Now brain researchers, in their own fashion, have begun to do so. A major result emerging from the new research is that the brain does not have just a single emotional center, as has long been believed, but that different emotions involve different structures. Another is that the brains of men and women seem to generate certain emotions with different patterns of activity.
The new advances are made possible by fast imaging methods that allow researchers to take snapshots of the brain in action. The snapshots are short enough that they roughly parallel the duration of an emotion, however fleeting. They have already resulted in a radical redrawing of the neurological map for emotion, showing regions of emotional activity both in and beyond the limbic system, a ring of structures around the brain stem, which for 50 years was considered the brain's emotional center.
One surprising result of the remapping is that emotional opposites, like happiness and sadness, are not registered that way in the brain, but rather entail quite independent patterns of activity, according to a report this month in The American Journal of Psychiatry.
"It's because happiness and sadness involve separate brain areas that we can have bittersweet moments, like when a child is leaving home for college and you're sad, but happy, too," said Dr. Mark George, a psychiatrist and neurologist at the National Institute of Mental Health in Bethesda, Md., and the lead author of the report.
When a woman feels sad, Dr. George discovered with a brain imaging method known as positron emission tomography, her brain shows increased activity in the structures of the limbic system near the face, and more activity in the left prefrontal cortex than in the right. His studies were conducted in women to avoid the confounding difficulty of possible differences between the sexes.
When his 11 subjects felt happy, the characteristic pattern was a decrease of activity in the regions of the cerebral cortex that are committed to forethought and planning. These regions are in the temporal-parietal area of the cortex, located just over and a bit behind the ears, and the right prefrontal lobe, just behind the forehead. "Those neocortical regions are used in complex planning -- it's interesting these shut down in happiness," Dr. George said.
The cue for sadness was to ask the subjects to recall personal events in their lives, such as deaths and funerals, or to look at a picture of a sad face. For happiness, the cue was to remember joyous times such as births and weddings, or to look at happy faces.
In earlier research, Dr. George found that the neocortical areas become even less active when volunteers received injections of morphine or cocaine. "There seems to be a continuum in brain activity in the same regions from transient happiness to ecstasy," he said.
Another key change was in the amygdala, a pair of almond-shaped structures in the limbic system. The amygdala area "activates during sadness," Dr. George said. But the structures change only slightly when a person is happy. "The left amygdala seems to shut down a bit, while the right goes up," he said.
Such findings are mapping out a new neuroanatomical atlas for the emotions, one that eventually may give psychiatrists new guides to treating mental illness. "The brain mechanisms of emotional change are perhaps the most central question in psychiatry," said Dr. Robert Robinson, chief of psychiatry at the University of Iowa Hospitals and Clinics in Iowa City.
Many serious psychiatric disorders, such as depression and panic attacks, are extremes of ordinary emotion. Studies of anxiety, for instance, show that the brain regions that are most active while people are anxious are even more active during panic attacks. Locating the primary sites of various emotions represents a major step toward understanding what is going wrong when these sites become overactive.
The recent findings on sadness offer a new twist: brain areas involved in ordinary sadness almost completely shut down when a person is clinically depressed. "Sadness and depression seem to involve the same brain region, the left prefrontal cortex, in different ways," said Dr. George. "It gets more active during ordinary sadness, but shuts down in people with clinical depression. Perhaps the left prefrontal cortex somehow burns itself out when sadness persists for several months."
Many people with severe depression no longer feel sadness or any other emotion. "They're emotionally numb," said Dr. George.
He has also studied the locations of happiness and sadness in men, though these studies have not been published. He has found that the processing of emotion is yet another aspect in which the brains of men and women apparently differ. "When they are sad, women activate the anterior limbic system much more than do men," said Dr. George. "At the same time, women seem to experience a more profound sadness than do men. It makes me wonder if this might be related to why women have twice the risk of depression as do men."
In a study still in progress, Dr. George is mapping anger and anxiety. "Other work on anxiety implicates the right temporal area of the cerebral cortex, and our findings seem to support that," Dr. George said.
For anger, a main area of increased activity appears to be the anterior septum, which is in the center of the brain. "In cats, if you stimulate this area with an electrode the cat lashes out in rage at anything nearby," said Dr. George.
