If sharp thinking appears effortless at times, that may be because it is, researchers reported yesterday at the annual meeting of the American Association for the Advancement of Science.

Using new imaging technology to peer into the brain while a person is thinking, researchers have found that there is hot thinking and cold thinking -- the kind that uses up lots of energy and the kind that does not.

It appears that when taking intelligence tests, those scoring high are also those who put the least energy into their cerebrations. Those burning up their brain circuits, by contrast, do poorly on tests.

Thus, efficiency may be a major part of sharp thinking, at least on intelligence tests.

Richard J. Haier from the University of California at Irvine measured the levels of energy use in the brain by feeding 17 people sugar labeled with slight radioactivity -- just enough to be detected by the brain scanner, called a positron emission tomographer, or PET scanner.

When the brain cells are working on a problem, they use sugar for energy. The more active the brain, the more sugar will be absorbed and used, Haier reported to AAAS, whose five-day annual meeting in Boston ends today.

The PET scan can spot minute amounts of the radioactively labeled glucose taken up by cells hours later.

Haier gave eight of 17 people the "Ravens" test of abstract reasoning -- which required them to look at complex geometrical figures and find one that would logically complete a series.

Those who scored well showed little glucose uptake, and those who did poorly showed far more energy use. In the extreme, a person showing great activity in the brain achieved a score one-third of that gained by someone whose scan showed little effort was expended.

In a control group of nine people who were paying attention to flashing visual images, there was no significant difference in glucose use.