measures of word recognition yet continue to suffer from the phonological deficit that makes reading less automatic, more effortful, and slow. For readers with dyslexia, the provision of extra time is an essential accommodation; it allows them the time to decode each word and to apply their unimpaired higher-order cognitive and linguistic skills to the surrounding context to get at the meaning of words that they cannot entirely or rapidly decode. Other accommodations useful to adolescents with reading difficulties include note-takers, taping classroom lectures, using recordings to access texts and other books they have difficulty reading, and the opportunity to take tests in alternate formats, such as short essays or even orally (Shaywitz, 1998).

Neurobiological Studies

To a large degree, advances in understanding dyslexia have informed and facilitated studies examining the neurobiological underpinnings of reading and dyslexia. Thus, a range of neurobiological investigations using postmortem brain specimens (Galaburda et al., 1985) and, more recently, brain morphometry (Filipek, 1996), and diffusion tensor MRI imaging (Klingberg et al., 2000) suggest that there are differences in the temporo-parieto-occipital brain regions between dyslexic and nonimpaired readers.

Rather than being limited to examining the brain in an autopsy specimen or measuring the size of brain regions using static morphometric indices based on CT or MRI, functional imaging offers the possibility of examining brain function during performance of a cognitive task. In principle, functional brain imaging is quite simple. When an individual is asked to perform a discrete cognitive task, that task places processing demands on particular neural systems in the brain. To meet those demands requires activation of neural systems in specific brain regions, and those changes in neural activity are, in turn, reflected by changes in brain metabolic activity, which in turn are reflected, for example, by changes in cerebral blood flow and in the cerebral utilization of metabolic substrates such as glucose. The term functional imaging has also been applied to the technology of magnetic source imaging using magnetoencephalography, an electrophysiological method with strengths in resolving the temporal sequences of cognitive processes.

Recent findings using fMRI may help reconcile the seemingly contradictory findings of previous imaging studies of dyslexic readers (Shaywitz, B. et al., in press; Brunswick et al., 1999; Helenius et al., 1999; Horwitz et al., 1998; Paulesu et al., 2001; Rumsey et al., 1992, 1997; Salmelin et al., 1996; Shaywitz et al., 1998, submitted; Simos et al., 2000). In addition, some functional brain imaging studies show a relative increase in brain activation in frontal regions and right hemisphere systems in dyslexics com-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement