Ouch! Our forehead and fingertips are the most sensitive to pain, suggests research that used lasers to give volunteers sharp shocks across their body.
The study was the first to look at how our ability to work out where something hurts – called “spatial acuity” – varies across the body. This hadn’t been done before because it is very difficult to activate the nerve fibers that process pain without simultaneously stimulating those that process touch, says lead author Flavia Mancini at University College London.
To overcome this hurdle, Mancini and her colleagues used two small lasers to deliver short painful stimuli to 26 volunteers. The distance between the two lasers was reduced until the volunteers couldn’t distinguish between them. The experiment was repeated across the whole body.
The volunteers could distinguish between the two lasers more accurately at areas closest to their trunk, and less accurately towards their extremities – the opposite of what is seen when researchers test how we feel touch. For example, pain acuity was much greater at the shoulder than the wrist. The volunteers were also more sensitive to pain in areas closest to the trunk. There were two exceptions, however: volunteers were most sensitive to the test on their fingertips and forehead.
This is unusual because our ability to perceive pain depends on how many nerve fibers that trigger the sensation of pain are present, says Mancini. The greater density of pain fibers you have in an area, the more accurately you perceive pain….