Project Summary Visual perception is mediated by complex interactions amongst neurons in the retina, visual cortex, and subcortical brain structures. The importance of vision to humans and other primates is reflected in the enormous percentage of cerebral cortex devoted to processing visual information. Thus, deficits in visual processing are particularly debilitating and arise from abnormalities not only in the eye, but also in cortical and subcortical circuitry. For example, strabismus or amblyopia during childhood can have long-lasting effects on the cortical circuits that process visual information. There is also evidence that some forms of dyslexia result from central visual system abnormalities. The function of the nervous system is dependent on complex interactions between networks of neurons composed of multiple neuron types across multiple cortical and subcortical areas. General principles about cortical feedback connections and corticothalamic networks learned from studies of the mouse visual system are likely to also apply to circuits mediating cognitive functions and may be impaired in disorders such as autism and schizophrenia. The proposed studies are aimed at revealing the organization and functional impact of: 1) cortical feedback connections to the primary visual cortex; and 2) cortico-thalamo-cortical circuits. These studies are conducted in mice to take advantage of the range of molecular, genetic, and viral tools that can be used to elucidate brain circuits and link them to function using optogenetic and imaging methods. The 4 aims will reveal: 1) the functional impact of feedback from layers 2/3 or layer 5 of cortical areas AL and PM to V1; 2) how feedback from layers 2/3 or layer 5 of cortical areas AL and PM to V1 integrate with specific inhibitory circuits within V1; 3) the input-output relationships of neurons in the pulvinar nucleus of the thalamus that project to identified visual cortical areas; 4) the functional impact of pulvinar inputs to the visual cortex.