As we age, we get progressively better at recognizing and remembering someone’s face, eventually reaching peak proficiency at about 30 years old. A new study suggests that’s because brain tissue in a region dedicated to facial recognition continues to grow and develop throughout childhood and into adulthood, a process known as proliferation. The discovery may help scientists better understand the social evolution of our species, as speedy recollection of faces let our ancestors know at a glance whether to run, woo, or fight.
The results are surprising because most scientists have assumed that brain development throughout one’s life depends almost exclusively on “synaptic pruning,” or the weeding out of unnecessary connections between neurons, says Brad Duchaine, a psychologist at Dartmouth College who was not involved with the study. “I expect these findings will lead to much greater interest in the role of proliferation in neural development.”
Ten years ago, Kalanit Grill-Spector, a psychologist at Stanford University in Palo Alto, California, first noticed that several parts of the brain’s visual cortex, including a segment known as the fusiform gyrus that’s known to be involved in facial recognition, appeared to develop at different rates after birth. To get more detailed information on how the size of certain brain regions changes over time, she turned to a recently developed brain imaging technology known as quantitative magnetic resonance imaging (qMRI). The technique tracks how long it takes for protons, excited by the imaging machine’s strong magnetic field, to calm down. Like a top spinning on a crowded table, these protons will slow down more quickly if they’re surrounded by a lot of molecules—a proxy for measuring volume.
Grill-Spector and Jesse Gomez, a graduate student at Stanford, along with a team of colleagues, used the technique to investigate the fusiform gyrus as well as a nearby area of the brain known as the collateral sulcus, which is involved in recognizing familiar places and locations. They recruited 22 children, ages 5 to 12, and 25 adults, ages 22 to 28, and asked them to look at images of faces and places. They then used functional MRI to map out the brain regions that lit up in response and qMRI to figure out the volume of the brain tissue in those regions across both age groups.
There was virtually no difference in tissue volume between the children and the adults in the place recognition areas of the brain, the team reports today in Science. In the facial recognition areas, however, the qMRI results revealed that adults had, on average, about 12% more volume than the children.
“It’s quite surprising that this part of the brain continues to develop and change after infancy and into adulthood,” Grill-Spector says, “especially when just 2 centimeters away, in the place-recognizing region, this doesn’t occur.”
To see whether this increase in brain volume might lead to better facial memory, she and Gomez gave a computer-based facial recognition quiz to the participants. Here, the children and adults saw a face from three different angles, then were asked to identify it within a panel of similar-looking faces. Those with higher tissue volume in their fusiform gyrus performed better than those with lower volume, the researchers found. The results suggest that, in combination with synaptic pruning, a major reason children improve so dramatically in their ability to recognize faces as they age is that their fusiform gyrus continues to develop into adulthood.
Grill-Spector speculates evolution might have bestowed this special developmental adaptation because precise facial recognition is critical to growing up in large, social communities. When you’re young, you might only need to know the faces of your parents and close relatives, but as you grow older, the number of important faces you encounter continues to increase, she says. Compared with other types of visual information, recognizing faces might require extra processing power because each visage has the same basic layout with only relatively minor differences between people, she adds.
So what’s driving this increase in brain tissue volume as people age? It’s not the growth of new neurons, Grill-Spector says. Numerous studies have shown that the number of neurons in the brain remains remarkably consistent from birth to death. One possible explanation is that dendrites—branches that extend outward from neurons and receive signals from other nearby cells—within the neurons of the fusiform gyrus are expanding and increasing its volume, Gomez says. He likens it to a forest maturing: Its overall size might remain the same, but the trees’ branches grow denser and more complex.
Further research into this brain region’s continued development might shed light on disorders such as prosopagnosia, also known as face blindness, and autism spectrum disorder, the researchers say.