Strange clues to pain perception
“A gentle rub really does help pain go away,” said the Daily Mail. The newspaper reported that British scientists have discovered that people experience far less pain when they touch a sore part of their body with their hand.
The research behind this news is of general scientific interest, showing that the nerve signals from light touches can interact with those transmitting heat and painful sensations. The study looked at heat perception after participants submerged their fingers in water of different temperatures and pressed their fingertips together in different combinations. The scientists involved concluded that touching an apparently painful part of the body will affect the way pain signals travel to the brain.
Pain is a very subjective experience, and many factors, including psychological and emotional factors, affect the way it is perceived. The effect of touch is likely to vary both between individuals and in the same person, depending on the cause and type of pain and the circumstances surrounding it. This experimental scenario provides interesting results, but cannot be considered directly representative of the real-life experience of pain or inform us of new ways to ease pain.
Where did the story come from?
This scientific research was carried out by researchers from University College London and institutions in New York and Paris. Individual researchers were supported by the Economic and Social Research Council, Medical Research Council, Biotechnology and Biological Sciences Research Council, and the Leverhulme Trust. The study was published in the peer-reviewed medical journal Cell .
The methods used in this research have been described in the news as a key way to ease pain. Rather than looking at medical pain relief the research is of more general scientific interest, furthering the understanding of what has been previously thought about pain and sensation: that pathways signalling light touch can interact with the transmission of painful sensations at the spinal cord level following some forms of injury.
What kind of research was this?
When there is pain from a peripheral site of the body, such as the hand or foot, this painful signal travels along a peripheral nerve until it arrives at the spinal cord for transmission to the brain. However, at the spinal cord level there may be many different types of sensations coming in from around the body (such as touch, vibration and heat) that will ‘compete’ for transmission to the brain. It is believed that the brain’s perception of pain could be reduced by multiple sensory signals arriving at the spinal cord at the same time.
This experimental research was also investigating this ‘gate’ theory of how different signals arriving at the spinal cord could influence the transmission of pain, either:
- allowing it to travel unaltered to the brain,
- preventing it from travelling to the brain, or
- altering it in some way so that the brain would perceive the painful sensation in a different way.
In this experiment the pain was a harmless, perceived pain generated by a paradoxical phenomenon in which placing different fingers in water of different temperatures leads the brain to think the body is in pain. To generate this phantom pain the middle finger is placed in cool water (14ºC) while the fingers on either side are placed in warm water (43ºC). This phenomenon is known as the ‘thermal grill illusion’.
The sensory pathway (i.e. the signal competing with the pain) was the gentle self-touch of the other hand. This involved touching the fingertips of each hand against the other.
What did the research involve?
Self-touch is said to ‘provide proprioceptive information’ (which means it informs our awareness of where body parts are positioned) as well as providing thermal and tactile signals that could be expected to affect pain signalling in the spinal cord.
The researchers investigated this using the thermal grill illusion (TGI), where participants immersed their index and ring fingers in warm water and the middle finger in cold water. With the TGI, the brain perceives the cool water as painfully hot.
They asked participants to judge the temperature of the cold middle finger by matching its perceived temperature to the temperature of a heat-producing device touching their face. They then investigated the effect of pressing the three fingertips of each hand against each other, to see how this influenced the perception of heat.
What were the basic results?
They found that self-touch reduced the TGI effects. With self-touch, instead of the middle finger feeling hotter due to the warmth of the other two fingers, it was perceived to be cooler again, and closer to its true temperature.
One combination of self-touch finger positions (ring to index, middle to middle and index to ring) caused a 64% reduction in perceived heat. This did not happen when the hand touched a neutral object, when the participants touched their own fingers in different combinations or when they touched their warm or cool fingers to an experimenter’s hand that had not been immersed.
The researchers say this could not be explained by heat transfer through touching alone, and is likely to involve some ‘cognitive response’ that occurred in the brain when one hand touched the other.
How did the researchers interpret the results?
The researchers of this scientific study conclude that self-touch ‘gates’ pain signals and stops them from reaching the brain. They theorised that this may be due not only to the touch sensation, but also to extra cognitive effects in the brain linked to the fact that we would be likely to grasp one hand with the other when we experience pain, but would not let another person grasp it. In other words, our brain may be ‘expecting’ this self-touching to have a beneficial effect on pain.
Conclusion
Pain is a complex and subjective experience and the findings of this study may explain some of the common behaviour seen in humans following pain, particularly to the hands. The research is of general scientific interest, furthering the understanding of what has been previously thought about pain and sensation: that pathways signalling light touch interact with those transmitting heat and those transmitting painful sensations.
It should be noted that participants were asked to record their sensation of temperature, not their experience of pain, so it is an extrapolation to say that self-touch took away pain (although, obviously, the two sensations are interlinked).
This research also examined how perceived heat was affected by the warmth of the surrounding fingers and the touch of the other hand. This experimental scenario provides interesting results, but cannot be considered representative of the real-life experience of pain following injury or due to other causes. Many other factors affect the way pain is experienced, including psychological and emotional factors. The effect of touch is likely to vary both between individuals and within the same person, depending on the cause and type of pain and the circumstances surrounding it.
While touch may help an individual to cope with pain to a variable extent, perhaps easing very mild pain from a minor injury, it is unlikely to obliterate the experience of pain entirely.
