| dc.description.abstract | Efforts to understand the evolution of modern mammalian brains are undertaken through comparative studies of cortical organization. Here, we have used a comparative approach in studying multiple species to understand more about the organization of the visual system across different mammalian orders. Primates are notable for their large brains, and every primate species has an average brain size larger than then 0.4 g mouse brain, the animal model most commonly used in laboratory studies. Neurons are the building blocks of neocortex, and an accurate estimate of the total number of neurons in a brain can reveal information concerning the specializations of cortex. In this collaborative effort, we determined the total numbers of cells and neurons within the neocortex of the adult chimpanzee and macaque brains. These are species that have relatively large brains compared to most mammals, and to the proposed brain size of our earliest mammalian ancestors. We found the same pattern of overall neuronal density described in all other primate species, in which primary visual cortex and primary somatosensory cortex contain higher-than-average neuron densities and primary motor cortex contains lower-than-average neuron densities. It is also important to look to species that also, independently of the primate lineage, came to evolve large brains. As such, we examined the visual neuroanatomy of the California sea lion and northern elephant seal using immunohistochemistry in coronal sections and other reconstruction methods. We found that the visual neuroanatomy is more similar in structure to other carnivores, such as cats, as opposed to primates, which is expected given these species’ phylogenetic position within the Carnivora order. | |
