Anatomical specialization of the anterior motor speech area: hemispheric differences in magnopyramidal neurons

The lateralization of motor speech function to the left hemisphere is supported by multiple lines of evidence, but relatively little is known about the anatomical basis of that specialization. In a preliminary study, we recently reported that area 45 of the left hemisphere (Broca's area) contained a subpopulation of magnopyramidal neurons which were significantly larger than any seen in the homotopic region of the right hemisphere (Hayes & Lewis, 1993a). In the present study we examined a larger sample of cases in order to determine how consistently this difference is present in the population, if it is specific to Broca's area or is a general feature of cortical regions mediating lateralized functions, and whether the subpopulation of large magnopyramidal neurons in left area 45 can be distinguished by their chemical phenotype. In Nissl-stained sections from 19 human brains, the mean (+/- SD) cross-sectional area of the largest layer III pyramidal neurons in area 45 was significantly (p < .0001) greater in the left hemisphere (522.1 +/- 128.3 microns2) than in the right (454.1 +/- 121.5 microns2). This interhemispheric difference appeared to be a unique characteristic of the largest neurons, since the mean size of all layer III pyramids in this area was not significantly different in the left (206.2 +/- 93.5 microns2) and right (213.3 +/- 103.9 microns2) hemispheres. In contrast to area 45, there was no interhemispheric difference in the mean cross-sectional area of the largest layer III pyramids in another lateralized region, primary motor cortex. In addition, in area 46, a region of prefrontal association cortex not known to be functionally lateralized, the mean somal size of the largest layer III pyramidal neurons was significantly (p < .001) smaller in the left hemisphere (402.4 +/- 84.9 microns2) than in the right (437.8 +/- 88.3 microns2). Finally, although the large layer III pyramids in area 45 were immunoreactive for nonphosphorylated neurofilament protein in both hemispheres, the mean cross-sectional area of the largest labeled neurons was significantly larger (p < .002) in the left hemisphere (525.2 +/- 149.0 microns2) than in the right (490.3 +/- 154.1 microns2). These findings demonstrate that layer III of Broca's area contains a distinctive subpopulation of neurons that may play an important role in the specific functional architecture of this region.