Left-right asymmetry in volume and number of neurons in adult Broca's area

Total neuron number in, and volume of, Brodmann areas (BA) 44 and 45 (Broca's area) were studied in Nissl-stained sections from the left and right hemispheres of five adult men and five adult women. The volume of BA 44 was greater in the left hemisphere than in the right in all ten cases, although asymmetry was only significant for the subgroup of male subjects. For six of the ten subjects (including all females), the volume of BA 45 was greater in the left hemisphere than the right. This asymmetry was significant only for the women. A significant left-over-right asymmetry has been found in total neuron number in male BA 44. Although the total number of neurons in left BA 45 was larger in all five female subjects, this asymmetry did not reach significant difference. In the male subjects no significant asymmetry difference in total neuron number was found in BA 45 either. There was no significant hemispheric asymmetry or gender interaction for neuronal number density, either in BA 44 or 45. This study is the first quantitative study of total number of neurons in BA 44 and 45 in adult subjects, and demonstrates that both the volume and the total neuron number of BA 44 and 45 on the left are generally greater than that of the right hemisphere, with the possible exception of the male BA 45. In addition, it shows that the inter-individual variability was also very large (more than twofold) in the numerical values of all variables.