"We have all met people who seem very sensitive to pain as well as those who appear to tolerate pain very well," said Robert C. Coghill, Ph.D., lead investigator. "Until now, there was no objective evidence that could confirm that these individual differences in pain sensitivity are, in fact, real."
Using magnetic resonance imaging (MRI) to assess brain function, Coghill and colleagues found that study participants who said that a heat stimulus was intensely painful had pronounced activation of brain regions that are important in pain. In contrast, people who said that the same stimulus was only mildly painful had minimal activation of these same areas.
"One of the most difficult aspects of treating pain has been having confidence in the accuracy of patients' self-reports of pain," said Coghill, an assistant professor of neurobiology and anatomy. "These findings confirm that self-reports of pain intensity are highly correlated to brain activation and that self-reports should guide treatment of pain."
For the research, 17 normal, healthy volunteers (eight women and nine men) had a computer-controlled heat stimulator placed on their leg. While their brains were scanned, this device heated a small patch of their skin to 120° Fahrenheit, a temperature that most people find painful. However, participants reported very different experiences of pain. Using a 10-point scale, the least sensitive person rated the pain around a "one," while the most sensitive person rated the pain as almost a "nine."
People who reported higher levels of pain showed increased activation in areas of the brain important in pain: the primary somatosensory cortex, which contributes to the perception of where a painful stimulus is located on the body and how intense it is, and the anterior cingulate cortex, which is involved in the processing the unpleasant feelings evoked by pain. However, there was little difference between subjects in activation of the thalamus, which is involved in transmitting pain signals from the spinal cord to higher brain regions.
"This difference between cortical and thalamic patterns of activation may help explain pain differences between individuals," said Coghill. "This finding raises the intriguing possibility that incoming painful information is processed by the spinal cord in a generally similar manner. But, once the brain gets involved, the experience becomes very different from one individual to the next."
Coghill believes that most individual differences in pain sensitivity are probably due to a combination of cognitive factors, such as past experience with pain, emotional state at the time pain is experienced, and expectations about pain. Coghill's current projects include looking at how a person's expectations about pain influence the pain they actually experience.
Coghill's colleagues on the reported research were John McHaffie, Ph.D., associate professor of neurobiology and anatomy at Wake Forest, and Ye-Fen Yen, Ph.D., from University of Western Ontario.
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