ABNORMAL PROCESSING OF VISUAL-MOTION IN DYSLEXIA REVEALED BY FUNCTIONAL BRAIN IMAGING

IT is widely accepted that dyslexics have deficits in reading and phon ological awareness(1,2), but there is increasing evidence that they al so exhibit visual processing abnormalities that may be confined to par ticular portions of the visual system(3,4). In primate visual pathways , inputs from parvocellular or magnocellular layers of the lateral gen iculate nucleus remain partly segregated in projections to extrastriat e cortical areas specialized for processing colour and form versus mot ion(5-10), In studies of dyslexia, psychophysical(3) and anatomical(4) evidence indicate an anomaly in the magnocellular visual subsystem, T o investigate the pathophysiology of dyslexia, we used functional magn etic resonance imaging (fMRI) to study visual motion processing in nor mal and dyslexic men, In all dyslexics, presentation of moving stimuli failed to produce the same task-related functional activation in area V5/MT (part of the magnocellular visual subsystem) observed in contro ls, In contrast, presentation of stationary patterns resulted in equiv alent activations in V1/V2 and extrastriate cortex in both groups, Alt hough previous studies have emphasized language deficits, our data rev eal differences in the regional functional organization of the cortica l visual system in dyslexia.