First report from large-scale study establishes norms for 6- to 18-year-olds.
Yes, there are gender differences in cognitive function, but they’re more limited than previously thought. And yes, income does affect cognitive performance – but less than expected when only healthy children are considered. And while basic cognitive skills steadily improve in middle childhood, they then seem to level off – questioning the idea of a burst of brain development in adolescence. These findings, published online on May 18 in the Journal of the International Neuropsychological Society, are the first data to emerge from the National Institutes of Health (NIH) MRI Study of Normal Brain Development, a large, population-based study that began in 1999 and is documenting structural brain development and behaviour from birth to young adulthood. The analysis, led by Deborah P. Waber, PhD in the Department of Psychiatry at Children’s Hospital Boston, focused on cognition and behaviour in healthy 6- to 18-year-olds enrolled at Children’s and five other metropolitan areas across the United States. Population-based sampling techniques used U.S. Census data to ensure demographic diversity. A rigorous screening process eliminated children with medical, neurologic or psychiatric disorders, familial risk factors for such disorders, or prenatal exposure to toxic substances, providing a glimpse of how a healthy brain develops. “This report – and many others that will follow – provides a comprehensive set of benchmark values that clinicians and scientists studying brain development can reference for many years to come,” said NIH Director Elias A. Zerhouni, MD. From an initial sample of more than 35,000 target families, the researchers were able to enrol approximately 450 children, of whom 385 were 6 years or older. Once enrolled, the children underwent MRI scans of the brain and completed a battery of behavioural and cognitive tests to ascertain their overall IQ, verbal ability, mental processing speed, spatial ability, memory, fine motor dexterity, psychosocial function, reading and calculation ability, and other measures of cognitive function. Most have returned two more times so that their development can be tracked. Overall, this healthy group performed better than previously reported norms. However, analysis of the first wave of data also found that:
- Sex predicted few aspects of cognitive function, with gender effects less prevalent than in some previous studies. Boys performed better on perceptual analysis, and girls performed better on processing speed and motor dexterity. Girls also showed a slight advantage on verbal learning, but by adolescence, this advantage had disappeared.
- Income predicted IQ and academic achievement. Lower income was associated with lower IQ scores (mean IQs were 105, 110, and 115 for low-, middle- and high-income children respectively). Lower-income children were more likely to be excluded from the study because of medical or developmental conditions; the healthy low-income children who qualified performed, on average, better than previously reported population averages. “We were pleasantly surprised by how well the lower-income children did when we focused on those who were healthy,” says Waber. Although income did not predict performance on basic cognitive tasks, such as memory or reading individual words, lower-income children did score lower on tests like reading comprehension and calculation. The authors suggest that such tasks, which require more reasoning and integration of cognitive abilities, are more vulnerable to the effects of poverty-related factors than are more basic skills.
- Age predicted performance on every measure of cognitive function. Performance climbed steeply from age 6, but levelled off overall for most tests between 10 and 12 years of age, then improved more slowly or not at all during adolescence. Waber cautions, however, that these data are “snapshots” at a single point in time, averaging the performance of a whole population. “We don’t know whether everyone’s performance improves more slowly in adolescence, or whether some children continue to improve while others do not, or whether our standard tests can measure what really changes in adolescence,” she notes. “As we follow these children over time, we will have a better understanding of what happens in adolescence.”
“In the past, studies of structural brain development and often studies of cognitive development were performed on samples of convenience that weren’t necessarily representative of the overall population,” Waber adds. “This study provides information on a much more diverse and representative sample, and a much larger one than previously available.” Other components of the NIH MRI Study of Normal Brain Development are structural brain imaging with MRI, magnetic resonance spectroscopy (MRS), and diffusion tensor imaging (DTI) to study the growth of different brain structures and the formation of connections between them, and even changes in brain chemistry. The ultimate goals of the project are to provide an atlas of the development of the healthy child’s brain and to link the imaging findings with neurobehavioral function. Waber’s co-authors were Carl de Moor and Peter Forbes, Department of Psychiatry, Children’s Hospital Boston; C. Robert Almli and Kelly Botteron of Washington University School of Medicine, St. Louis; Gabriel Leonard and Denise Milovan of McGill University, Montreal; Tomas Paus of McGill University and the University of Nottingham (UK), and Judith Rumsey of the National Institute of Mental Health. The six Pediatric Study Centers are Children’s Hospital Boston; Children’s Hospital Medical Center of Cincinnati; Children’s Hospital of Philadelphia; University of California at Los Angeles; University of Texas, Houston; and Washington University, St. Louis. The study was funded by the National Institute of Neurological Disorders and Stroke, the National Institute of Mental Health, the National Institute of Child Health and Human Development and the National Institute on Drug Abuse. For more information see