Scripted Brains: Learning to read evokes hemispheric trade-off

It seems that the brain takes sides in promoting the skills necessary for proficient reading.

The extended process of learning to read elicits a hemispheric trade-off in which left-brain structures get increasingly tuned to reading-specific tasks, a new study finds. Meanwhile, the right brain’s more general contributions to deciphering text grow weaker.

From childhood through adolescence, budding readers display gradually intensifying neural activity in parts of the brain’s left hemisphere that discern relationships between sounds and letters, say neuroscientist Guinevere Eden of Georgetown University in Washington, D.C., and her coworkers. During this same period, activity declines in right-brain areas that supply novice readers with word-related visual knowledge, the scientists report in an upcoming Nature Neuroscience.

These findings would have come as no surprise to the late neurologist Samuel T. Orton. In 1925, Orton proposed that childhood reading problems arise when visual information from the right hemisphere isn’t dampened, thus interfering with word-identification efforts of the left hemisphere.

Although neuroscientists have shown a keen interest in the severe reading disorder called dyslexia (SN: 3/7/98, p. 150), little is known about how the brain supports efficient reading, Eden says.

“The data derived from [Eden’s] study have the potential to tell us a great deal about neurobiological foundations of both skilled reading and dyslexia,” remarks psychologist G. Reid Lyon, chief of the Child Development and Behavior Branch at the National Institute of Child Health and Human Development in Bethesda, Md.

Eden’s group used functional magnetic resonance imaging (fMRI) to measure blood flow in the brains of 41 young people, ages 6 to 22. All of them could read at age-appropriate levels and scored similarly on IQ tests. While being scanned, participants identified randomly placed, especially tall letters within both simple words and nonsensical letter strings.

Earlier research had shown that people read the words included in the task even though they’re not asked to. By focusing on this unintentional reading, the researchers avoided triggering contrasting brain responses displayed by beginning and accomplished readers as they consciously decipher words.

Even among the youngest readers, reading induced elevated blood flow–an indirect sign of neural activity–in left-brain areas that match printed letters with speech sounds. This activity intensified in older readers and was greater for the more advanced readers within each age group, the scientists say.

Other left-brain areas concerned with recognizing words and discerning speech sounds also displayed greater activity during word reading as age and reading skill increased.

In contrast, word-inspired activity in a right-brain region used to recognize forms progressively declined with increased age of the volunteer and better reading skills. This brain area may fill in the visual context of words, such as associating the word “stop” with a red, octagonal sign, Eden proposes.

The new study represents a first step toward understanding how brain development makes it possible to read. In future work, Eden’s group hopes to gather reading-related fMRI brain data from a sample of healthy children as they progress through their schooling.

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