The extrageniculate visual pathway, which carries visual information from the retina through the superficial layers of the superior colliculus and the pulvinar nucleus, is poorly understood. Several studies have implicated this pathway, in particular the pulvinar, in cognitive tasks such as selective attention, but no specific mechanism or circuit has been described that could support such a function. In order to better understand what role this secondary visual pathway serves, here we examine the basic anatomical and functional properties of cells in the pulvinar, and what driving or modulating effect these cells have on visual cortex. In the first study, we made extracellular recordings in the pulvinar nucleus of lightly anesthetized rats. We identified visually responsive cells that have large receptive fields on the order of 80 degrees of visual field, fire selectively in response to low spatial frequencies, and can sometimes be selective to the direction or orientation of moving grating stimuli. We also determined that the anatomically-defined lateral subdivision encodes a greater diversity of temporal frequency stimuli than the more medial subdivision, which has better direction selectivity. In the second study, we used modified rabies viruses to retrogradely trace connections between rat pulvinar and visual cortex in order to better understand from where pulvinar receives its visual information and to which cortical areas it is best connected. We found a significantly weaker projection from pulvinar to primary visual cortex (V1) than from pulvinar to higher visual cortex (V2). In the final study, we designed and tested a new modified rabies virus to optically excite and silence populations of the more numerous projections to V2 from the rat pulvinar. We found that pulvinar modulates cortical size tuning and suppresses flash responses, but doesn’t drive activity in V2. This dissertation builds a foundational understanding of the role of the pulvinar nucleus in rodents, illustrating how pulvinar integrates spatiotemporal information from visual cortex and the superior colliculus, and regulates firing rates in rodent V2. The new rabies virus variant described here is well suited to test theories of pulvinar function, in addition to its wide potential applications in non-transgenic animals such as cats or non-human primates, where models of pulvinar are already well developed
