Seeking a link between neural anatomy and mental ability, researchers at the National Institute of Mental Health and McGill University in Montreal discovered it where they least expected — not in sheer brain size or special structures, but in the patterns of childhood growth.
Brain development in children with the highest IQ peaked four years later than among average children, the researchers reported Wednesday in the journal Nature.
"Smart children really do develop differently, and here is the first physical evidence of that," said UCLA neuroscientist Paul Thompson, an expert on imaging and brain development. "You'd think they'd develop faster and earlier than normal kids. The surprise is they don't."
Philip Shaw at the NIMH child psychiatry branch and his colleagues periodically scanned the brains of 307 healthy children from age 5 to age 19.
To monitor the living brains, they used magnetic resonance imaging, which can detect the anatomical differences between gray matter, composed of neurons and other brain cells, and white matter, composed of the nerves that connect them. They gauged intelligence by giving each child standard IQ tests.
In general, every brain blossoms from a single cell in the womb, growing at a rate of 250,000 cells a minute until, by early childhood, it has more neurons and more connections between them than the average adult brain. Unused cells and synapses then atrophy and die.
When the researchers analyzed their images, they discovered patterns of brain development that differed depending on the child's age and IQ.
The scans revealed tell-tale waves of change that coursed through the brain's prefrontal cortex — a thick wrinkled carpet of cells that orchestrates memory, attention, perceptual awareness, language, reason and consciousness. "The story of intelligence is in the trajectory of brain development," Shaw said. "What differs with intelligence is the rate of these changes."
Among average children — those with an IQ measuring 83 to 108 — the growth of the cortex peaked at age 8, while among those with high intelligence — rated with an IQ of 109 to 120 — growth peaked at age 9.
The smartest children — those with IQs measuring 121 to 145 — displayed a pattern of brain growth that peaked at age 11 or 12, the researchers said.
The anatomical scans revealed that among the most intelligent children, the cortex displayed the most prolonged period of growth and the most rapid rate of change. The cortex also was thinner in early childhood, grew thicker, then thinned more rapidly.
"There is something very dynamic about these brains," said Judith L. Rapoport, chief of the NIMH child psychiatry branch. "What the intelligent children have is a very malleable brain."
By the teen years, however, the cortex could be seen to be thinning in all three groups and, by adulthood, the brains could no longer be distinguished by IQ differences, said NIMH brain imaging expert Jay Giedd.
"Even though they end up at pretty much the same place, the shape of the [development] curve and the age at which they peak is very different between the three groups," he said. "We would have missed it if we had looked at adults."
No single brain scan could reveal a child's IQ. The patterns only revealed themselves across a large group. The differences are measured in fractions of a millimeter of brain tissue that emerge over a decade or more.
"These are tiny changes," Shaw said. "But in brain terms, it is a lot."
No one knows whether such subtle developmental changes in the cortex are caused by the genes inherited from a child's parents, by the biochemical influences of life experience, or by the interplay of both.
"It is tempting to assume that this developmental change in brain structure is determined by a person's genes," said psychologist Richard Passingham at the University of Oxford, who wrote a commentary accompanying the Nature paper. "But one should be wary of such a conclusion."