How do brain mechanisms develop from childhood to adulthood? There is extensive debate if brain development is due to pruning of excess neurons, synapses, and connections, leading to reduction of responses to irrelevant stimuli, or if development is associated with growth of dendritic arbors, synapses, and myelination leading to increased responses and selectivity to relevant stimuli. Our research addresses this central debate using cutting edge multimodal imaging, obtaining multiple measurements of brain function using functional magnetic resonance imaging (fMRI), and brain anatomy using quantitative MRI (qMRI) and diffusion MRI (dMRI) in each of 27 children (ages 5-12) and 30 adults (ages 22-28). We use the face recognition system as a model system to study brain development as it is a well understood cortical system that shows particularly protracted development throughout childhood and adolescence, into adulthood.
Both functional and anatomical measurements provide compelling empirical evidence supporting the growth hypothesis. Functionally, results reveal (1) age-related increases in the size of face-selective regions, (2) age-related increases in responsiveness and selectivity to faces, and (3) a developmental increase in neural sensitivity to face identity, which is correlated with an increase in perceptual discriminability of faces. Importantly, this development is specific, occurring in face- but not object- and place-selective regions and cannot be explained by differences in data quality or measurement noise across age groups. Anatomically, we find (1) age-related decreases in T1 relaxation that are associated with increases in macromolecular tissue volume in face- but not place-selective regions, which we validate in histological slices of postmortem brains, (2) this tissue development is correlated with specific increases in functional selectivity to faces, as well as improvements in face recognition, and (3) the largest developmental decreases in both T1 relaxation and mean diffusivity occur close to the gray-white matter boundary of face-selective regions, suggesting that in addition to dendritic complexification increased myelination may contribute to tissue growth. Together, these data suggest a new model by which emergent brain function and behavior during childhood result from cortical tissue growth rather than from pruning.