When we think about brain development and plasticity, most of us think about the birth and death of neurons or changes in the number or strength of synapses. Yet, no part of the brain works in isolation. Brain maturation and plasticity also depends upon the large nerve fiber tracts that connect different brain regions together. These include the corpus callosum that link the two cerebral hemispheres, tracts that connect the cerebral cortex and lower brain areas and spinal cord, and "association" tracts that connect different parts of the cerebral cortex together. Strikingly, all long association fiber pathways have a terminus in the frontal lobes of the cerebral cortex, a brain region involved with emotional regulation, social behavior, attention, and planning. No other part of the brain is so richly connected.
When do the large fiber tracts mature in the human brain? The axons of long fiber tracts are surrounded by myelin, a fatty material that speeds up the conduction of nerve impulses. Since the presence of myelin gives the axons a whitish appearance in preserved specimens of the brain, these large fiber tracts are also called "white matter." Sensitive brain imaging methods, such as diffusion tensor imaging, can measure changes in myelination throughout the lifespan.