A new study shows for the first time how the brain is able to identify where pain is located in the body without the sense of touch.
This honing process, called spatial acuity, varies across the body and is most sensitive in the forehead and fingertips.
The research, published in the journal Annals of Neurology, involved 26 healthy volunteers who experienced pain-producing lasers to create the first systematic map of how pain acuity is distributed across the body.
The researchers, from University College London (UCL), found that spatial acuity tends to get stronger toward the center of the body — with the exception of the hairless skin on hands –whereas the acuity for touch is greatest at the extremities. These results were highly consistent among all participants.
Also involved in the study was a rare patient who lacked a sense of touch, but who normally feels pain. The results for this patient were consistent with those for healthy volunteers, showing 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.”
For the study, pairs of lasers were targeted at various parts of each participant’s body. These caused a brief sensation of pinprick pain. Sometimes only one laser would be activated, and sometimes both, but patients were not told either way.
Participants were asked whether they felt one “sting” or two at varying distances between the two lasers. The researchers recorded the minimum distance between the beams at which volunteers were able to accurately say whether it was one pinprick 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.” Iannetti said.
“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.”
Acuity for both touch and pain tend to correlate with the density of nerve fibers in each part of the body. However, fingertips remain extremely sensitive despite having a low density of pain-sensing nerve cells.
“The high pain acuity of the fingertips is something of a mystery that requires further investigation,” said Mancini. “This may be because people regularly use their fingertips, and so the central nervous system may learn to process the information accurately.”
Source: University College London