A comparison of kid brains and grownup brains may explain why our ability to recognize faces keeps getting better until about age 30.
Brain scans of 25 adults and 22 children showed that an area devoted to facial recognition keeps growing long after adolescence, researchers report in the journal Science.
The area didn't acquire more neurons, says Jesse Gomez, a graduate student in neurosciences at Stanford University and the study's lead author. Instead the brain region became more densely populated with the structures that connect and support neurons.
"You can imagine a 10-foot by 10-foot garden, and it has some number of flowers in there," Gomez says. "The number of flowers isn't changing, but their stems and branches and leaves are getting more complex."
To see whether that sort of change occurred elsewhere in the brain, the researchers also looked at a nearby area that responds to places, instead of faces. In this area, there was no difference between children and adults.
The results suggest that brain development is more varied than researchers once thought.
For years, scientists have focused on a process known as synaptic pruning, which shapes the brain by eliminating unused connections among neurons. Most synaptic pruning takes place in the first few years of life.
"After age 3, the textbooks are pretty silent about what's going on in the brain," Gomez says.
The continuing growth in facial recognition areas may be a response to the need to recognize more and more faces as children grow older, says Kalanit Grill-Spector, a professor in the psychology department at Stanford.
"When you're a young child, you need to recognize your family and a handful of friends," she says. "But by the time you've reached high school or college your social group has expanded to hundreds or even thousands of people."
And recognizing all those people requires a lot of brain power, Grill-Spector says, because "all faces have the same features and the same configuration."
Ongoing changes in the brain may also help children focus on different sorts of faces at different stages of development, says Suzy Scherf, an assistant professor of psychology at Penn State University.
"Children's face recognition early on is very much tuned to adult faces," Scherf says. "In adolescence it changes to be highly tuned toward adolescent faces."
Understanding how facial recognition develops throughout childhood could make it easier to figure out why some people have difficulty recognizing faces, researchers say.
Gomez hopes to scan the brains of people with "face blindness," a disorder that can leave a person unable to recognize even familiar faces.
And Scherf wants to know whether people with autism, who often struggle to recognize faces, have abnormal development in the facial recognition area of their brains.