Spatio-temporal Patterns of Brain Growth and Cortical Thinning May Help to Explain Cognitive and Behavioral Changes Occurring During Adolescence:

Background: Over the last few years, there has been a surge of scientific interest in teenage brain development, notably because of the general belief that the largest, most important changes in brain structure occur prior to birth and during the first few years of life. While brain structural changes occurring during teenage years are more subtle than earlier maturational changes, the former are more robust and likely to be involved in the transition from notorious teenage cognitive and behavioral patterns (i.e., impulsivity, emotional lability, poor planning) to more mature adult-like functioning. In vivo magnetic resonance imaging (MRI) studies of brain development during adolescence have revealed subtle total brain volume increases, along with regional patterns of gray matter (nerve cells and connections) volume reduction, and white matter (myelin insulating nerve fibers) volume increases. Yet, little is known about the relationship between brain growth and tissue density changes. Understanding these complex spatio-temporal relationships could help shed new light on important biological processes contributing to the brain maturation observed via MRI.

Advance: An international team of scientists evaluated the relationships between brain growth and tissue density changes, using high-resolution MRI combined with novel computational image analysis techniques. They mapped continued postadolescent brain growth, demonstrating for the first time that it occurs primarily in the dorsal aspects of the frontal lobe bilaterally. Notably, anatomic maps of the spatial distribution of postadolescent cortical gray matter thinning were highly consistent with maps of spatial distribution of postadolescent brain growth, showing an inverse relationship between cortical gray matter thinning and brain growth, primarily the brain's superior frontal regions, which controls executive cognitive functioning.

Implications: This exciting new finding that the brain continues to grow in frontal regions where the cortex gray matter is thinning, opens the possibility that regressive cellular changes, that would eventually have to result in a net volume loss, cannot solely account for the reduction in cortical thickness observed during and after adolescence. Rather, it appears that increased myelination, which would seem to necessarily result in a net brain volume increase, is the more likely candidate cellular event, resulting in the reshaping of the frontal cortex that occurs during the postadolescent years. Thus, examination of the spatio-temporal patterns of brain growth and cortical thinning could help to explain the cognitive and behavioral changes occurring during adolescence, as well as to provide better understanding of the relationships among different cellular maturational events. One can further speculate that improved accuracy in thinking performance may result from regressive changes such as the pruning of nerve connections during this age range. On the other hand, increased efficiency might result from increased myelination observed as brain growth, given that myelinated fibers improve conduction speed of electrical impulses between various brain regions. By looking at brain growth and gray matter density at the cortical surface simultaneously, scientists can now test these hypotheses and parse out the relative contributions of these various factors to functional and structural brain maturation.

(A.W.. Toga)