Scientists who have spent the last decade charting the human brain have quietly debuted the fruit of their labors: a mammoth digital atlas that maps in multiple dimensions thousands of examples of the most complex of organs.

Researchers designed the atlas, housed at the University of California, Los Angeles, to be used in probing both the brain's form and function, as well as the interplay of both as influenced by age, gender and health.

Its compilers first made it available over the Web, free to registered users, a few weeks ago, said Dr. Arthur Toga, who leads the International Consortium for Brain Mapping with his colleague, Dr. John Mazziotta.

Like a geographic atlas, it too traces a material world, including that of the densely convoluted folds of gray matter that make up the cerebrum, the brain's most instantly recognizable feature.

It's more than mere landscapes, though: The atlas also maps brain activity, pinpointing the seat of functions such as speech, memory, emotion and language and highlighting how those locations can vary between individual people and populations.

It's as if the geography of a traditional atlas was overlaid with weather patterns, ocean temperatures, socio-economic data and the ebb and flow of vehicle traffic.

Early cartographers of the brain typically relied on a single example to create atlases purported to be universal in scope; the latest amassed data on 7,000 brains, including those of 342 twins.

"This is a project born of frustration, basically. For many years, all of us who study brain structure and function have struggled with the fact that no two brains are the same - not in shape or size and certainly not in function," Mazziotta said. "But how different they were and how to compare them was not known."

Such conclusions cannot be drawn from study of a single individual, researchers said. Brains are stunningly diverse: no two brains are identical - not even those of otherwise identical twins.

That diversity complicates efforts to draw even basic conclusions about how the nerve-packed organ should look, much less act.

Gathering a large number of brains together in the database allows researchers to capture their differences and quantify them in a sophisticated game of neurological compare-and-contrast, Toga said.

"You want to quantify the differences between brains because it tells you how variable we are. And understanding how variable we are normally also gives us a good index between normal populations and a diseased population," Toga said.

The database also contains images and data on many hundreds of diseased brains, among them those of people suffering from Alzheimer's, fetal alcohol syndrome, autism and schizophrenia. More data are continually being added.

Housed in a humming supercomputer on the UCLA campus, the atlas contains more than 100 terabytes of data, or roughly five times the information in all the books in the Library of Congress.

Much of that data is stored on cartridges, arrayed like 45s in an old-fashioned jukebox, that a robot arm plucks at from behind a glass window.

The data were collected from anonymous subjects living in seven nations on four continents. The bulk are between the ages of 20 and 40, although some are as young as 7 and as old as 90.

The database contains extensive files on those subjects, including details of their age, nationality, education, profession, eating habits and recreational interests. Also included are details of their intelligence, spatial ability and whether they are right- or left-handed.

Added to the mix are multiple images that show each brain's anatomy, as well as blood oxygen flow, electrical activity and where radioactive tracers collect within the organ.

Together, the latter three give an indication of brain activity as the subjects performed a variety of tasks, including tests of their ability to recall number sequences and recognize objects.

A researcher using the atlas can call up any number of combinations of brains - say, those of a group of left-handed, 20-year-old Asian women - and compare them with another population to illustrate their similarities and differences.

"Might they be different from 90-year-old, left-handed men?" Toga said.

Use of the brain atlas also could lead in some surprising directions, including a tweaking of primary school curricula, Toga said.

Empirical data suggest the brain is more receptive to learning some skills at certain stages during development. Mapping the growth of regions of the brain known to be involved in the acquisition of those skills could validate those observations.

Since all of the brain images in the atlas have been stretched, shrunken or otherwise manipulated to fit a standard reference, researchers can make apples-to-apples comparisons.

That sort of big-picture view gives clarity that individual snapshots cannot, said Michael J. Ackerman, of the National Library of Medicine.

"One and one is three: All these data out there is cool but when you're able to put it together and work with it, you begin to see things that weren't there when it was in individual pieces. That's the beauty of an amalgamated project," said Ackerman, who spearheaded the Visible Human Project, a digital atlas of detailed, three-dimensional representations of the normal male and female human bodies.

Pinpointing the same region on multiple brains results in locations that are, literally, all over the map. That underscores the differences inherent in each of us, researchers said.

Those differences may foil even the broadest efforts to generalize, said Stephen Hanson, of Rutgers University, who is not connected with the brain atlas project.

"Finding Africa versus America - that's not a good metaphor for this trillion-synapse thing," said Hanson, co-director of the Rutgers' Mind/Brain Analysis Center. "It's very hard to say 'That part of tissue is doing that and that is not doing that.' Different tissues might be recruited at different times - like Tinkertoys."

The atlas can tell a researcher where a certain structure or activity is most likely to occur. That statistical analysis can allow researchers to be sensitive to the subtlest of differences in exploring the complexity of the brain.

"It has statistical power. We know what our confidence level is - that's where it becomes important. To guess without confidence isn't much use," Toga said.