Functional MRI Maps the Development of Neurobiologic Changes in Children with Dyslexia
Kristina Woodworth
*Medical Writer, SciMantis Communications, Inc, Pen Argyl, Pennsylvania.Address correspondence to: Kristina Woodworth, SciMantis Communications, Inc, PO Box 3, Pen Argyl, PA 18072. E-mail: kristina@scimantis.com.
By studying differences in brain patterns between nonimpaired individuals and those with dyslexia, researchers have been able to locate specific neural areas within the brain that are important for reading in both children and adults. However, little is known about the development of these neural systems. In the April issue of the Annals of Neurology, Bennett A. Shaywitz, MD, a specialist in pediatric neurology at the Yale University School of Medicine, and colleagues reported functional magnetic resonance imaging (MRI) findings that characterized the development of neural systems for reading in dyslexic and nonimpaired children.1
Previously Identified Neural Systems for Reading
In previous brain imaging studies comparing individuals with and without dyslexia, researchers identified important neural systems used in reading, including an anterior system around the inferior frontal gyrus that has been linked to word analysis, or decoding; a posterior system in the parietotemporal region that is also linked to word analysis; and another posterior region, more inferior in the occipitotemporal region that accounts for reading fluency. Dr Shaywitz et al cited multiple studies that pointed to functional deficiencies in left hemisphere posterior systems in individuals with dyslexia, but noted that little was known about the developmental patterns of these systems.2-13 They also highlighted recent research that has identified a language system deficit as the central problem in dyslexia, specifically relating to phonology, or the ability to understand underlying sound structures in words.14
Basis for this Analysis
The researchers hypothesized that the development of a neural system in the left occipitotemporal region is necessary for nonimpaired fluent reading, and that this system does not develop with age in dyslexic readers. To compensate for this lack of development, the researchers conjectured, dyslexic individuals develop an alternative neural pathway for reading. The authors emphasized that the identification of an alternative reading pathway in dyslexic children may provide practical insight into different mechanisms used for reading in dyslexic children, leading to better teaching methods for these individuals.
Subject Selection, Tasks, and MRI Analysis
A total of 232 right-handed children ages 7 to 18 years participated in the analysis, including 113 dyslexic readers and 119 nonimpaired readers. Individuals were asked to answer questions requiring phonologic analysis, including an assessment of line orientation and rhyme with the use of "pseudowords" (Sidebar), while undergoing MRI analysis.
Imaging was performed with a 1.5-Tesla Signa LX imaging system from General Electric Medical Systems. Neural activation maps for each subject were generated by comparing the images from the nonword rhyme (NWR) task and Line task. Each individual subject's activation map was combined to form composite activation maps for the dyslexic and nonimpaired groups. A general linear model was used to assess the effect of age on neural activation within each group. The researchers also analyzed age and in-magnet performance on the NWR task in each study group to account for variations in reading skill within each group.
Reading and Task Performance
As expected, subjects with dyslexia demonstrated a significant impairment in reading ability (P <.001). Older children within each group demonstrated better phonological awareness and scores on 3 reading indices (word identification, fluency, and comprehension) compared to younger children. Likewise, older children performed better on both the line orientation and NWR tasks in both groups.
Age-Related Imaging Findings
In nonimpaired readers, the left anterior lateral occipitotemporal region became more active with increasing age (P = .0003). In younger nonimpaired readers, neural systems in both the right and left superior and middle frontal regions were more active compared to older readers (P = .0001). In the dyslexic group, older readers demonstrated greater activity in the left inferior frontal gyrus and right and left posterior medial occipitotemporal regions (P = .0002) compared to younger readers. Maps demonstrating the correlation between age and activation in nonimpaired and dyslexic children are presented in the Figure. As illustrated in the Figure, a significant degree of asymmetry of activity was observed in the anterior lateral occipitotemporal region in nonimpaired readers with increasing age (P <.0003), but no significant degree of asymmetry was observed in dyslexic readers (P <.9). Also, nonimpaired readers demonstrated a significant correlation between asymmetry and age when compared to dyslexic readers (P <.009). Although older, nonimpaired readers demonstrated asymmetric activity in the left lateral region, older dyslexic readers did not demonstrate a greater degree of asymmetry compared to younger dyslexic readers.
Advancing the Understanding of How Dyslexics Read
Dr Shaywitz et al concluded that the development of neural systems, particularly the anterior lateral occipitotemporal region that has been linked to reading fluency, differs considerably between nonimpaired and dyslexic children. They noted that the most significant differences occur in the left occipitotemporal area, and that older nonimpaired readers exhibit a greater degree of activity in the left anterior lateral occipitotemporal region compared to younger nonimpaired readers and dyslexic readers of either age.
Interestingly, the authors noted a study in which Japanese individuals demonstrated a similar activation of the left anterior lateral occipitotemporal region when reading Japanese in the Kana writing system, whereas the posterior medial occipitotemporal area was activated when reading Japanese in Kanji script.15 The Kana system uses symbols linked to sounds or phoneme, similar to English script, whereas Kanji is a system of characters that are not linked to sounds. Dr Shaywitz et al explained, "the Kanji script uses ideographs in which each character must be memorized, suggesting that the posterior medial occipitotemporal region functions as part of a memory-based system." Dyslexic children may demonstrate an activation of this neural area when reading, the authors hypothesized, because they rely on memorization rather than sound-symbol linkages to understand and communicate written words.
These findings also emphasize the presence of one neural reading system that develops with age—this system is based in the anterior lateral area in nonimpaired readers, and in the posterior medial occipitotemporal area in dyslexic readers, according to the authors. They explained that these findings support another study that literacy is best understood as a single unitary construct as opposed to a system composed of separate components, such as phonological awareness, word reading, and passage comprehension.16
The authors emphasized that the age-related differences in neural activity found in their analysis would need confirmation in prospective, longitudinal trials that can better elucidate the developmental course of reading systems. In the meantime, these findings "provide a plausible and useful account of the development of the neural systems for reading and suggest a possible neurobiological explanation of the different mechanisms for reading relied on by each group," according to the authors.
References
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