Verbal Brains: Neural word paths take a mature turn
By Bruce Bower
Kids have less experience with words than adults do. Specific neural correlates of that difference have now shown up in a new series of brain scans conducted on members of the two age groups by neuroscientist Bradley L. Schlaggar of Washington University in St. Louis and his colleagues.
Experience isn’t everything, however. Some of the adults’ neural responses on word tasks stem from a physical maturation of the brain that hasn’t yet occurred in school-age youngsters, the scientists say.
In a brain-scan investigation of adults and 7-to-10-year-old children who came up with single-word responses to single-word cues, Schlaggar’s group teased apart the influence of the brain’s physical maturation from that of the gap in verbal knowledge between youngsters and their elders.
The researchers used functional magnetic resonance imaging to measure blood flow in the brains of 21 adults, ages 18 to 35, and 19 children as they performed three word tasks. Blood flow provides an indirect sign of neural activity. On successive trials, each participant was told to verbally respond to individual words presented on a computer screen either with a related verb, a rhyming word, or an opposite word.
Overall, children’s answers were neither as accurate nor as quick as those of adults. During the word tests, the kids exhibited greater activity in parts of the visual cortex than adults did. Adults displayed greater activity than children did in a section of the frontal cortex, indicating the use of a more complex thought process. The results appear in the May 24 Science.
The researchers couldn’t discern from these brain scans whether the observed changes in blood flow stem from specific physical changes that occur in brain maturation or from a general superiority in word knowledge among adults.
To distinguish between these possibilities, the scientists studied 10 adults and 10 children who performed the word tasks with comparable speed and accuracy. In this comparison, differences in brain activity between adults and children during the word tasks largely disappeared, Schlaggar says. Still, a small frontal area was more active in adults than in kids, and a narrow patch of visual cortex reacted more strongly in children than in adults.
“Our findings suggest that maturation of the pattern of regional brain activations involved in [single-word] tasks is incomplete at age 10,” Schlaggar says.
The new study uses “elegant methods” to separate the effects of brain maturation from those of increased word knowledge with age, comments neuroscientist B.J. Casey of Cornell University’s Sackler Institute for Developmental Psychobiology in New York City. Further investigations need to examine brain activity in children and adults before and after extended practice on various word tasks, he says. This strategy will illuminate whether the same neural mechanisms for acquiring verbal knowledge operate in immature and mature brains, or whether different mechanisms become operative at the different stages of life.
In related work, the Sackler Institute’s Bruce D. McCandliss is now conducting brain scans of children with severe reading problems before and after they take a course in how to decode the speech units, or phonemes, that make up novel words.