The study from the University College London defines for the first time how our ability to identify where it hurts, called 'spatial acuity', varies across the body, being most sensitive at the forehead and fingertips. (Agencies)
Using lasers to cause pain to 26 healthy volunteers without any touch, the researchers produced the first systematic map of how acuity for pain is distributed across the body.
With the exception of the hairless skin on the hands, spatial acuity improves towards the centre of the body whereas the acuity for touch is best at the extremities. This spatial pattern was highly consistent across all participants.
The experiment was also conducted on a rare patient lacking a sense of touch, but who normally feels pain. The results for this patient were consistent with those for healthy volunteers, proving that acuity for pain does not require a functioning sense of touch.
"Acuity for touch has been known for more than a century, and tested daily in neurology to assess the state of sensory nerves on the body. It is striking that until now nobody had done the same for pain," said lead author Dr Flavia Mancini of the UCL Institute of Cognitive Neuroscience.
"If you try to test pain with a physical object like a needle, you are also stimulating touch. This clouds the results, like taking an eye test wearing sunglasses.
"Using a specially-calibrated laser, we stimulate only the pain nerves in the upper layer of skin and not the deeper cells that sense touch," Mancini said.
Volunteers were blindfolded and had specially-calibrated pairs of lasers targeted at various parts of their body. These lasers cause a brief sensation of pinprick pain.
Sometimes only one laser would be activated, and sometimes both would be, unknown to participants. They were asked whether they felt one 'sting' or two, at varying distances between the two beams.
The researchers recorded the minimum distance between the beams at which people were able to accurately say whether it was one sting or two.
"This measure tells us how precisely people can locate the source of pain on different parts of their body," said senior author Dr Giandomenico Iannetti of the UCL Department of Neuroscience, Physiology and Pharmacology.
"Touch and pain are mediated by different sensory systems. While tactile acuity has been well studied, pain acuity has been largely ignored, beyond the common textbook assertion that pain has lower acuity than touch.
"We found the opposite: acuity for touch and pain are actually very similar. The main difference is in their gradients across the body. For example, pain acuity across the arm is much higher at the shoulder than at the wrist, whereas the opposite is true for touch," Iannetti said.
The work is published in the journal Annals of Neurology.
The study from the University College London defines for the first time how our ability to identify where it hurts, called 'spatial acuity', varies across the body, being most sensitive at the forehead and fingertips.