we can feel social touch
Social touch is so important for our well-being that we have specific cells in our skin to detect it.
Our skin gives us the power of discriminative touch, which allows us to feel the pressure, texture and vibration of objects. But our skin also has sensors known as C-tactile fibers or afferents that are specifically sensitive to social touch from people and the caress of a loved one.
C-tactile fibers innervate hairy skin and are optimized to detect the gentle stroking touch of 1 to 10 centimeters (half- to four inches) per second that many people say is pleasant. If the movement is too fast or too slow, not only are the C-tactile fibers less responsive but people also find the sensation less pleasing.
Interestingly, they are also more attuned to warmer temperatures of around 32 degrees Celsius (almost 90 degrees Fahrenheit), akin to the warmth of a person’s skin.
There are individual differences, however, in the need and craving for social touch. “I think touch is very important,” said Mariana von Mohr, a researcher specializing in social cognition at Royal Holloway University of London. “But at the same time, we have to be a bit careful about these individual differences because there are people that might prefer to connect in some other way.”
But in general, intimate touch between loved ones is important for emotional regulation and social buffering, where being together with others helps us handle and recover from stress.
The benefits of social touch
Social touch causes the release of the social-bonding hormone oxytocin in the brain, which is thought to reduce anxiety and pain.
Von Mohr’s research has found that romantic partners felt less pain when receiving a slow, stroking touch compared with a faster touch. Other studies have found that touch enhances intimacy between couples.
In one study, 84 adult women participated in an experiment showcasing the soothing power of affective touch on feelings of social exclusion. Each participant played Cyberball, an online ball-tossing game, with two other players who, unbeknown to the subjects, were bots that would eventually stop throwing the virtual ball to the participant.
When that happens, it can produce a profound sense of social exclusion, and even led to “one participant actually smashing the computer,” von Mohr said.
But when half of the participants received a slow, affective touch from the experimenter afterward, their feelings of social exclusion were partially mitigated. The other half of participants who received a fast-stroking touch, which would not activate C-tactile fibers, did not experience similar relief.
This is akin to a mother comforting a child after a similar experience of social hurt, von Mohr said. “We do this sometimes without realizing how good it is,” she said.
All mammals that have been studied have C-tactile fibers, suggesting that these sensory cells — and the ability to detect social touch — are evolutionarily conserved and essential, said Ishmail Abdus-Saboor, a biological scientist specializing in touch research at Columbia University’s Zuckerman Mind Brain Behavior Institute.
From skin to brain for pleasurable social touch
Research suggests that social-touch-sensitive neurons may be key for making the touch of a loved one feel good, which in turn helps bind us.
A study in Cell this month found that directly stimulating neurons in female mice — similar to C-tactile fibers in humans — can release dopamine, a neurochemical associated with reward, in their brains. These Mrgprb4 neurons are also necessary for female mice to be receptive to sexual advances from male mice.
Using optogenetics, a now-mainstay neuroscience technique that allows researchers to manipulate the activities of specific neurons by shining a light on them, researchers activated the Mrgprb4 sensory cells on the back, an important area for social touch in mice where they groom and huddle together.
The researchers saw that the female mice would lower their backs in response to the light, which is lordosis, a female mouse sexual behavior indicating receptivity to a male mouse.
The finding that activating cells in the skin can trigger a “social touch-like behavior even without other sensory cues” such as the presence of another mouse “was pretty surprising,” said Abdus-Saboor, who was an author of the new study.
Stimulating Mrgprb4 neurons was pleasurable, and female mice spent more time in designated sections of the cage where researchers activated the cells. Intriguingly, stimulating these sensory neurons and engaging in sexual behavior both released dopamine in the nucleus accumbens, a key brain area associated with reward.
But when the researchers genetically ablated these Mrgprb4 sensory neurons, sexual experiences no longer released dopamine and the female mice began rejecting the male mice’s advances after their first experience. The male mouse’s touch was no longer rewarding, and the male’s advances were no longer welcome. The female mouse would even become combative with her paws up, making herself inaccessible to the male.
“I sometimes think she throws a punch,” said Leah Elias, a postdoctoral researcher at Johns Hopkins University who conducted the study as a graduate student at the University of Pennsylvania.
Though this study examined the importance of pleasurable touch to sexual behaviors in mice, the researchers say the Mrgprb4 cells also play a role in other forms of affiliative social touch such as play.
Before the touch signals reach the brain, “even at the first relay station in the skin, there are already dedicated cells that are competent to engage social stimuli,” Abdus-Saboor said.
Their research opens up a potential target for future therapeutics that can use the skin to access reward circuits to help treat trauma or depression.
“Just by activating these neurons in the skin, you kind of have a highway to the brain,” Elias said. “It’s kind of a gold mine.”
With just a hug, a caress or a gentle squeeze of the hand, we can already take advantage of the power of social touch.
To feel better, healthier and more connected, skin deep is a good place to start.
Do you have a question about human behavior or neuroscience? Email BrainMatters@washpost.com and we may answer it in a future column.
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