Temporal lobe asymmetry and dyslexia: an in vivo study using MRI

Three measures of the right and left temporal lobes were taken with magnetic resonance imaging (MRI) in groups of dyslexics (N = 17), retarded readers (N = 6), and normal controls (N = 12). The most pronounced differences among the groups were found with measures on coronal slices of the cross sectional area of the temporal cortex with subcortical white matter--in particular lateral to insula. While most of the normal and the retarded readers (13 of 18) had left asymmetry (left area larger than right), most of the dyslexics (14 of 17) had symmetry or right asymmetry. The degree of left asymmetry, (L-R)/[0.5(L + R)], was found to correlate with both reading skills and skills in phonemic analysis of spoken language. These results were also found when differences in non-verbal IQ were accounted for. The implications for definitions of dyslexia, for psycholinguistic research on dyslexia, and for remedial teaching are discussed.