Frontal cortex Area 10 cytoarchitecture
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Brodmann Area 10 (BA10) is a prefrontal cortical area responsible for several executive functions associated with planning of future actions, taking initiative, and the processing of working memory and attention. In terms of size, BA 10 is proportionally twice as large in humans as it is in the other great apes. Additionally, while primates have greater cell density in cortical layer IV, throughout the entire BA 10, humans stand out as having decreased density overall. Furthermore, humans stand out as having significantly lower density in the supragranular layers compared to other primates. Lower density indicates greater neuropil space, which implies an increase in space available for connections. The supragranular layers are associated most with cortico-cortical connections, and so having an increased amount of space in these layers may indicate increased connection between BA 10 and other association areas.
The human BA10 is located at the rostral end of the brain in the frontal pole region. It was initially defined on cytoarchitectonic traits. BA10 forms the entire frontal pole in humans, bonobos, chimpanzees, and orangutans, the orbital sector of the frontal pole in gibbons, and has two separate components in the macaque brain. Cytoarchitectonically, this area is fairly consistent across the hominoids, although there are some species differences, most likely reflecting functional specializations, have been observed. Humans, chimpanzees, and bonobos all have larger supragranular layers, which are associated with cortico-cortical connections, and smaller infragranular layers, which are associated with subcortical connections. This is in contrast with the rest of the hominoids, for whom this trend is reversed, and with macaques, whose supragranular-infragranular layer ratio is nearly equal. The six cortical layers of human BA10 are considered remarkably homogeneous. Functionally, it is involved in higher cognitive skills like planning future actions, decision-making, taking initiatives, and to some extent, working memory and attention.
The size of the human BA10 is larger when compared to other hominids, both absolutely as well as relatively. BA10 occupies 1.2% of the entire brain, which is 14 cm3 by volume. The closest hominid to humans is the bonobo, with a 2.8 cm3 volume occupying 0.74 % of its whole brain. Other great apes lie below 0.74% and above 0.46% with orangutan having the smallest BA10 region. Additionally, human BA10 drastically exceeds the expected size estimated based on linear regression. Cytoarchitectural comparisons show human BA10 has the lowest neuron density in primates, which is attributed to higher connectivity.
Note: Linear regression comparisons of brain size have been criticized and it has been argued that ballooning of cortical volumes can be due to algometric constrains on connectivity. Log-log scale comparisons of BA10 size matches the expected value for human, but small sampling size makes definitive conclusions difficult.
Comparative studies do not find any difference in BA10 size of humans across hemispheres and subjects. There is no specific study showing variance of BA10, however overall brain size varies within a population. Therefore, BA10 volume also likely varies with total brain size.
Most proposed mechanisms are general hypotheses for overall increase in human brain size. However, some theories do have direct connection with functionalities of BA10. One major selective force is increased social complexity. It has been argued that cognitive abilities required for successful social exchanges evolved in response to increasing social complexity. As skills like decision-making, planning future actions, etc are involved in social exchanges, brain regions like BA10 were likely under positive selective pressure in the hominin lineage. Other factors that may have influenced BA10 selection were better nutrition, sexual selection of intelligent individuals, habitat domination, dependence on tools, and increased reliance on problem solving skills.
Enhanced social structure improves survivability of a species but requires enhanced cooperation. These circumstances could drive the selection for cognitive skills like future planning of actions, decision-making, initiative taking, etc. This interplay of social interaction and complexity of social structure might be a possible selective process for expansion of BA10.
Lesion studies show that individuals with impaired BA10 regions have flawed decision-making, and unsuccessful social skills.
Brodmann areas are defined only for humans and primates, therefore studies for BA10 are restricted to the primate lineage. All mammals have frontal lobes and homologous regions to Brodmann Areas, but their functional similarities are not established.
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