PROJECT SUMMARY / ABSTRACT The concept of parallel pathways that code different aspects of the visual scene has led to many key insights regarding the functional organization of the visual system. Inspired by this concept, the proposed studies focus on parallel visual pathways from the retina to the superior colliculus (SC) through the pulvinar nucleus (PUL). Projections from the SC to the PUL originate from motion-detecting widefield vertical (WFV) cells, and their synaptic organization defines two distinct PUL subdivisions: one that receives ipsilateral topographic WFV projections (“specific”), and one that is innervated by bilateral convergent WFV projections (“diffuse”). These two WFV innervation patterns are correlated with distinct cortical and subcortical connections, as well a variety of histochemical criteria, suggesting that the tectorecipient PUL may be organized into separate visual movement processing streams. However, we currently lack a functional framework that allows us to test this hypothesis and decipher the modular organization of the PUL. We plan to address this gap in knowledge by defining PUL cell types and synaptic inputs in the context of their functional properties. Our guiding hypothesis is that the PUL is composed of two distinct modules that coordinate visual perception with body movements or motivational state to initiate appropriate motor commands. To begin to test this theory, with mice as our animal model, we will use anatomical intersectional viral vector approaches and in vitro whole cell recordings coupled with dual optogenetic activation of cortical and WFV synaptic inputs to define circuit mechanisms that can alter firing properties within each PUL module (Aim 1). We will use in vivo extracellular recordings coupled with optogenetic activation and silencing of synaptic inputs to determine how circuit interactions within each PUL module adjusts receptive field properties (Aim 2). A key innovation of our experiments is the ability to identify PUL neuron subtypes by their unique frequency-dependent responses to optogenetic activation of WFV inputs (“neuron identification via single input dynamics”). This new method will allow us to link detailed in vitro circuit dissection techniques with in vivo recording of visual response properties, providing a framework of PUL function that has thus far been elusive. By comparing two parallel PUL modules, our goal is to understand how visual motion signals are parsed to initiate appropriate behavioral responses.