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 order to obtain area measurements of the left and right planum temporale and the midsagittal corpus callosum. Comparisons between these left-handed males and an archival sample of age-matched right-handed males were also performed. Results demonstrated a strong leftward asymmetry in the planum temporale among subjects with left-hemisphere speech representation, regardless of handedness, but no consistent planum temporale asymmetry among subjects with right hemisphere speech representation. The results suggest that reversed speech lateralization is not necessarily accompanied by a concomitant reversal of planum temporale asymmetry. Examination of callosal areas revealed that left-handed subjects with left hemisphere speech functions had a larger corpus callosum than either left-handed subjects with right hemisphere speech functions or right-handed subjects. Increased interhemispheric communication may be required when the neural systems underlying speech and handedness are represented in opposite hemispheres.