Dyslexia tied to disrupted brain network

Science News, March 7, 1998 by Bruce Bower

Children and adults who exhibit the reading disability known as dyslexia have a difficult time applying appropriate sounds to the letters that make up written words. A new brain-imaging study indicates that a widespread network of brain regions critical to this ability malfunctions in people with dyslexia.

Further work is needed to determine whether this brain disturbance acts as a "neural signature" for the fundamental problem in dyslexia, say pediatrician Sally E. Shaywitz of Yale University School of Medicine and her colleagues.

"These brain activation patterns now provide us with hard evidence of a disruption in the brain regions responsible for reading-evidence for what has previously been a hidden disability," Shaywitz holds.

The Yale researcher's group recruited 14 male and 15 female dyslexic readers, as well as 16 male and 16 female volunteers with no reading impairments. Participants ranged in age from 16 to 63.

During a series of tasks that tapped into progressively more complex manipulations of letter sounds in words, a functional magnetic resonance imaging scanner measured changes in oxygen use in the participants' brains. Brain cells use more oxygen as they work harder.

Dyslexic readers had the most trouble when asked to identify nonsense words that rhyme, such as "leat" and "jete." Actual words that rhyme, many of which had already been memorized by the dyslexic readers, usually proved less vexing for them.

Compared to unimpaired readers performing these tasks, dyslexic readers lacked neural activity in several regions toward the back of the brain, Shaywitz and her coworkers report in the March 3 Proceedings of the National Academy of Sciences. These include Wernicke's area, which contributes to language comprehension, as well as parts of the visual cortex and a section of the association cortex considered pivotal to integrating the sight of printed letters with their corresponding sounds.

At the same time, dyslexic readers showed greater than average activity in areas at the front of the brain. The most prominent such sites were the inferior frontal gyrus, especially during nonsense word rhyming, and Broca's area, a crucial location for speech processing.

The areas affected in dyslexic readers largely correspond to sites of brain damage in adults who developed severe reading problems after having suffered a tumor or stroke, the scientists contend.

Other studies of the brain have suggested that dyslexia derives from perceptual disturbances, such as underactivation of a structure called V5. This brain structure facilitates the visual processing of printed letters and other fast-moving images (SN: 2/17/96, p. 104). The extensive reading network identified in the new study appears to maintain anatomical connections to that area, according to Shaywitz and her coworkers.

"This is an interesting and exciting new study," comments neuroscientist Guinevere Eden of Georgetown University Medical Center in Washington, D.C. "But the relationship of the Shaywitz data to the V5 findings is unclear." Eden directed the V5 research.

Jonathan Demb, a neuroscientist at the University of Pennsylvania School of Medicine in Philadelphia, has found reduced brain activity in several visual regions linked to reading problems. Many dyslexia-related disturbances cited by the Shaywitz group occur within or near primary sensory areas of the brain, he notes. Scientists need to clarify the proposed reading network's relationship to auditory and visual regions already implicated in dyslexia, he remarks.

Increased activity in the Broca's area of dyslexic readers is another intriguing and unexplained finding in the new study, Eden adds.