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Deborah Giaschi, Veronica Edwards, Simon Au Young, Bruce Bjornson; Asymmetrical cortical activation by global motion in children with dyslexia. Journal of Vision 2005;5(8):848. doi: 10.1167/5.8.848.
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Several groups have reported elevated motion coherence thresholds on global motion tasks in children and adults with dyslexia (eg. Edwards et al., 2004; Raymond & Sorensen, 1998; Talcott et al., 1998). The nature of the relationship between motion perception and reading deficits, however, has not been established. We used functional MRI to study the neural basis of the global motion deficit in 12 right-handed children with dyslexia and 12 age-matched controls. Area V5/MT+ was identified with a localizer task in which blocks of dots in expanding/contracting radial motion alternated with blocks of stationary dots. Activation in the V5/MT+ region was observed in 23 of 24 hemispheres in the control group and in all 24 hemispheres in the dyslexic group. This result is contrary to previous reports of reduced or no activation in response to moving stimuli in V5/MT+ in adults with dyslexia (Demb et al., 1998; Eden et al., 1996). Global motion direction discrimination was assessed using blocks of discrete trials of horizontally moving dots alternating with blocks of stationary dots. The coherence level was 85% or 30% on alternate motion blocks. Both groups showed more widespread activation when the coherence level was 30% than when the coherence level was 85%. At 30% coherence, activation was bilateral and symmetric in the controls. In contrast, the dyslexic group showed asymmetric activation with significantly reduced left hemisphere activation in V5/MT+, posterior occipital (putative V3A, V1, V2) and posterior parietal cortex. This finding, on motion tasks, is notable because, on reading tasks: 1) normal young readers show increasing left hemispheric lateralization as their reading fluency increases (Turkeltaub et al., 2003), and 2) children with dyslexia show reduced activation in left posterior regions compared to control children (Shaywitz et al., 2002). These results implicate left posterior cortex in both reading and global motion deficits in children with dyslexia.
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