Deficient orthographic and phonological representations in children with dyslexia revealed by brain activation patterns.

Dyslexia is perhaps the most common neurobehavioral disorder in children, with prevalence rates ranging from 5 to 17.5% (Shaywitz et al., 1998). Converging behavioral evidence suggests that a central problem in patients with dyslexia is a deficit in phonological processing, especially in identifying and manipulating the sound structure of a word (Brady & Shankweiler, 1991; Bruck, 1992; Stanovich & Siegel, 1994; Shankweiler et al., 1995). Neuroimaging studies show that patients with dyslexia exhibit less intense activation than controls in left temporo-parietal regions, including the posterior part of the superior temporal gyrus and inferior parietal cortex (Rumsey et al., 1997a, 1997b; Shaywitz et al., 1998; Brunswick, McCrory, Price, Frith, & Frith, 1999; Helenius, Uutela, & Hari, 1999; Pugh et al., 2000b; Simos et al., 2000b; Shaywitz et al., 2002, 2003). Under-activation in these regions may reflect deficits that patients with dyslexia have in phonological processing in superior temporal gyrus and in mapping from orthographic to phonological representations in inferior parietal cortex. Neuroimaging studies have also found that patients with dyslexia show less activation than controls in left fusiform gyrus (Rumsey et al., 1997b; Shaywitz et al., 1998; Brunswick et al., 1999; Paulesu et al., 2001; Shaywitz et al., 2002, 2003). Research suggests that left fusiform gyrus is important for processing the orthographic structure of well-learned visual word forms (Price, Wise, & Frackowiak, 1996; Tarkiainen, Helenius, Hansen, Cornelissen, & Salmelin, 1999; Cohen et al., 2000; Xu et al., 2001; Cohen et al., 2002). Although there is a debate on whether the left fusiform gyrus is sensitive to lexicality or just familiarity, most agree that this region includes orthographic representations that are linguistically structured (Pugh et al., 2000b). Although reduced activation in left fusiform gyrus for patients with dyslexia may reflect a deficit in orthographic processing, some studies have shown normal activation in this region during letter rhyming judgment (Temple et al., 2001) or non-word reading (Georgiewa et al., 1999, 2002). In addition to the activation deficits described above, neuroimaging studies of adults with dyslexia show either greater activation (Shaywitz et al., 1998; Brunswick et al., 1999) or no difference (Rumsey et al., 1997b; Paulesu et al., 2001; Shaywitz et al., 2003) from controls in left inferior frontal gyrus. This over-activation in the left inferior frontal gyrus has been interpreted as a compensatory mechanism involving sub-vocal phonological rehearsal by adults with dyslexia. In children with dyslexia, however, only one study found greater activation in the left inferior frontal gyrus (Georgiewa et al., 2002), whereas most studies show either less activation (Georgiewa, 1999; Shaywitz et al., 2002) or no difference (Georgiewa et al., 1999; Temple et al., 2001; Shaywitz et al., 2002). One study showed that there was a positive correlation between activation in left inferior frontal gyrus and age during a nonword rhyming judgment task in children with dyslexia (Shaywitz et al., 2002). Taken together, these studies suggest that adults with dyslexia show over-activation whereas children with dyslexia show under-activation in the left inferior frontal gyrus. In the current study, we manipulated the difficulty of a visual rhyming task by including word pairs with conflicting (e.g., pint-mint, jazz-has) or non-conflicting (e.g., gate-hate, press-list) orthographic and phonological information in children. Research generally shows that spelling and rhyming judgments are more difficult for conflicting than for non-conflicting pairs (Polich, McCarthy, Wang, & Donchin, 1983; Johnston & McDermott, 1986; Kramer & Donchin, 1987; Rugg & Barrett, 1987; Levinthal & Hornung, 1992) and that patients with dyslexia have greater difficulty with conflicting pairs as compared to non-conflicting pairs (Rack, 1985; McPherson, Ackerman, & Dykman, 1997), but no neuroimaging studies have compared patients with dyslexia to controls on conflicting versus non-conflicting pairs. The goal of this study was to determine if trials that placed greater demands on orthographic and phonological processing would produce larger group differences. We expected the more difficult conflicting trials to generate greater behavioral and activation differences between children with dyslexia and controls as compared to the non-conflicting trials. Consistent with previous reports, we expected children with dyslexia to show less activation in left temporo-parietal cortex, left fusiform gyrus, and left inferior frontal gyrus.