Neural Risk Factors for Dyslexia.

Abstract:
Phonological decoding and text comprehension contribute independently to reading success. This study provides evidence that these two processes depend on different neural substrates. Computerized measurements were made on volumetric MRI scans administered to 28 (15M/13F) college students, 13 of whom received special services for reading disability (RD). Nine of the 13 (5M/4F) had nonword reading scores in the bottom 27th percentile (phonological deficit: PD). The remaining 4 NP-RD students (2M/2F) had nonword reading scores at or above the 50th percentile. The 9 PD had a number of cerebral risk factors: (1) duplication of the left Heschl's gyrus; (2) extreme rightward asymmetry of the cerebral hemispheres; (3) a small right anterior lobe of the cerebellum; and (4) leftward asymmetry of the planum temporale in nonrighthanders. A discriminant analysis using these four variables discriminated PD from NP-RD and controls (1 PD, 1 control misclassified). Risk factors for NP-RD were small brain size, symmetrical hemispheres and rightward planar asymmetry. Brain size predicted passage comprehension but not phonological decoding skill. These data suggest that different developmental disturbances of asymmetry are associated with different types of RD. Persistent phonological deficits are associated with aberrant development of the left hemisphere and circuits involving auditory cortex and the right anterior lobe of the cerebellum. brains. "Garden variety" reading disabilities, on the other hand may be associated with small symmetrical brains. Supported by DCD R01 2922.