A PICTURE OF THE DEVELOPMENT OF THE ADOLESCENT BRAIN: A STRUCTURAL AND FUNCTIONAL ASSESSMENT
Laurence Stanford, Ph.D.
National Institute on Drug Abuse
David Shurtleff, Ph.D.
National Institute on Drug Abuse
Maturation of the human brain is a complex, protracted process that is being extensively studied across several levels of analysis using neuroimaging, as well as cognitive and behavioral techniques. There is now considerable evidence that points to the adolescent period as a time during which critical neurobiological and cognitive capacities undergo maturation. Speakers will describe research looking at cortical and subcortical brain and cognitive changes in adolescence using magnetic resonance imaging, taking advantage of the capabilities of MRI for demonstrating structural, chemical, connectional, and functional patterns. Structural, chemical, and cognitive changes observed in healthy children, will be contrasted with developmental alterations related to early onset substance use disorder (SUD) and attention deficit hyperactivity disorder (ADHD) among others. Research on the development of basic cognitive control processes will describe how the maturation of cognitive processes can be related to the development and differential activation of frontostriatal brain regions. This symposium provides an overview of the changes in brain development and cognition associated with normal development and early onset behavioral and psychiatric disorders.
Mapping Adolescent Brain Maturation With Structural Magnetic Resonance Imaging
Elizabeth R. Sowell, Ph.D.
This talk highlights our recent work evaluating maturational change in brain structure during the childhood and adolescent years and how these structural changes relate to changing cognitive abilities and brain activation. The primary focus of this talk is on recent studies using cortical pattern matching (CPM) techniques to assess age-related changes in gray matter distribution and brain growth during the childhood and adolescent years. Specifically, we have found cortical thinning over large regions of the dorsal, frontal, and parietal lobes, and increased cortical thickness in primary language cortices in three independent samples of normal individuals studied both cross-sectionally and longitudinally. The spatial and temporal pattern of results, with incomplete development of frontal cortices during adolescence, is consistent with observations of increased risk taking behaviors during this period. We have also used CPM methods to assess relationships between cortical thickness and cognitive function on tests of general verbal intellectual functioning and have recently found cortical dissociations between improved phonological processing and motor skills within children studied longitudinally. Finally, we discuss findings of relationships between cortical thickness and brain activation on tests of language and executive function in normally developing children and adolescents with combined functional and structural MRI. An integration of these findings highlights the great variability in brain structure and function that occur during normative adolescent development and how brain structure may be a better predictor of cognitive function and brain activation than chronological age.
Magnetic Resonance Spectroscopy Studies of Human Brain Development
Perry F. Renshaw, M.D., Ph.D. and Young-Hoon Sung, M.D.
Magnetic Resonance Spectroscopy (MRS) provides a unique in vivo assessment of the chemistry of the human brain. In addition, the role of MRS as a tool for characterizing human brain development has become more significant due to recent technical advances. Proton (1H) and phosphorus (31P) MRS detectable metabolites provide information with respect to neuronal viability from N-acetyl aspartate (NAA); membrane metabolism from choline containing compounds (Cho); phospholipid metabolism from anabolites (phosphomonoesters; PME) and catabolites (phosphodiesters; PDE); and bioenergetics from high-energy phosphate metabolites.
A MEDLINE literature search was done from 1989 to 2006 and initially 54 articles relevant to human brain development were identified. A much smaller number of these articles described changes occurring during adolescence.
White matter NAA levels, which reflect axonal development and myelination, increase steeply throughout the first decade of life and reach a peak around adolescence. Gray matter NAA levels decrease in adolescence relative to childhood, perhaps due to synaptic/neuronal pruning that mainly occurs in adolescence.
The Cho resonance shows a decreasing concentration with age early in life, but not during adolescence. Cho serves as an intermediate for cell membrane formation. PME and PDE levels generally reflect anabolic and catabolic states of freely mobile membrane phospholipids, respectively. Several study results suggest increasing PME levels or phospholipid turnover in adolescent brain white matter. This finding may reflect both increased glial and neuronal membrane synthesis.
MRS is a useful tool for exploring the maturational process and provides valuable in vivo information for further investigating the developmental and diseased human brain. Knowledge of dynamic metabolite changes with age using MRS, especially during the adolescent period, is important for monitoring and evaluating normal brain maturation.
Risk Taking and the Adolescent Brain: Who is at Risk?
B.J. Casey, Ph.D.
Adolescence is a developmental period characterized by suboptimal decisions and actions that give rise to an increased incidence of unintentional injuries and violence, alcohol and drug abuse, unintended pregnancy, and sexually transmitted diseases. Traditional neurobiological and cognitive explanations for adolescent behavior have failed to account for the nonlinear changes in behavior observed during adolescence, relative to childhood and adulthood. This presentation provides a biologically plausible conceptualization of the neural mechanisms underlying these nonlinear changes in behavior as a heightened responsiveness to incentives and emotional reactivity, while impulse control is still relatively immature during this period. Recent human imaging and animal studies provide a biological basis for this view, suggesting differential development of limbic systems relative to top-down control systems during adolescence, compared to childhood and adulthood. This developmental pattern may be exacerbated in those adolescents with a predisposition toward risk taking, increasing the risk for poor outcomes.