Comparative analysis of Broca's area in hominoids

At the heart of the study of the evolution of language ability is the question how to produce testable hypotheses regarding the antecessors of language in our hominid ancestors and the presence or absence of such traits in our closest living relatives, the apes. One region that is consistently implicated in modern functional studies of language, and was implicated even in early lesion studies, is the region known as Broca's area (Brodmann's areas 44 and 45). The region's involvement in language function in humans is the reason that Broca's area is expected to express different anatomical characteristics from other prefrontal areas and to have unique characteristics in humans. In order to assess the degree to which the human brain has changed in relation to other hominoids, we need to have detailed knowledge, not only of the human brain, but also as many closely related species as possible. Previous studies have reported data on the comparative gross morphology of the inferior frontal gyrus, in which Broca's area is located in chimpanzees, bonobos, and gorillas (Cantalupo and Hopkins, 2001), but studies of the cytoarchitecture of Broca's area are limited to humans (Brodmann, 1909; von Economo, 1929; Sarkissov et al., 1955; Amunts et al., 1999; 2003) and chimpanzees (Bailey et al., 1950; Sherwood et al., 2003). The purpose of this dissertation was to assess, both qualitatively and quantitatively, the cytoarchitectonic structure of Brodmann's areas 44 and 45 in human, chimpanzee, bonobo, gorilla, orangutan, and gibbon specimens. Quantitative measurements of minicolumnar organization, volume, neuron number, and neuron density revealed only limited differences between humans and apes. While volume of and neuronal number in Brodmann's areas 44 and 45 were larger in humans, neither were larger than expected. Furthermore, the relative size of the areas was not larger in humans than in apes and did not differentiate among ape species. Neuron density and grey level index (within minicolumns) were decreased in humans relative to apes, and only limited evidence for any asymmetry was observed. The variation among species in Brodmann's areas 44 and 45 suggests that this region behaves much as expected in response to changes in brain size. Despite Broca's area's involvement in language related behavior in humans, the current data provides no evidence that there are differences in information processing occurring in this region, and no pattern emerges that requires explanations other than those related to increases in brain size. This suggests Broca's area involvement in non-linguistic activities that are just as crucial to apes as they are to humans. Alternatively, there may be differences among species that are not distinguishable using the parameters examined in this research. Future research should focus on expanding the sample to encompass individual variation within species and expand the parameters examined to subpopulations of neurons to gain a more detailed view of the anatomical structure of Broca's area within and among specie