"Where does it hurt?" is probably the first question asked to anyone affected by pain.
A new UCL study 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.
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. The work is published in the journal Annals of Neurology
and was funded by the Wellcome Trust.
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," says 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."