Scientists have recently found a brain area that keeps growing unexpectedly into adulthood.
Image credit: Jesse Gomez and Kalanit Grill-Spector at the Vision and Perception Neuroscience Lab
Human brain development is thought to start in the third gestational week. Then, neural progenitor cells start to differentiate into specific neural structures and functions - a process influenced by both genes and the environment.
This process of fetal development carries on until birth, when the basic structures of the central and peripheral nervous system are roughly established.
After birth, the brain continues to develop. During the preschool period, the brain grows four times in size and reaches almost 90 percent of its adult volume by the age of 6.
As children, our brains produce an excess of synaptic connections between neurons. During adolescence, the brain continues to morph into its adult form by discarding these unnecessary synapses.
The process - which lasts well into our 20s and is known as "synaptic pruning" - is thought to be largely responsible for the brain's development and is crucial for normal social behavior. However, a new study suggests that growth in size, not synaptic pruning, is what helps the brain to mature.
The new study was published in Science, a journal of the American Association for the Advancement of Science.
Development of facial and spatial recognition brain areas
A team of international researchers - led by Jesse Gomez from Stanford University School of Medicine in California - set out to better understand the brain's ability to recognize faces - a critical component of social behavior and normal social interaction.
Gomez and team used anatomical, quantitative, and functional magnetic resonance imaging (fMRI) to compare brain tissue across study participants.
Using MRI scans, researchers examined 22 children between 5 and 12 years of age, and 25 adults aged between 22 and 28. They also tested the participants' ability to recognize faces and places.
The face recognition task consisted of the Cambridge Face Memory Test, and it used child faces instead of adult ones. Place recognition was assessed using an "old-new" recognition task developed by the scientists.
The team measured cortical thickness - the macromolecular and lipid tissue volume - as well as the composition of the tissue, including the lipid and cholesterol content of cell walls and myelin. Myelin is the fatty white substance that covers the axons of some nerve cells and ensures the speedy conductance between neurons.
Gomez and team corroborated the results of these in vivo measurements with postmortem analyses of adult brains. They also used brain modeling techniques to uncover the mechanisms responsible for the observed changes in brain tissue volume.
Facial recognition brain area continues to grow in adults
The measurements revealed that the cortical tissue developed differently in the face and place recognition areas of the right brain hemisphere.
In adults, the brain region that enables facial recognition was found to increase in size, whereas the region responsible for recognizing places remained the same.
The region identified as dealing with facial recognition is the fusiform gyrus. Tissue development in this area was correlated with improvements in functional face selectivity and face recognition.
Development of the face-selective regions - but not the place-selective ones - was found to be dominated by microstructural proliferation. These findings were confirmed by the cytoarchitectonic measurements taken in postmortem brains.
The scientists also analyzed postmortem brains to see if the changes in size were due to increased myelination. They concluded, however, that changes in myelination cannot be the sole explanation for the expansion of the brain region.
Therefore, the authors suggest that this unexpected increase may be caused by a combined increase of the cell body, dendritic structures, and myelin sheath.
The authors conclude:
"These data suggest a new model by which emergent brain function and behavior result from cortical tissue proliferation rather than from pruning exclusively [and] a rethinking of the anatomical development of cortex throughout childhood."