Some people really are more sensitive to pain, scientists have found.
Scientists observed pain responses in the brain
US researchers say their findings mean doctors can rely on patients' reports of the pain they are in to determine treatment.
A team from the Wake Forest University Baptist Medical Center studied how people reacted to heat.
They carried out magnetic resonance imaging (MRI) brain scans on17 volunteers, each of whom had a computer-controlled heat stimulator placed on their leg.
The stimulator was heated to 120° F (49°C), which most people find painful, while the MRI scan was being carried out.
Each of us has different thresholds and tolerances for different pain stimuli
Professor Henry McQuay, Oxford Pain Relief Unit
The participants were also asked to rank the pain they were feeling on a scale of one to 10, with 10 being the highest pain rating.
The least sensitive person rated the pain around one, while the most sensitive person rated the pain as almost a nine.
In those who reported more pain, scientists saw more activity in two areas of the brain.
The looked at the primary somatosensory cortex, which contributes to the perception of where a painful stimulus is located on the body and how intense it is.
The researchers also examined the anterior cingulate cortex, which is involved in the processing the unpleasant feelings evoked by pain.
Those who ranked the sensation as only mildly painful had minimal activity in those areas.
But there was little difference in activity in the thalamus, which helps transmit pain signals from the spinal cord to higher brain regions.
The researchers suggest incoming painful information is processed by the spinal cord in a generally similar manner, but that how the brain responds varies between individuals.
Robert Coghill, assistant professor of neurobiology and anatomy, who led the research, said: "We have all met people who seem very sensitive to pain as well as those who appear to tolerate pain very well.
"Until now, there was no objective evidence that could confirm that these individual differences in pain sensitivity are, in fact, real."
He added: "One of the most difficult aspects of treating pain has been having confidence in the accuracy of patients' self-reports of pain.
"These findings confirm that self-reports of pain intensity are highly correlated to brain activation and that self-reports should guide treatment of pain."
He said people probably respond to pain differently because of their past experience with pain, their emotional state at the time pain is experienced, and their expectations of pain.
But Henry McQuay, professor of pain relief, University of Oxford, said: "Pain is treated and has been for centuries by people saying how much they hurt and being given, or taking themselves, enough medicine to give some relief.
"If it still hurts after a small dose then you take a bigger dose."
He added: "Each of us has different thresholds and tolerances for different pain stimuli - some find ice cold water noxious but don't have injections at the dentist.
"You would expect the pain areas to light up more in those who find the cold water test painful than in those who don't."
The research is published on-line by the Proceedings of the National Academy of Sciences journal.