Anatomical risk factors for phonological dyslexia

Successful behavioral genetic studies require precise definition of a homog enous phenotype. This study searched for anatomical markers that might rest rict variability in the reading disability phenotype. The subjects were 15 college students (8 male/7 female) diagnosed with a reading disability (RD) and 15 controls (8 males/7 females). All subjects completed a cognitive an d reading battery. Only 11 of the RD subjects had a phonological deficit [p honological dyslexia (PD): pseudo word decoding scores < 90 (27th percentil e)]. Thirteen RD (9 PD) and 15 controls received a volumetric MRI scan. Fou r anatomical measures differentiated the PD group from the remainder of the subjects: (i) marked rightward cerebral asymmetry, (ii) marked leftward as ymmetry of the anterior lobe of the cerebellum, (ii) combined leftward asym metry of the planum and posterior ascending ramus of the sylvian fissure, a nd (iv) a large duplication of Heschl's gyrus on the left. When these four measures were normalized and summed, the resulting variable predicted short - and long term phonological memory. By contrast, oral and written comprehe nsion skills were predicted by a different anatomical variable: low cerebra l volume. These findings provide neurobiological support for an RO phenotyp e characterized by phonological deficits in the presence of normal or super ior comprehension. The study of individual variation in cortical structure may provide a useful link between genotype and behavior.