White matter tissue properties are highly correlated with reading proficiency; we would like to have a model that relates the dynamics of an individual's white matter development to their acquisition of skilled reading. The development of cerebral white matter involves multiple biological processes, and the balance between these processes differs between individuals. Cross-sectional measures of white matter mask the interplay between these processes and their connection to an individual's cognitive development. Hence, we performed a longitudinal study to measure white-matter development (diffusion-weighted imaging) and reading development (behavioral testing) in individual children (age 7-15 y). The pattern of white-matter development differed significantly among children. In the left arcuate and left inferior longitudinal fasciculus, children with above-average reading skills initially had low fractional anisotropy (FA) that increased over the 3-y period, whereas children with belowaverage reading skills had higher initial FA that declined over time. We describe a dual-process model of white matter development comprising biological processes with opposing effects on FA, such as axonal myelination and pruning, to explain the pattern of results.R eading requires efficient communication within a network of visual, auditory, and language-processing regions that are separated by many centimeters. Hence, the white-matter fascicles that connect these regions are critical for proficient reading (1). Only in the last decade has it become possible to study the microstructural properties of the white matter in the living human brain, and only in recent years has there been an opportunity to trace the developmental progression of these fascicles systematically. During this decade it has been shown that learning to read is associated with corresponding changes in sensory and language circuits in the brain (2-5).Diffusion measurements of the white-matter pathways (3, 5-7) and neurological case studies (8, 9) suggest that in typical development and education both the left hemisphere arcuate fasciculus and inferior longitudinal fasciculus (ILF) carry signals important for reading. Studies in adults, adolescents, and schoolage children have reported differences between good and poor readers in white-matter volume and diffusion properties in the vicinity of these pathways (10-14). One possibility is that these differences are present from an early age, remain constant through development, and constrain children's aptitude for reading. An alternative possibility is that there is an interaction between the biological development and timing of instruction. Two case studies support this hypothesis. When damage to the arcuate fasciculus occurs at birth, normal reading skills can develop (15); when damage to the arcuate fasciculus occurs later, for example after reading instruction has begun, it can result in severe reading impairment (9). These cases suggest that learning depends on the circuits' current state and capacity for plasti...

For more than a century, neurologists have hypothesized that the arcuate fasciculus carries signals that are essential for language function; however, the relevance of the pathway for particular behaviors is highly controversial. The primary objective of this study was to use diffusion tensor imaging to examine the relationship between individual variation in the microstructural properties of arcuate fibers and behavioral measures of language and reading skills. A second objective was to use novel fiber-tracking methods to reassess estimates of arcuate lateralization. In a sample of 55 children, we found that measurements of diffusivity in the left arcuate correlate with phonological awareness skills and arcuate volume lateralization correlates with phonological memory and reading skills. Contrary to previous investigations that report the absence of the right arcuate in some subjects, we demonstrate that new techniques can identify the pathway in every individual. Our results provide empirical support for the role of the arcuate fasciculus in the development of reading skills.

The development of skilled reading requires efficient communication between distributed brain regions. By using diffusion tensor imaging, we assessed the interhemispheric connections in a group of children with a wide range of reading abilities. We segmented the callosal fibers into regions based on their likely cortical projection zones, and we measured diffusion properties in these segmented regions. Phonological awareness (a key factor in reading acquisition) was positively correlated with diffusivity perpendicular to the main axis of the callosal fibers that connect the temporal lobes. These results could be explained by several physiological properties. For example, good readers may have fewer but larger axons connecting left and right temporal lobes, or their axon membranes in these regions may be more permeable than the membranes of poor readers. These measurements are consistent with previous work suggesting that good readers have reduced interhemispheric connectivity and are better at processing rapidly changing visual and auditory stimuli.corpus callosum ͉ diffusion tensor imaging ͉ magnetic resonance imaging ͉ reading R eading is a crucial skill in modern industrialized societies. Therefore, it is important to understand how people learn, or fail to learn, this skill. A substantial behavioral and neuroscience literature characterizes the neural basis of reading in both children and adults. Functional neuroimaging studies of reading have identified a distributed functional network where neural activity correlates with reading skill (1-5).Such a distributed network of cortical regions requires a set of long-range white matter tracts to communicate essential signals. Structural neuroimaging studies using diffusion tensor imaging (DTI) have identified white matter properties that vary with reading skill. Klingberg et al. (6) were the first to apply DTI in the study of reading disabilities. They compared fractional anisotropy (FA) (a measure of the diffusion anisotropy within the white matter) in reading-impaired and normal-reading adults. They found a statistically significant difference primarily in a left temporal-parietal region of the white matter. This basic result has been replicated and extended to children by three independent groups (7-9).In addition to these recent measurements of the white matter using DTI, there is also a long history of measurements and theories arguing for a relationship between interhemispheric communication and reading (10-15). Orton (10) proposed that the lack of lefthemisphere language dominance was the cause of dyslexia. Geschwind and colleagues (11, 16) also argued that reduced left-hemisphere dominance was a cause of dyslexia. More recent measurements using structural MRI suggest that the corpus callosum has a different shape in dyslexic adults compared with those with normal reading development (17-19). Other research on the callosum supports the basic idea that increased functional (20) and structural (21) hemispheric asymmetry is associated with sparser interhemis...

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