UC San Diego

Much can be learned about the functional organization of the visual system by examining its anatomy. In traditionally-studied cat and primate species, certain morphological types of retinal ganglion cells project to specific thalamic and then cortical layers, separated by eye in each region. In addition, physiology shows that information is segregated by functional cell type; each layer is a complete or near-complete map of some aspect of the world such as form and motion. This results in a system that can easily distribute specialized information without disrupting the larger visuo-spatial map. While the layers of the macaque retino-recipient thalamus are easily discernible due to the correlation between cytoarchitecture and function, the rat geniculate appears to have no such patterning. As the rat is a favorable research model, it is important to understand how its brain is set up to tackle the same visual problems as that of higher mammals. The purpose of this project was to describe substructure within the rat dorsal lateral geniculate nucleus of the thalamus, with the goal of learning more about how the anatomical organization might give insight into function. To visualize the anatomy, I intra-occularly injected retrograde tracer to map retinal termini throughout the nucleus. Based on three-dimensional reconstructions of termination zones from each eye, I identified multiple ipsilaterally-recipient zones within the largely contralateral thalamic nucleus. These ipsilateral subregions are more reminiscent of the layering in the primate thalamus than previously described in the rat. Furthermore, the nucleus appears to be well segregated by eye, suggesting that, like in these other species, the rat thalamus passes on information without binocular mixing. My findings also support the hypothesis that, unlike in the primate, rat cell bodies throughout the layers appear to be cytoarchitecturally homogeneous. While we can glean insight into the function of macaque thalamic layers by looking at cell-body statistics such as size and density, no such patterns can be seen in the rat. Among other possibilities, this could mean that rat projection neurons have similar morphologies, or that functional cell types are intermingled and not spatially organized as they are in other species