Researchers from University College London use lasers to stimulate pain in a volunteer's hand. 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. The ability to accurately discriminate the source of pain is known as 'spatial acuity' for pain.