Project Summary How experience influences the formation of neural circuits during mammalian development remains among the most fascinating questions confronting developmental neurobiologists. Over the past 50 years, studies of vision-dependent circuitry forming in the binocular visual cortex in carnivores, non-human primates and the mouse has led to a deep appreciation of this phenomenon. New technological developments, including the imaging of neural activity of individual cells in awake behaving animals, single cell labeling and transcriptomics, and large-scale electrophysiology, has opened the door to revisit classic questions in the field by tracking the development of individual neurons and cell types. In a series of recent studies, we established that visual experience soon after eye opening most profoundly shapes the development and refinement of cortical responses to the ipsilateral eye and that this experience-dependent plasticity sets the upper limit on binocular acuity. The locus of plasticity in the circuitry of primary visual cortex, and the role played by inhibition, remain unknown. The goal of this proposal is to systematically interrogate the visual pathway from thalamus to cortex and within cortex to gain a mechanistic understanding of how visual experience informs the establishment of cortical responses to the ipsilateral eye and, thus, binocular vision. There are three Specific Aims. In Aim 1, we will examine the development of the thalamocortical projection to V1 in layer 2/3 and 4 for both eyes. We will determine whether there are laminar differences in this organization as a function of eye and visual experience. In Aim 2, we will examine how this geniculate input is integrated by recipient cortical neurons. We will determine if there are differences in this integration for ipsilateral and contralateral eye inputs to excitatory and inhibitory neurons and whether visual experience is necessary for its establishment. And in Aim 3, we will explore and test the role of intracortical inhibition in refining ipsilateral eye input as a function of vision. None of these measurements have been made for binocular neurons in any species but are critically needed to obtain an understanding of the origins of high acuity binocular vision. We envision that these studies will establish general principles by which experience influences the development of the mammalian cortex.