Human brains are stunningly diverse. No two are identical, not even those of identical twins. So when scientists are looking at a brain, how do they know when they are looking at one that is normal?

It's a question that has baffled scientists for years. But now, researchers say a newly compiled online atlas that contains digitally mapped images of 7,000 brains will be instrumental in helping them understand what constitutes a "normal" brain, and, in the process, could lead to important discoveries about how students learn.

Use of the atlas will allow researchers to compare and contrast these brain images, captured from all sorts of people living in seven nations on four continents. Most are between the ages of 20 and 40, but some are as young as 7 and as old as 90.

Along with the brain images of "normal" people are those of people suffering from Alzheimer's, fetal alcohol syndrome, autism, and schizophrenia. More data are continually being added.

"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," said Dr. John Mazziotta of the International Consortium for Brain Mapping, based at the University of California, Los Angeles. "But how different they were and how to compare them was not known."

His colleague, Dr. Arthur Toga, says the plan is to quantify the differences between brains. Understanding the variations should provide "a good index between normal populations and a diseased population," Toga said.

A decade in the making, the brain mapping project quietly debuted this summer. The resulting atlas—freely available to registered users over the internet—maps the brains in multiple dimensions. It charts brain activity, pinpointing the seat of functions such as speech, memory, emotion, and language and highlighting how those locations can vary among individuals and populations.

A researcher using this resource 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 asked.

Findings from other types of comparisons also might lead in some surprising directions, Toga suggests, such as tweaking primary school curricula.

Some data suggest the brain is more receptive to learning some skills at certain stages of development, so it should be useful to map the growth of regions of the brain connected to specific skills, he said.

And because all of the brain images 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.