Gentle touch - the science of the perfect hug

Touch stimulus is so important for developing babies, but it's importance doesn't stop there. Physical contact has a part to play throughout life, including into old age, and alongside the many advantages that our increasingly virtual world brings us, perhaps a consequential lack of physical touch could be a downside. Katie Haylor spoke to Francis McGlone, firstly asking if we are living in a less tactile world than previously...

Francis - In my experience… probably yes. And I think modern technology calls itself “social media” but it’s actually quite antisocial in terms of the fact that very little physical contact is mediated now between people, it’s done through a touchscreen. So, yeah possibly there is a less physical contact in terms of the way we socially mix.

Katie - Why is touch important for our health?

Francis - When I described touch earlier, I described the sense of touch that most people know about. Now that sense of touch is coded by what’s called fast myelinated nerve fibres, so when somebody touches you you feel it immediately. But, in the late 1980s early 1990s, another touch nerve was discovered in human skin, which is a C fibre. Now we’ve heard about C fibres earlier. C fibres are nociceptors; these are the nerve fibres that code for itch and for pain and they conduct information into the brain very slowly at sort of walking speed. So they can have no function in terms of altering you to something that’s going on in the world.

For 20 years now we’ve been characterising this C fibre that responds to gentle touch, and have built a whole litany of research understanding that this is the nerve fibre that basically activates when you’re cuddled or stroked or nurtured. It is probably, I would suggest, one of the most important nerve fibres we have developmentally and even across the lifespan because it basically drives the reward of physical touch between a mother and a baby, or two people.

Katie - It’s easy to understand why developing a response to pain is important. You have a stimulus that might be dangerous so stop touching that thing. From an evolutionary point of view, why is this idea of gentle touch important?

Francis - If we look at these C fibres, we know the C nociceptor is fundamental to survival. We know the the C pluriceptor is fundamental to survival and I think the C tactile afferent is equally fundamental to survival because this promotes the benefit of physical contact and social grouping.

These three C fibres that evolved before the fast ones by the way. The first systems to go down that basically protect, and once you have those systems in place you can get off your rock and start exploring. So the C tactile one is the one that basically drives the benefit of social contact.

Katie - It’s not just with children that touch is important is it? It’s important to mention other stages of life and in the elderly population as well.

Francis - Oh absolutely, across the lifespan. There’s the scientific evidence that elderly people, who are just touched on the shoulder in an old people’s home, eat more more food - it’s called the Midas Touch. So, yes, across the lifespan these C tactile afferents are playing a fundamental role in engaging social interactions between humans. And the less we do this, I think there’ll be a cost.

Katie - If I get a hug from my friend because I’ve had a horrible day, emotionally that makes me feel better. But why is that; why does touch have such a strong psychological impact?

Francis - Well that’s basically because the nerve fibres that are optimally responding to that gentle touch are C fibres. We spoke earlier about the fast touch nerves that tell you that something’s touched your body, but the C fibres basically project into areas of the brain that process emotion. The classic C fibre in the nociceptor, so the reason that pain is so distressing is the C fibres project into brain areas that are basically produce an affective state. And the C tactile afferent, this gentle touch nerve, also plug into brain networks that are processing emotion, so they’re encoding that affiliative affective state that we get when we’re cuddled, stroked, or hugged.

Katie - Is there a certain frequency of gentle touch? What makes a touch gentle?

Francis - There is indeed. Well we record from these nerve fibres. We’ve got the only lab in the country that’s using this technique of microneurography. We put a small electrode into a nerve bundle and then we can record from a C tactile afferent. And when we do finally get one, if you stroke across its receptive field, it’s tuned for a particular velocity, basically roundabout 3 to 5 centimetres per second stroking velocity that nerve fibre is firing optimally. If you go faster or slower, the nerve fibre basically responds less.

Now if you ask people at a psychophysical experiment, and you produce different velocities of stroking over the skin again, they will say that roughly 3 to 5 centimetres per second is far more pleasant than say one at 30 or one a less than 1.

Katie - Me and Georgia are here in the studio practicing…

Francis - Stroking each other.

Katie - Wow, that’s amazing.

Francis - This is a beautiful system. The first neuron responds optimally to exactly the stroking velocity that you would stroke your baby or your partner.

Katie - So it’s basically the formula for the perfect hug?

Francis - Absolutely! I mean these nerve fibres respond optimally to a gentle stroking, but they will also respond to a gentle static touch. But the optimally prefer something which is moving across the skin in a stroking movement rather than just a physical contact.

Georgia - Francis, I have a quick question: why is it some people sort of like the sensation, other people if you run a finger down their arm they will lose their mind and curl up into a ball because it tickles them so much? Do we know why some people just cannot deal with it?

Francis - Well, I’m probably a victim of that being brought up by my parents in the 50s, where hugging was not a particularly common thing to do. There’s some developmental influences in terms of physical contact between parents and kids. Some people don’t like being stoked or touched and other people can’t get enough of it, but I think to a large extent it may be experience. That’s one of our sort of theories that neglect is so devastating to the developing social brain.