Morphology of the planum temporale and corpus callosum in left handers with evidence of left and right hemisphere speech representation

In the present study we investigated planum temporale asymmetry and corpus callosum morphology in a sample of young adult left-handed males, using MRI , Two subgroups of left-handed males were identified on the basis of their differing speech lateralization patterns, which were inferred from results of the Fused Dichotic Words Test. These individuals then underwent MRI in o rder to obtain area measurements of the left and right planum temporale and the midsagittal corpus callosum, Comparisons between these left-handed mal es and an archival sample of age-matched right-handed males were also perfo rmed. Results demonstrated a strong leftward asymmetry in the planum tempor ale among subjects with left-hemisphere speech representation, regardless o f handedness, but no consistent planum temporale asymmetry among subjects w ith right hemisphere speech representation. The results suggest that revers ed speech lateralization is not necessarily accompanied by a concomitant re versal of planum temporale asymmetry. Examination of callosal areas reveale d that left-handed subjects with left hemisphere speech functions had a lar ger corpus callosum than either left-handed subjects with right hemisphere speech functions or right-handed subjects. Increased interhemispheric commu nication may be required when the neural systems underlying speech and hand edness are represented in opposite hemispheres.