| This dissertation compares the superior colliculus of the California ground squirrel, a highly visual diurnal sciurid rodent, with that of other highly visual vertebrates from primates to birds and reptiles. In the first study, retinal projections were mapped with intraocular injections of Cholera toxin B-subunit. Retinorecipient structures were found to be large, highly differentiated, and readily comparable to visual structures in tree shrews, primates, birds and reptiles. The dense innervation of the superficial superior colliculus revealed 50 μm-wide columnar terminal arborizations. In the second experiment, single neurons of the tectopulvinar pathway were filled by iontophoretic injection of biocytin in slice preparations. These neurons displayed 2000 μm wide dendritic fields ending in monostratified arrays of spiny endings. Such neurons were morphologically comparable to avian tectorotundal neurons with whom they share the property of having large motion-responsive receptive fields. In the final study, sublaminar and morphological patterns were studied with intracellular methods and Golgi's method to reveal a previously unknown sublaminar and columnar organization within the superficial superior colliculus. Each of the three major categories of tectal neurons, horizontal, narrow-field and wide-field vertical cells, displayed dendritic arborizations respecting 50 μm-wide cylindrical volumes within the superficial layers. These three neuronal categories exhibited distinct patterns of intracollicular axonal arborizations and are known to participate in different extrinsic projections or express distinct neurotransmitters. The deep collicular layers contained similar categories of neurons but their organization was not as clearly revealed. Broad comparisons suggest that these three neuronal categories are ubiquitous to the vertebrate optic tectum. Their morphology and connections are highly conserved and ideally suited to mediate conserved tectal functions. Against this framework of constancy appear novelties highlighting the evolution of the visual system. |
