Project Summary Understanding how visual information is processed and transformed by cortical and subcortical pathways to generate visually guided behavior is fundamental for visual research. As a critical sensorimotor center, the superior colliculus (SC) in the mammalian midbrain mediates visually guided behaviors in a context- dependent manner and holds great promise for understanding neural principles for the processing and transformation of visual signals into appropriate behavioral responses. The visuosensory layer of SC (or SCs) receives direct bottom-up relay of visual information from the retina, as well as inputs from multiple other visual structures. How the information from different sources is integrated by SCs neurons to differentially influence the processing and transformation of visual signals for context-dependent behavioral control remains largely unclear. In this project, by focusing on visual processing and transformation underlying two salient SC-mediated visually guided behaviors, defense behavior induced by high-field looming visual stimuli and approach behavior induced by low-field moving dots, we will investigate how the two major inputs of SC, the glutamatergic input from the primary visual cortex (V1) and GABAergic input from the ventral geniculate nucleus (vLGN), contribute to the shaping of SC processing. We will test an overarching hypothesis that V1 and vLGN inputs act together to regulate the gain and selectivity of visual features of SC neurons to achieve context-dependent modulation of visual behaviors. In Aim 1, we will examine how V1 input modulate visual response properties of SCs neurons in a lamina-dependent manner and how it contributes to selection of defensive reactions in response to high-field threatening visual stimuli though eliciting competitive interactions between SCs subcircuits. In Aim 2, we will examine the functional role of vLGN input in modulating SCs neuron responses to various visual stimuli and how it plays a role in the context-dependent modulation of size preference in approach behavior. Though these proposed studies, we hope to greatly enhance our understanding of functional organization, signal integration, intracollicular/interlaminar interactions as well as functional modulation in SC circuits.