Human brain reveals the anatomy of pain.

A preliminary study suggests the human brain processes physical pain in three specific and unexpectedly localized areas, including one previously linked only to emotions and motivation. On brain scans, these regions display marked jumps in blood flow as volunteers perceive "painful but tolerable" heat on the right forearm, researchers report in the March 15 Science.

All three structures lie in the brain's outer layer, or cerebral cortex, where they apparently interpret information about pain intensity and about the side of the body on which the painful stimulus occurs, says study director Gary H. Duncan of the University of Montreal.

Scientists have debated the role of the cerebral cortex in pain processing, Duncan says. Moreover for ethical reasons, most studies of the brain's response to pain have focused on people with specific types of brain damage or disease. For instance, some epileptics with disturbances of the primary or secondary somatosensory areas - two structures involved in bodily sensation and located near the brain's midpoint - experience pain during seizures.

Duncan's team conducted a rare investigation of pain processing among healthy people, each of whom received $100 to participate. Eight right-handed men between age 25 and 31 under underwent six or seven positron emission tomography (PET) scans, each lasting 1 minute, in a single session. The PET scans tracked the disintegration of a short-lived radioactive oxygen isotope injected in a water solution, and converted the data into images of blood flow in the brain.

During two of the scans, a probe delivered a 5-second pulse of moderately painful heat to six spots, one after another, on each man's forearm. The heat reached 48 degrees to 49 degrees C, roughly comparable to that of a mug of hot coffee, Duncan says. During the next two scans, the volunteers endured warm but not painful pulses of 41 degrees to 42 degrees C at the same six forearm spots. The remaining no-stimulus scans served as controls.

To identify areas of blood flow change, the researchers combined PET data with information from magnetic resonance images of each man's brain anatomy.

Heat pain activated blood flow in the two somatosensory areas, but only on the brain's left side, the team reports. One other region - the left side of a structure called the anterior cingulate gyrus, considered critical for controlling emotions - also showed a substantial blood flow rise in response to heat.

The anterior cingulate activation reflects a pain response, not simply anxiety or emotional arousal, Duncan asserts. Each man received heat pulses within his range of pain tolerance and underwent practice sessions to reduce anxiety, Duncan notes, and the volunteers' heart rates stayed normal during the experiment. Moreover, anxiety would have activated other regions linked to the cingulate cortex, he says.

Duncan and his co-workers now plan to investigate brain responses to heat pulses on the left forearm - controlled by the brain's right side - to determine whether blood flow rises only on the right side of the three critical regions. "We're still at the preliminary stage, particularly in studying the cingulate cortex's role in pain processing," he says.