Objective: To examine for structural asymmetries in the posterior superior temporal lobe at the microscopic level in an effort to explain the gross anatomical and functional asymmetries of this brain region.
Background: The posterior superior temporal lobe is typically larger on the left and damage to this area frequently results in an aphasia. This has led to the hypothesis that the structural asymmetry determines the functional asymmetry, but no definite confirmation of this hypothesis exists.
Methods: Sixteen men were studied at postmortem. Posterior superior temporal lobe dimensions, gray matter volume, white matter volume, SMI-32 immunopositive neuronal density, and glia cell volume were measured for both the left and right hemispheres. In a subset of eight subjects, myelin sheath and axon diameters were measured with electron microscopy.
Results: Posterior superior temporal lobe white matter volume was greater on the left (p = 0.003, t test for dependent samples). This asymmetry did not appear to be the result of an isolated proliferation of glia (p = 0.46, t test for dependent samples), nor the density of cortical to cortical projections neurons in the overlying cortex (p = 0.71, t test for dependent samples). In a subset of eight subjects studied with electron microscopy, axons of the left posterior superior temporal lobe were more thickly myelinated (57 nm [SD = 27] left, 46 nm [SD = 24], p < 0.001, ANOVA).
Conclusions: As axons with thicker myelin sheaths conduct faster and require a greater volume, these results suggest asymmetry of myelination as an explanation for both a left hemisphere dominance for rapid sensory signal processing, leading to a functional asymmetry for language, and a larger left planum temporale.