FIST bumps and bumslaps, high fives and back pats – most sports teams can’t keep their hands off each other. Watch a group of players on a winning streak and you’ll see a lot of touching. Keep a tally and it might even give you a way to pick the champions. The teams at the top of the rankings at the end of the US National Basketball Association season, for example, engage in more hands- on interaction from the start than those who ended up at the bottom, according to work by a group at the University of California, Berkeley. Not only does touch seem to signal trust and cooperation, it creates them.
Examples like this are showing that our sense of touch does much more than help us navigate the world at our fingertips. It is becoming clear that touching each other plays a fundamental role in our lives. It isn’t just a sentimental human indulgence, says Francis McGlone at Liverpool John Moores University, UK. “It is a biological necessity.”
Such touching gives the world an emotional context. It builds trust and promotes teamwork, wins friends and influences people. But that’s not all. Beginning in the womb, it may guide the development of regions in our brain that govern social behaviour. It could even give us our sense of self. The touch of others makes us who we are.
Compared to the other senses, however, touch often gets a raw deal. It receives less attention than sight or hearing, say. And yet the skin – our touch detector – is our biggest organ. An average-sized man has some 5 or 6 kilograms of it – roughly the weight of a bowling ball. As well as regulating our temperature and shielding us from infection and injury, our skin is a communication interface with the outside world. And just as we can lose our sight or hearing, we can go touch-blind.
The nerves that carry signals from the surface of our skin to the brain run at different speeds. In the fast lane, we have A fibres, heavy duty cables that carry breaking news to the brain in an instant – detailed information that helps us safely navigate our environment. In the slow lane, however, we have C-fibres, thinner wires that deliver messages at a more languid pace. Moving at a sedate 7 kilometres an hour, information carried by one of these nerves takes about a second to travel from a caressed ankle up to the brain.
Our high-speed nervous system is relatively well understood. For years, we also thought the vocabulary of our skin was limited to messages of pressure or vibration, temperature, itches and pain. The slower C fibres were just thought to convey the less immediate components of pain – throbs and aches, rather than pricks, stings and burns. But in the late 1990s, researchers identified a type of C fibre in humans – dubbed C-tactile fibres or CT fibres – that seemed to be activated by soft caresses.
Most touch receptors are concentrated in places like the lips and fingertips. However, CT fibres are found only on hairy skin – almost everywhere except the lips, palms of the hands and soles of the feet – and are concentrated
on the top of the head, upper torso, arms and thighs. Like other touch highways, CT fibres are wired up to the brain region that lets us construct a model of the physical world around us – the somatosensory cortex. But they also plug into areas like the insular cortex, which is linked to emotions.
“CT fibres activate this whole network of brain regions involved in thinking about other people and trying to understand what their intentions might be,” says Kevin Pelphrey of Yale University. These same regions also respond to other social cues, such as facial expressions. “We think this touch system is another way to communicate social intentions,” he says.
Or as David Linden – a neuroscientist at Johns Hopkins University in Baltimore, Maryland, and author of new book Touch – puts it: fast fibres are all about the facts, slow ones the emotional vibe.
What’s more, they seem to be primed to the touch of others. “These nerve fibres respond optimally to low force, low velocity, stroking movements of around 3 to 5 centimetres per second,” says McGlone. In other words, a gentle stroke. This kind of touch – variously called social, emotional or affective touch – also seems to be activated more by warm temperatures, meaning a touch from cold hands is less rewarding. “They are exquisitely tuned to exactly the type of affiliative touching that you see between parents and baby, or between two lovers,” says McGlone.
But what for? It is probably to do with social bonding, if clues from our primate cousins are anything to go by. Robin Dunbar of the University of Oxford and his colleagues at Aalto University in Helsinki, Finland, have preliminary results suggesting that gentle stroking activates similar brain pathways in humans to those that fire in non-human primates such as rhesus macaques during a grooming session. It also triggers the same release of endorphins. Interestingly, in humans, the density of an individual’s endorphin receptors seems to correspond with the size of their social network.
Since gentle touching is rewarding, Dunbar thinks it encourages individuals to spend time together to develop relationships of trust and obligation. “We probably have as much physical contact within our core relationships as monkeys do within theirs,” he says.
Touch lets us communicate a range of emotions. Gratitude, sympathy and love can all be conveyed with the briefest of touches. “I have always argued that touch is worth a thousand words in terms of understanding how somebody really sees you,” says Dunbar.
Indeed, given our complex social behaviour, the group behind the NBA basketball study believe touch between individuals may be even more vital to humans than it is for other primates. And it’s almost certainly not limited to sports. “There is nothing special about the NBA,” says Linden. “Social touch is important in developing trust and cooperation in a large number of group activities.”
Touch builds relationships. But it can also be used to fake a relationship where there is none. Salespeople use it to build trust, waiters use it to boost their tips. If you fancy a free bus ride, try touching the driver when you ask.
Emotional touch is hugely influential in our lives, but some think it shapes a lot more than our day-to-day interactions. In fact, touch may be a driver of what makes you you. Studies of the rubber hand illusion in adults suggest that emotional touch plays a role in body ownership, for example. In this experiment, people begin to believe that a rubber hand is their own, if it is stroked and prodded at the same time as their hidden real one. Katerina Fotopoulou at University College London recently showed that the illusion is all the more convincing if the stroking occurs at the speed known to activate CT fibres.
Setting boundaries
“The psychological sense of being oneself seems to be linked to being touched in this emotional way by another person,” she says. “This may have a crucial role in teaching us the psychological boundaries of our own body, what is mine and what is not.” Fotopoulou also has new results suggesting that people who have had a stroke can recover a lost sense of limb ownership if the arm or leg is stroked on a regular basis.
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