The optic tectum has emerged as a tractable visuomotor transformer, in which anatomical and functional studies can allow a better understanding of how behavior is controlled by neuronal circuits. We plan to examine the formation and function of the visual system in zebrafish larvae using in vivo time-lapse microscopy and state-of-the-art “connectomic” and “optogenetic” approaches to monitor and perturb neuronal activity. We will apply complementary cellular and molecular analyses to dissect this circuit and identify the neuronal substrate of visual behaviors. We will start by analyzing the function, development and connectivity of a newly characterized class of inhibitory interneurons located in the superficial part of the tectal neuropil named SINs (superficial inhibitory interneurons) that I have previously identified. Our work based on functional imaging has placed SINs at the center of a tectal micro-circuit for size tuning of visual stimuli. We will dissect this working model by analyzing the physiological properties of SINs. We also will investigate their development and connectivity at the level of single synapses by imaging these cells in vivo using fluorescent reporters in transgenic animals. We will then study how SINs migrate to their final position in the superficial tectum away from the zone where they are initially generated and how their processes direct tectal synaptic lamina formation. SINs are the only tectal cells expressing Reelin and we will analyze the role of this pathway in tectal development and proper synaptic lamination in the tectal neuropil. Our multidisciplinary approach aims to describe in great detail the formation and function of a neuronal circuit crucial for visual function, establishing this model for neural circuits studies in vertebrates.