Normal planum temporale asymmetry in dyslexics with a magnocellular pathway deficit

DEVELOPMENTAL dyslexia has been associated with both abnormal hemispheric s ymmetry of the planum temporale (PT) and a deficit in the magnocellular vis ual pathway. We examined the relationship between these two abnormalities. Using sagittal magnetic resonance images and three methods, we measured the PT in dyslexic subjects with a documented magnocellular deficit and contro ls. Dyslexic subjects did not deviate from normal leftward PT asymmetry, bu t both groups became less left-lateralized with methods that excluded sulcu l tissue. Results suggest that dyslexic subjects with a magnocellular defic it do not always have abnormal symmetry of the PT. PT symmetry may instead be related to a different subtype of dyslexia. In addition, PT asymmetry in any subject group depends on the measurement method. (C) 1999 Lippincott W illiams & Wilkins.