Researchers at University College London (UCL), using lasers to cause pain to 26 healthy volunteers without any touch, 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. 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.

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.

"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," said lead author of the study Flavia Mancini from UCL.

"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 concluded. The findings were published in the journal Annals of Neurology.


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