A Comparative Quantitative Analysis of Cytoarchitecture and Minicolumnar Organization in Broca's Area in Humans and Great Apes

Broca's area was identified in the inferior frontal gyrus of chimpanzee, bonobo, gorilla, and orangutan brains through direct cytoarchitectonic comparison with human brains. Across species, Broca's area comprises Brodmann's areas 44 and 45. We found that these areas exhibited similar cytoarchitectonic characteristics in all species examined. We analyzed the minicolumnar organization of cells in layer III of Broca's area in 11 human and 9 great ape specimens. A semiautomated method was used to analyze digitized images of histological sections stained for Nissl substance. Horizontal spacing distance and gray level index (GLI; or the area fraction occupied by cells) were quantified in all images. In contrast to area Tpt, the only cortical area for which comparative minicolumnar data have been published previously for humans and one of the great apes, we found no population-level asymmetry, for either horizontal spacing distance or GLI. Only human females exhibited a leftward asymmetry in GLI. GLI was lower in humans than in great apes (P < 0.001), allowing more space for connectivity in layer III. In humans, horizontal spacing distance was greater than in great apes but smaller relative to brain size

Spatial organization of neurons in the frontal pole sets humans apart from great apes

Few morphological differences have been identified so far that distinguish the human brain from the brains of our closest relatives, the apes. Comparative analyses of the spatial organization of cortical neurons, including minicolumns, can aid our understanding of the functionally relevant aspects of microcircuitry. We measured horizontal spacing distance and gray-level ratio in layer III of 4 regions of human and ape cortex in all 6 living hominoid species: frontal pole (Brodmann area [BA] 10), and primary motor (BA 4), primary somatosensory (BA 3), and primary visual cortex (BA 17). Our results identified significant differences between humans and apes in the frontal pole (BA 10). Within the human brain, there were also significant differences between the frontal pole and 2 of the 3 regions studied (BA 3 and BA 17). Differences between BA 10 and BA 4 were present but did not reach significance. These findings in combination with earlier findings on BA 44 and BA 45 suggest that human brain evolution was likely characterized by an increase in the number and width of minicolumns and the space available for interconnectivity between neurons in the frontal lobe, especially the prefrontal cortex

Parallels between machine and brain decoding

We report some existing work, inspired by analogies between human thought and machine computation, showing that the informational state of a digital computer can be decoded in a similar way to brain decoding. We then discuss some proposed work that would leverage this analogy to shed light on the amount of information that may be missed by the technical limitations of current neuroimaging technologies