Objective: To examine for structural asymmetries in the posterior superior
temporal lobe at the microscopic level in an effort to explain the gross an
atomical and functional asymmetries of this brain region. Background: The p
osterior superior temporal lobe is typically larger on the left and damage
to this area frequently results in an aphasia. This has led to the hypothes
is that the structural asymmetry determines the functional asymmetry, but n
o definite confirmation of this hypothesis exists. Methods: Sixteen men wer
e 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 meas
ured with electron microscopy. Results: Posterior superior temporal lobe wh
ite 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 pro
liferation of glia (p = 0.46, t test for dependent samples), nor the densit
y 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 lob
e 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 fa
ster and require a greater volume, these results suggest asymmetry of myeli
nation as an explanation for both a left hemisphere dominance for rapid sen
sory signal processing, leading to a functional asymmetry for language, and
a larger left planum temporale.