Faces are important to us. From the moment we are are born, we prefer to look at faces than at other, inanimate objects, and, being social animals, we encounter faces every day of our lives. The face is the first thing we look to when identifying other people; faces also convey emotions, informing us of peoples’ mood, and from them we can usually determine a person’s sex and, sometimes, roughly how old they are. Eye movements can also reveal to us something about another person’s intentions.

Faces are so important for human social interactions that our brains contain a region that is specialised for processing them. This region, the fusiform face area, lies on the lower surface of the temporal lobe, and is thought to be unique to humans and other primates. Our childhood ability to recognize faces improves as we get older, in line with development of this region, but we still know very little about how the emergence of such abilities is linked to anatomical changes.

A surprising new study by researchers at Stanford University now shows that the size of this face-selective brain continues to increase well into the second decade of life. The findings, published in the latest issue of the journal Science, challenge our assumptions about the patterns of brain development in adolescence.

Jesse Gomez and his colleagues recruited 22 children and 25 adults, and scanned their brains using two different techniques – functional magnetic resonance imaging (fMRI), which measures brain activity indirectly by detecting changes in cerebral blood flow, and quantitative MRI (qMRI), a newly developed method that gives precise measurements of both the structure and composition of human brain tissue. The researchers focused on the fusiform gyrus in underside of the temporal lobe, which contains not only the FFA, but also, immediately adjacent to it, another region that is selective for scenes and places.

The researchers first identified both regions in the fMRI scans, confirming that one is activated only in response to images of faces, and the other only in response to images of places. Next, they used qMRI to produce detailed maps of both brain regions in all the participants. This revealed that the size of the fusiform face area increased with age – it was larger in the adult participants than in the children, and the older the individual, the larger it was. By contrast, no such difference was seen in the adjacent place-selective region, whose size remained stable in all the adults.

Gomez and his colleagues also tested the participants’ face and place recognition memory, and found that their ability to recognise faces was closely related to the size of their FFA – the bigger their FFA, the better the memory for faces. Their ability to remember places, on the other hand, was not at all related to FFA size. This suggests that the FFA continues to grow into adulthood, and that this growth is tightly linked to an improved face recognition abilities.

To confirm their anatomical findings, and try to establish how these size differences might occur, the researchers examined post-mortem brain tissue obtained from 10 adults. Sure enough, they found that the size of the FFA increased with age. Their analyses also suggested that these age-dependent size increases are likely due to a number of factors, especially the branching of dendrites, the growth of new dendritic spines, and the formation of myelin, a fatty substance that envelops nerve fibres to insulate them and hasten their conductance of impulses.

Until recently, it was believed that brain development ended at around 16 years of age. Although the brain has reached its full size by then, we now know that parts of it continue to mature until at least, and perhaps beyond 25 years of age. Most notably, the prefrontal cortex undergoes an extended period of maturation, during which huge numbers of synaptic connections are eliminated. This synaptic ‘pruning’ refines the prefrontal neural circuitry, making it more efficient at performing executive functions such as planning and decision-making.

The new findings seem to show that the FFA continues to grow into early adulthood, and therefore challenge the view that later stages of brain development are characterised by volume reductions caused by synaptic pruning. The findings are, however, somewhat limited by the small number of participants, and also by the fact that the oldest participants were just 28 years of age. It is at around this age that brain development is thought to end, and so we cannot say conclusively that the FFA continues to grow in adulthood until the findings are replicated in people of a much broader age range.

The findings nevertheless seem to provide yet another example of experience-dependent neuroplasticity, the process by which the things we do alter brain structure and function. Childhood, adolescence, and early adulthood are periods of life in which most of us expand our various social circles. The size of the FFA may therefore increase relative to the number of new faces we see and remember, and so it would also be interesting to whether its size does actually differ according to the number of friends we have.

Gomez, J., et al. (2017). Microstructural proliferation in human cortex is coupled with the development of face processing. Science, 355: 68-71.

Mo Costandi trained as a developmental neurobiologist and now works as a freelance science writer. His work has appeared in Nature, Science, and New Scientist, among other publications. He is also the author of Neuroplasticity (MIT Press, 2016) and 50 Human Brain Ideas You Really Need to Know (Quercus, 2013).

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