Not only does touch seem to signal trust and cooperation, it creates them. 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."
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.
People can communicate several distinct emotions through touch alone, including anger, fear, disgust, love, gratitude, and sympathy. Accuracy rates ranged from 48 per cent to 83 per cent, comparable with those found in studies of emotions shown in faces and voices. "The evidence indicates that humans can communicate several distinct emotions through touch," said Dr Hertenstein. "Our study is the first to provide rigorous evidence showing that humans can reliably signal love, gratitude and sympathy with touch. These findings raise the interesting possibility that touch may convey more positive emotions than the face.''
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.
Touch helps to build trust
"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.
"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.
One question that fascinates many researchers is when this sense of identity develops. It might be that a parent's touch teaches infants about where they stop and others begin. "We aren't born with a fully fledged sense of body and self, and so we believe that what parents do is important for building it," says Fotopoulou.
Our sense of touch kicks in early. It is the first sense to develop, starting about eight weeks after conception, when the fetus is 1.5 centimetres long and brain activity is just beginning. "We know that babies are learning a great deal about touch in the uterus," says Tiffany Field, director of the Touch Research Institute at the University of Miami School of Medicine in Florida. "They suck on their thumbs, grab the umbilical cord and are constantly bumping against the walls of the mother's abdomen."
Emotional touch may also be at work in the womb. McGlone thinks that the swirl of amniotic fluid around the downy, lanugo hairs that cover a fetus may provide crucial stimulation to a developing brain, guiding the construction of areas such as the insular cortex. For McGlone, such caresses provide a kind of scaffold for the "social" parts of the brain, promoting the growth of synapses and connecting networks in key areas. And this building work continues after birth.
Ongoing stimulation is crucial for the development of other sensory systems, such as vision. If you deliberately block vision in one eye soon after birth, for example, the parts of the brain that process vision from that eye will never develop, even if it is unblocked later on.
"My hunch is that the natural interaction between parents and the infant -- that continuous desire to touch, cuddle and handle -- is providing the essential inputs that lay the foundations for a well-adjusted social brain," says McGlone. "It's more than just nice, it's absolutely critical."
We can lose our sense of touch in a number of ways. When Ian Waterman was 19, his immune system attacked his nerves and he lost his sense of proprioception -- a kind of internal touch that helps us locate our body in space. He could still feel pain and temperature, and his motor nervous system still functioned. But without knowing where his limbs were unless he looked, he couldn't move. It took years of mental retraining to learn how to will his arms and legs into action.
Other cases have been reported in which people lose the ability to feel prods and pokes, with similarly debilitating results. There is also a community of people in Norrbotten, Sweden, who have a genetic condition that makes them largely insensitive to pain.
Touch at a distance
Touch is a fundamental part of human communication, but in this era of remote digital interaction are we missing out? Some obviously think so, because there are now devices that can help us connect with colleagues or loved-ones remotely.
One gadget on the market is the Hug Shirt. By donning a sensor-laden sweater, you can record a cuddle by giving yourself a hug. This is then sent to a shirt worn by a recipient, which recreates the strength, duration and shape of your embrace. Other devices in development include objects that send strokes and squeezes over the internet by making companion devices respond with vibrations and temperature changes.
Can remote touches replace the real thing? Maybe. Michiel Spape at the Helsinki Institute for Information Technology in Finland and his colleagues recently invited volunteers to play a game of trust called Ultimatum remotely. In the game, two players must decide how to divide a sum of money. One is asked how the money should be split, the other accepts or rejects the proposal. If an agreement is reached, both players receive their share; if not, neither gets anything.
Being touched by someone often makes us feel more altruistic towards them, so Spape's team strapped a vibrating "tactor" to the players' hands to recreate the sense of being touched. This made it slightly more likely that players would agree when divvying up their spoils.
Hug actually affect the entire body to such an extent that many scientists claim it is equivalent to the effect of many different drugs operating on the body simultaneously. Even seemingly trivial instances of interpersonal touch can help people deal with their emotions with clarity and more effectively.
How hugs can heal
* Hugging your partner could lower his or her blood pressure.
* Researchers have found that in younger women, the more hugs they get, the lower their blood pressure.
* Researchers at the University of North Carolina who investigated 69 pre-menopausal women showed that those who had the most hugs had a reduced heart rate.
* Exactly what could be responsible is not clear, but the psychiatrists who carried out the work also found that blood levels of the hormone oxytocin were much higher in the women who were hugged the most.
* Other research finds that oxytocin is released during social contact and that it is associated with social bonding, while a study at Ohio State University shows that when it is put into wounds in animals, the injuries heal much more quickly.
* Work at the Swedish University of Agricultural Sciences suggests that oxytocin can induce anti-stress-like effects, including reduction in blood pressure and levels of the stress hormone cortisol: "It increases pain thresholds and stimulates various types of positive social interaction, and it promotes growth and healing. Oxytocin can be released by various types of non-noxious sensory stimulation, for example by touch and warmth," they say.