The difference in mental maturity between an 8-year-old and a 22-year-old is typically easy to identify in terms of emotions and behavior. The changes in the brain at the root of this difference, however, are much more difficult to see.

Researchers from the School of Engineering and Applied Science and the Perelman School of Medicine are revealing this process via a series of brain scans of hundreds of children ages 8-22.

The study, led by Danielle Bassett, the Skirkanich Assistant Professor of Innovation at Penn Engineering, used data from the Philadelphia Neurodevelopmental Cohort, a larger effort to understand brain development through brain scans, cognitive tests, and genetic assays conducted on nearly 10,000 local children and young adults.

Bassett is interested in the “community architecture” of the brain, or how its different parts “talk” to one another. By tracking similarities between activity patterns in different regions in the brains of 780 members of the cohort, Bassett and her colleagues could map out the various neural subsystems that are employed in memory, attention, motor control, and other cognitive tasks.

“If you perform this mapping when someone is simply resting in the MRI scanner,” Bassett says, “you see that there are modules, or parts of the brain that are very densely interconnected. These modules seem to map onto known cognitive functions like memory or attention. Thus, we can infer the different functions that the brain performs just from this connectivity map.

“We wanted to know how this connectivity map develops as the brain matures from childhood to adulthood,” she adds.

In plotting the community architecture of the brain against the age of the subjects, two trends emerged. Modules, which start more evenly sized, become less homogeneous, and connectivity between them decreases.

“The modules are essentially crystallizing,” Bassett says. “Cognitive function is becoming more specific, more focused. This is especially true of higher order cognitive systems, which become less connected to other parts of the brain as children age.”

There were two notable exceptions: Modules dealing with sensory-motor function did not change, and the “default mode” module became more diffuse and connected with other modules. This meshes with expectations, however. Children’s sensory-motor function is already well-developed by age 8, and the “default mode” is thought to play the role of a sort of a mental transportation network, allowing people to switch between the many different modes of thought found in everyday experience.

This neural map adds weight to existing theories of neurodevelopment, as well as provides additional context for what might be going wrong in certain cognitive disorders.