Normally, our brain actually loses neurons between early childhood and puberty in a process called "pruning." That applies to visual parts of the brain that identify things like cityscape or hallways, but not faces.

The researchers used two different MRI machines to scan both brain activity and density in two different parts of the brain: the region responsible for identifying faces, and an area used for other types of visual recognition. They then compared those structures in the brains of children (aged five to 12) to adults between 22 to 28. It turned out that adults had thicker fusiform gyrus regions than kids, different levels of proteins and cells and more activity. By contrast, the other visual regions showed lower levels of development.

Jesse Gomez and Kalanit Grill-Spector/Stanford Vision and Perception Neuroscience Lab

"We actually saw that tissue is proliferating," said grad student Jesse Gomez. "Many people assume ... that tissue is lost slowly as you get older. We saw the opposite –- that whatever is left after pruning in infancy can be used to grow." The researchers figure that the region expands because humans start with very poor facial recognition skills in infancy, but as we hit adolescence and meet more people, the region has to grow to keep pace.

The technique is the first to directly see such cell changes in living subjects, and could easily be adapted for other types of research and diagnoses. The next step, the team says, is to see if other regions of the brain also grow. The study could have a more immediate benefit, too, helping the two percent of the adult population that suffers from facial blindness, a disorder that makes it difficult to identify faces.