MRI Captures Differences in Pain Perception : Statistically significant differences in brain activity found beyond the thalamus but not within it.

The investigators determined odors each patient did not like and then performed essentially the same comparison test as before, but using two odors with the two painful heat stimuli.

They found that regardless of where the subjects' attention was focused, if the odor was pleasant, they were in a good mood, but a bad odor always put them in a bad mood.

Attention significantly affected subjects' ratings of pain intensity but not of pain unpleasantness, whereas odor, the more emotionally fraught stimulus, had a strong and significant effect on unpleasantness and thus mood but no such effect on intensity.

She concluded that psychological factors might contribute to many pain states–for example, allodynia, in which a light touch can elicit severe pain. In such cases–including in experiments conducted by her pain clinic–the patient's perception is borne out by what is happening in the brain.

When asked in a panel discussion whether the ability to divert attention from one's pain could be affected by cognitive factors, she alluded to research findings that chronic-pain patients with more solicitous spouses reported more pain and had more activation of the pain region in the brain than did those with less solicitous spouses.

“By constantly asking a patient about their pain, you're focusing their attention on their pain,” Dr. Bushnell said.

Roger B. Fillingim, Ph.D., of the University of Florida, Gainesville, presented a discussion of the individual factors affecting pain perception, which can include gender, ethnicity, physiological and psychological states, and genetics. He cited previous research from his laboratory showing that men were able to tolerate ischemic pain longer than women could.

Dr. Fillingim also discussed an analysis showing that for ischemic pain, heat, pressure, and temporal summation of heat pain, the group with the highest pain sensitivity was more heavily populated by women and ethnic minorities. He cautioned, however, that such studies rely on self-reported pain thresholds.

To avoid this limitation, his group performed a study in which they induced a leg-muscle reflex normally correlated with pain. They found that African Americans responded to a lower stimulus intensity than whites did.

In the area of gender differences, Dr. Fillingim's group found that for all pain measures (heat, cold, ischemic, and pressure pain), men had greater-than-average self-reported tolerance, whereas women had less than the mean. This may be influenced by attitudes toward pain, such as catastrophizing, he noted.

But interestingly, positive affect was correlated with decreased pain sensitivity only in men. Analgesic responses were less consistent; morphine or pentazocine showed only insignificant sex differences in a study by his group. However, negative affect in men predicted less analgesia but had no effect on women's responses to pain medication. The same was true for catastrophizing.

There are also genetic factors that research has shown might influence pain and analgesia, Dr. Fillingim said. In particular, an allele of the OPRM1 gene predicts lower pressure-pain sensitivity in men. This allele is rare in African Americans but more common in whites and Hispanics, he noted. But only in whites did this allele seem to confer less pain sensitivity; the opposite was true in Hispanic subjects.

It's important to determine which factors predict individual differences in pain response, he emphasized, so that future treatment approaches can be tailored to each patient.

Activation (yellow) is visible during pain stimulation; the subject expected the administered 50° C stimulus.

There was less pain-related activation during the same stimulation when the subject expected a reduced stimulus. Images courtesy Dr. Robert C. Coghill/Wake Forest University School of Medicine