Project Summary Neural circuits of the primary visual cortex (V1) are critical for generating perceptions of our external world. In V1, most neurons exhibit potent modulation by stimuli that are outside their receptive fields, a process termed `surround modulation'. Importantly, the magnitude and sign of surround modulation depend on the orientation of the center and surround – leading to the notion that flexible surround modulation contributes to scene segmentation, salience detection, contour integration, and figure/ground segregation. The specific neural circuits in the visual cortex that explain the orientation dependence of surround modulation are largely unknown. By combining two photon calcium imaging, two photon holographic optogenetics, in vivo patch clamp electrophysiology, and the first use of a two photon holographic mesoscope, we aim to reveal the precise synaptic and circuit architecture in the mouse visual cortex that mediates the earliest stages of image segmentation. First, we will measure the visually evoked synaptic conductances in cortical interneurons that explain their highly differentiated tuning properties to contextual visual stimuli. Next, we will precisely map the local and long-rate connectivity onto specific subtypes of cortical interneurons in vivo using two photon photo- stimulation. Finally, we will probe how two photon holographic co-activation of surround co-tuned ensembles in V1 or higher visual areas give rise to figure/ground modulation in V1 principal neurons. Together, our aims will establish a highly detailed mechanistic understanding for a visual computation responsible for object recognition.