Neuroscientists at Washington University School of Medicine in St. Louis have assembled a first-of-its kind atlas of the folds of the cerebral cortex, the wrinkled surface layer of the brain credited with many of the higher cognitive functions that make us human.
The atlas, known as the Population-Average, Landmark and Surface-based (PALS) Atlas, links brain functions to the various peaks and valleys of the cortex.
PALS is the first brain atlas that accurately portrays the complex folds of the cerebral cortex not just from a single individual but from a group of individuals. It is available online both to provide a resource for neuroscientists seeking to determine the functions of an interesting brain area and as a repository for adding new data that expand and fine-tune the atlas' maps.
The creators hope it will be useful in a wide variety of research projects. Among other studies, PALS is already helping scientists understand how an inherited disorder changes the brain and how brain function adjusts in response to blindness.
A paper on the creation of PALS appears online this week in the journal Neuroimage.
Senior investigator David Van Essen, Ph.D., the Edison Professor of Neurobiology and head of the Department of Anatomy and Neurobiology, has been compiling cortical cartography for two decades. He compares scientists' current knowledge of the details of human brain structure to 17th-century mapmakers' grasp of the surface of the Earth.
"We know a lot, and we're learning much more all the time, but some features that we would like to be able to definitively pinpoint are actually rife with uncertainty," he says. "And large fractions of the cortex have areas of controversy or outright error."
In all fairness to cortical cartographers, Van Essen notes, mapmakers charting the Earth never had to deal with the tremendous variability found in billions of individual human brains. Genetic and environmental differences, reactions to injury, and inherited disorders can all change the topography of the brain in minor and major ways.
To compensate for these variations, neuroscientists have developed a process called registration that lets them mathematically adjust the results of a brain scan to either match it with an atlas or enter it into the atlas.
This approach previously has been used to assemble atlases based on study of the volume of different brain areas.
"The cerebral cortex has been crumpled up so that it can fit snugly inside the skull, like a beach ball crumpled to fit inside a cardboard box," Van Essen explains. "Volume-based registration is like squeezing and twisting each cardboard box so that they all end up the same size. That's a good start, but it doesn't allow us to bring different folds from the individual beach balls into alignment."
Surface-based registration sets the box aside, according to Van Essen.
"That lets us inflate each cortex into a perfectly spherical shape, and then it becomes much easier to rotate and locally deform each beach ball so that they are all consistently aligned to the target atlas," he explains. "We can do this with much more accuracy than is possible when both the atlas and the individual brain are in the crumpled state, and this leads to a dramatic improvement in the quality of the results."
Initially based on MRI scans of 12 healthy people gathered by Randy Buckner, Ph.D., associate professor of neurobiology and of radiology, PALS now includes data from 60 additional scans. To give the atlas its first test run, Van Essen conducted a study of the structural differences between the left and right hemispheres of the brain.