Hemispheric differences in layer III pyramidal neurons of the anterior language area.

OBJECTIVE The major objective of this study was to determine if the lateralization of motor speech functions may be associated with hemispheric differences in the size of layer III pyramidal neurons in Brodmann's area 45. DESIGN A case series design involving postmortem human specimens was used to compare the cross-sectional area of NissI-stained layer III pyramidal neurons of Brodmann's area 45 from the left and right hemispheres. SUBJECTS A convenience sample consisting of seven cases with no known neurological or psychiatric disorders was obtained at autopsy. MAIN OUTCOME MEASURES The cross-sectional area of layer III pyramidal neurons in both left and right hemispheres was measured in four fields per hemisphere per brain using a computerized image analysis system. Measurements of both the largest layer III pyramids and of all layer III pyramids were done. RESULTS The largest layer III pyramidal neurons of area 45 were significantly larger in the left than in the right hemisphere in both an unblinded and a blinded series of measurements. However, this hemispheric difference appeared to be restricted to the largest neurons, since the mean size of all layer III pyramids in this area was not significantly different in the left and right hemispheres. CONCLUSIONS The presence of a unique population of large pyramidal neurons in left Brodmann's area 45 may be related to the involvement of this region in the circuitry that mediates motor speech functions.