A technique that may help them measure pain from the outside, is being developed by scientists.
Tara Renton of King's College London has come up with an alternative way of analyzing functional MRI scans called arterial spin labeling (ASL) to measure how much oxygenated blood is flowing through particular areas.
Renton and her team scanned the brains of young men who were fresh out of the operating theater after getting their wisdom teeth removed, New Scientist reported.
The experts found the amount of oxygenated blood correlated with the intensity of pain in regions of the brain associated with feeling pain.
However, critics have cast a shadow of doubt over the ASL technique that Renton describes as the first objective measure of ongoing pain intensity.
David Borsook, who leads the Pain and Analgesia Imaging and Neuroscience group at McLean Hospital in Boston, said: "Whilst it offers a reasonable guesstimate of the amount of pain a person is in, it's not objective, and there is great variation in responses."
Stuart Derbyshire, who researches pain at the University of Birmingham, added: "The hunt for an objective measure of pain is a fool's errand. We will always need to rely on subjective measures."
Richard Gracely at the University of Michigan, Ann Arbor, agreed: "It's like saying you can measure love, or the beauty of a painting, objectively. Pain is such a private, personal experience. You can only validate what you've measured by asking patients how much pain they're in, so why not just ask them in the first place?"
But not all were against the possibility with Jeffrey Mogil, who researches pain at McGill University in Montreal, Canada, pointing out a technique like Renton's could be used to measure the level of pain in patients with locked-in syndrome or who are in a vegetative state.
Renton described the findings at King's College but is yet to publish them.