The question itself seems audacious: Can the human brain understand what goes on inside itself? Can people, by thinking about thinking, discover what happens when they think?
Although all -- or even most -- of the answers are not in, some researchers believe they are on their way to understanding thought.
The theory of thinking "is a very big area now," said Dr. J. Allan Hobson, a Harvard University psychiatrist who studies brain changes when people dream. "We have to regard it as mainly promising, but the methods are there."
Others agree. Dr. Mark Raichle, a neurologist at Washington University in St. Louis, said that recent work with brain imaging "has been more revealing than I ever would have imagined."
Neurologists are exploiting techniques for watching the brain in action to see what parts of it are used when people ponder different sorts of questions. And computer scientists are asking if they can apply hypotheses about human thought to make a machine that thinks.
The neurology research uses new techniques like positron emission tomography, or PET, scans that let scientists see the brain at work.
Before that, they could only look at dead brain tissue obtained at autopsy, or at brain structures, seen through computerized tomography, or CT scans, which produce X-ray pictures of living brains.
The PET scan work started several years ago, but because it is slow and difficult, involving thinking tests and scans of multitudes of people, the most intriguing results are only now being published.
PET scans use small doses of radioactive glucose, injected intravenously, to indicate on a monitor where blood flows in the brain.
When a small area of the brain is unusually active, that area
becomes engorged with blood, and that engorgement registers on the monitor.
PET scans, Dr. Raichle said, "tell us about the chemistry and pharmacology of the brain in ways we'd never thought of."
Dr. Michael Posner, a researcher at the University of Oregon, said he was struck by the way PET scans showed that very distinct small areas of the brain are turned on when people think.
That means rather than being a diffuse process, involving
scattered cells throughout the brain, thinking is more concentrated. That means it may be amenable to biochemical analysis.
"It's turned out, at least so far, that there is a dramatic localization," Dr. Posner said. "A lot of people hoped that that would happen, but I don't think anyone really expected it."
Dr. Raichle and his colleagues began studying thinking by using the PET-scan technique to see what happens in the brain when people are given words or nonsense strings of letters to read that look like English words but are not.
They went on to more complex PET-scan experiments, studying, for example, what happens in the brain when people are asked to quickly find a verb that goes with a noun.
They may be given the noun "hammer," for example. Many people will respond with the verb "pound." As people are given more nouns, they become faster at finding verbs to associate with them.
This association task uses parts of the brain that the investigators found, in their initial work, also come into play when people tried to recognize words -- a portion on the top along the left side and an area along the midline of the brain, where the two lobes come together.
But it also involved a part of the right side of the cerebellum, a JTC part of the brain that was thought to be used only to control movements.
Dr. Raichle and his colleagues recently found a lawyer in his 50s who had a stroke involving that exact part of the cerebellum.
Though the lawyer has returned to work and appears to be functioning normally, he cannot learn the noun-verb association task, the researchers found; he never gets faster at finding verbs, no matter how much he practices.
"In the context of his daily life, he's fine," Dr. Raichle said. "But if he tried to learn something entirely new, he might have a problem."
Asked whether different people might think in different ways or whether a person who is taught new and better thinking methods might have new areas of their brains start to light up in PET scans, Dr. Raichle said that he and others were still at the very beginning of the PET scan studies and were not yet able to answer the questions.
"These are the sorts of things that you talk about when you put your feet up Friday afternoon when the week is over," he said. "Sure, we think about them, but at the moment, we are only thinking about them."
While scientists like Dr. Raichle probe the human brain in action, computer scientists are trying to understand thought by looking at hypotheses about how people might think and then testing those hypotheses by building computers that will mimic human reasoning.