Before imaging, neuroscientists' principal method of mapping the sites of emotions in the brain rested on analyzing what was missing in patients who had had brain injuries or strokes. The technique is like drawing a diagram of a house's electrical wiring by pulling out the fuses one by one.
But the brain imaging techniques are still far from perfect. PET scans require subjects to be injected with a mildly radioactive chemical and make images that are averaged from multiple readings instead of from a single scan. And a serious problem with the magnetic resonance imaging technique is that a patient must lie in a metal cylinder, an experience that has been likened to being trapped in a coffin. "While you're in those machines it's very difficult to have reactions other than those to the machine itself," said Dr. Paul Ekman, director of the Human Interaction Laboratory at the University of California at San Francisco, who recently spent three hours in such a machine.
To overcome the problem, researchers have gotten their subjects into the desired emotional states with such prods as a scene from the film "The Godfather" in which a decapitated horse's head is found in a bed, a joyous scene from "On Golden Pond" in which Jane Fonda dances with her father, Henry Fonda, and even -- to elicit fear in people with a snake phobia -- a live (but friendly) python perched atop the doughnutlike PET scan equipment that surrounds the head. Patients having magnetic resonance imaging can watch films through a kind of periscope.
Dr. Ekman and colleagues plan to study how emotions are evoked by seeing which brain areas light up in response to stimuli like remembering an upsetting event, seeing an image of a sad face or putting one's facial muscles in the configuration typical of various emotions.
The investigation is of crucial importance for researchers, since the means they employ to evoke an emotion while capturing brain images may itself affect the image they get.
A team led by Dr. Richard Lane at the University of Arizona in Tucson compared the brain areas involved when people either watched film clips that evoked happiness or sadness, or called to mind happy or sad moments. In all cases there was heightened activity in the thalamus and the prefrontal cortex, suggesting a role for these regions in each of these emotions, no matter how it was evoked.
"The prefrontal cortex monitors a person's emotional state, no matter what it is, to generate an appropriate response," said Dr. Lane, "while the thalamus participates in how that response is executed."
During the film -- not during emotional memories -- two key areas of the limbic system were active: the amygdala and the hippocampus, suggesting that these structures are involved in evaluating whether a situation is of emotional import, Dr. Lane said.
On the other hand, during the recall of sad or happy events there was more activity in the anterior insular region, an area of the cortex with strong connections to the limbic system, implying a special role for this area in emotional memories. "The anterior insular region seems to be involved in investing thoughts or memories with emotional significance," said Dr. Lane.
That same region is also active during anticipatory anxiety, such as when someone is waiting to receive a mild electric shock, according to new findings by Dr. Eric Reiman, a psychologist at the University of Arizona in Phoenix, and a colleague of Dr. Lane's.
"The vast majority of findings until now about the brain's emotional regions has been based on emotions in laboratory animals, all of which are induced by external means, such as shocks," said Dr. Reiman. "But the results with people who are evoking emotions through memories are showing that a very different set of brain areas are active than had been thought from the animal research."
The exploration of the brain's topography for emotions through imaging is at its earliest stages and, like any forays into new terrain, may produce distorted maps, researchers warn.
"We need to be cautious about interpreting these findings on the regions involved in emotions," said Dr. Richard Davidson, a psychologist at the University of Wisconsin and a participant in some of the research. "We're just beginning to work out the technical difficulties in capturing neuroanatomical images from people during something so private and fleeting as an emotion."
Photo: Dr. Mark George, a Federal researcher, with photographs that were used as cues in studying how the brain processes different emotions. (Marty Katz for The New York Times) (pg. C9) Diagrams: "How the Brain Computes Tears and Laughter" The brain handles happiness and sadness in different areas, not in a single emotional center as was thought. New fast scanning methods show that happiness is marked by a decrease in activity in the cortex, in areas responsible for forethought and planning. Sadness is associated with enhanced activity in regions of the limbic system. PET scans below show changes in activity of subjects experiencing the two emotional states. (pg. C1) "Mapping the Brain: Let Your Mood Be Your Guide" The diagrams, which show the brain sliced down the middle and from the outside, show areas that researchers say are associated with happiness and sadness. The areas asscoiated with happiness showed a decrease in activity and those associated with sadness showed an increase. (Source: Dr. Mark George, Biological Psychiatry Branch, National Insitute of Mental Health; Principles of Neural Science and Ultimate Visual Dictionary) (pg.C9)