Microglia, the primary immune effector cells in the brain, are critically involved in many pathological conditions including injury, stroke, and Alzheimers and Parkinsons diseases.Under physiological condition, microglia spend most of time on a resting state with highly branched processes surveying surround brain area.However, little is known about what are sensed by resting microglia and what are their physiological functions.Using in vivo two-photon morphology and calcium imaging in the optic tecturn of zebrafish larvae, we found here that the bulbous contact between neuron and microglial process was positively correlated with spontaneous neuronal activity.Local increase of neuronal activity by two-photon based focal glutamate uncaging effectively induced oriented-motility of resting microglial processes and its bulbous contact with neurons in uncaged area.Reciprocally, microglial contact decreased spontaneous neuronal activities.Thus, resting microglia dynamically surveying high neuronal activity are important for neuronal activity regulation and homeostasis in vivo.