| dc.description.abstract | These studies are part of a larger effort to understand how the human brain evolved by specifically addressing questions about the evolution of the visual system in early primates and closely related mammals. Both cortical and thalamic connections of the superior colliculus (SC) were studied in three highly visual mammals; grey squirrels (Sciurus carolinensis), tree shrews (Tupaia glis), and prosimian galagos (Otolemur garnetti). These species were chosen for several important reasons. All three species have well-developed visual systems, share similar ecological environments, and are members of the same phylogenetic mammalian radiation, the Euarchontoglires. Therefore, these species likely share many common brain features, but also have divergent specializations reflecting differences in their evolutionary history. An emphasis was placed on understanding the connections of visual brain structures with the SC, a key structure within the extrageniculate pathway that is also involved in producing orienting movements. Injections of anatomical tracers were placed within the SC, while various histochemical and immunohistochemical procedures were used to reveal architectonic characteristics of the pulvinar complex and cortical areas. Histochemical techniques included those that reveal Nissl substance, cytochrome oxidase, myelin, and acetylcholinesterase, while immunohistochemical methods included those that label the vesicle glutamate transporter 2 (VGLUT2). These procedures, in conjunction with our analysis of anatomical connections, revealed multiple subdivisions of the pulvinar complex, while similarities in SC projection patterns and immunohistochemical staining of the pulvinar complex allowed us to identify homologues within the pulvinar of rodents and primates for the first time. Many features of the cortical projections to the SC are similar in all three species. Our results showed that superficial layers of the SC receive projections from visual areas within the occipital and temporal lobes, while deeper layers receive projections from visuomotor areas within parietal and frontal cortex. However, more complex patterns of projections were observed in galagos than in tree shrews and squirrels, reflecting the relative expansion of extrastriate visual and visuomotor cortical areas in primates. Together, these findings reveal common evolutionary patterns of cortical and subcortical SC circuitry, as well as specializations in primates due to an emphasis in visual and visuomotor processing. | |
