Principles of presynaptic networks for single layer 2/3 neurons in ferret visual cortex Single neurons in neocortical circuits are driven by presynaptic networks composed of excitatory and inhibitory neurons. Each neuron's population of presynaptic partners determines how incoming information is processed. A longstanding view of cortical circuits is that a majority of synaptic inputs originate from local networks through horizontal (recurrent) connections. However, the mechanisms by which recurrent networks shape the activity of cortical neurons is largely unknown. Additionally, synaptic and cellular mechanisms proposed by theoretical models rely on studies of the rodent visual cortex, which is increasingly shown to differ from that of carnivores and primates in organization and function. This proposal aims to address these problems by mapping presynaptic excitatory and inhibitory cells of single layer 2/3 neurons and dissect how they act to selectively modulate neural activity in ferret V1 in vivo. This proposal uses a novel combination of advanced optical techniques and electrophysiology, building off the candidates optical create has a deep background in in vivo physiology and imaging in a wide variety of mammalian species and training during the K99 phase. This proposal will the foundation for an innovative, multidisciplinary, and independent research program to establish fundamental principles of cortical circuits, ultimately providing a scaffold for understanding disorders, such as schizophrenia and autism, which show profound impairments in the processing of sensory signals.