1. Scientists have mapped how sensitive different parts of the human body are to pain.
The researchers, based at University College London, used lasers to cause pain to 26 healthy volunteers. It turns out, perhaps unsurprisingly, that the most pain-sensitive parts of our body are our fingertips and forehead.
2. Experiments testing how well people can identify where on their body they’ve been touched are routine, but the equivalent for pain has never been assessed before.
“We don’t know much about pain,” says Dr Giandomenico Iannetti of the UCL Department of Neuroscience, Physiology and Pharmacology, whose lab the research took place in.
That’s because to study a sensory system you need to be able to isolate the input, in this case pain, from other inputs like touch. This is especially important because touch is actually a pretty effective painkiller (that’s why you rub your knee after you fall on it).
It’s been difficult to find stimuli that provide pure pain without touch. Until a pair of high-powered lasers came along.
3. So I went to visit the lab at UCL to see just how much pain these lasers could inflict. This was the set up.
There were two lasers pointing down to where journalists were going to put their hands. The wires lead off to a dial that the scientists used to decide how much they want to hurt you. Everyone in the room had to wear safety goggles. Selfies were tolerated, but not encouraged.
(This set up wasn’t exactly the same as it was for the volunteers who participated in the study. They wore blindfolds, for example, so they couldn’t tell if one or two lasers were activated at any one time.)
4. The lasers in this experiment allowed the researchers to trigger two types of pain.
When something happens to trigger pain, there are two pathways the signal takes to travel from your skin to your brain. The first is quicker and causes you to feel a sharp stinging pain, a bit like a pinprick. The second pathway causes a dull burning sensation that lasts longer.
5. This is my face before the scientists even turned the lasers on.
7. “Oh, is that it?”
It turns out that the laser pain doesn’t hurt that much. Our photographer then asked lead author on the new study Dr Flavia Mancini to turn the pain levels up in an effort to get more of a reaction from me. She looked at me, worried, but I told her to go ahead. Eek.
8. And… it does kind of hurt more the next time.
There’s a part of pain that is purely physical, but a lot of it is in our heads. Anticipating pain makes it worse, as I learnt when Mancini started slowly counting down before she activated the laser. Turns out, when you’re waiting what feels like forever for someone to laser your hand and expecting it to hurt more than the last time, it does. Even when the input is essentially the same.
9. In the actual study, pain was inflicted on 26 volunteers using lasers and the volunteers’ sensitivity on different parts of their bodies was measured.
The volunteers were asked to distinguish whether just one or two lasers were being used during each test. The distance between the two lasers was changed and the researchers measured the minimum distance at which volunteers could tell there were two pinpricks of pain instead of just one.
The researchers found that, in general, our ability to identify the source of pain is better towards the centre of the body and worse at the extremities. This matches the higher density of nerve cells towards the centre.
But our fingertips were an exception, being very sensitive to pain with few pain-sensing nerve cells. “The high pain acuity of the fingertips is something of a mystery,” says Dr Mancini. “It may be because people regularly use their fingertips, and so the central nervous system may learn to process the information accurately.”
10. Here’s the full map, based on data from the research paper.
The longer a line is, the further apart the lasers had to be for the volunteers to distinguish them and the less sensitive that part of the body is to pain.
11. Eventually the results should be able to help people with chronic pain.
Almost ten million people in the UK suffer pain almost daily.
Teaching chronic pain patients how to differentiate between two points of pain could form part of the treatment for some chronic pain conditions. “There’s some soft evidence that doing it in the tactile field can help with complex regional pain syndrome, one of the rare pain syndromes that is very difficult to treat,” says Dr Roman Cregg, who treats patients and was not involved in the research.
The laser technique could also allow doctors to monitor nerve damage, a common cause of chronic pain, across the body, giving them a quantitative way to see if a condition is getting better or worse. “I’m excited about the prospect of taking this to the clinic,” says Dr Cregg.