Planum temporale, planum parietale and dichotic listening in dyslexia

A reduction or reversal of the normal leftward asymmetry of the planum temp orale (PT) has been claimed to be typical of dyslexia, although some recent studies have challenged this view. In a population-based study of 20 right -handed dyslexic boys and 20 matched controls, we have measured the PT and the adjacent planum parietale (PP) region in sagittal magnetic resonance im ages. For the PT, mean left and right areas and asymmetry coefficients were compared. Since a PP area often could not be identified in one or both hem ispheres, a qualitative comparison was used for this region. The total plan ar area (sum of PT and PP) was also compared between the two groups. A dich otic listening (DL) test with consonant-vowel syllables was administered to assess functional asymmetry of language. The results showed a mean leftwar d PT asymmetry in both the dyslexic and the control group, with no signific ant difference for the degree of PT asymmetry. Planned comparisons revealed however, a trend towards smaller left PT in the dyslexic group. In control children, but not in the dyslexic children, a significant correlation betw een PT asymmetry and reading was observed. A mean leftward asymmetry was al so found for the total planar area, with no difference between the groups f or the degree of asymmetry. Significantly fewer dyslexic children than cont rol children showed a rightward asymmetry for the PP region. Both groups sh owed a normal right ear advantage on the DL task, with no significant diffe rence for DL asymmetry. No significant correlation was observed between PT asymmetry and DL asymmetry. The present population-based study adds to rece nt reports of normal PT asymmetry in dyslexia, but indicates that subtle mo rphological abnormalities in the left planar area may be present in this co ndition. (C) 2000 Elsevier Science Ltd. All lights reserved.