GABA is the main inhibitory neurotransmitter in the adult brain. By contrast, at early stages of cortical development GABA is presumed to depolarize immature nerve cells and, consequently, facilitate action potential generation and network activity. However, a fundamental question remains currently unanswered: What are the long-term functional consequences of a depolarizing, rather than hyperpolarizing, mode of GABA action in early life? Using the mouse primary visual cortex as a model system, we will address this question in an interdisciplinary approach. More specifically, our collaborative research proposal pursues two main objectives: 1) How do depolarizing actions of GABA orchestrate the refinement of neuronal network activity during postnatal development? 2) To which extent is depolarizing GABAergic transmission required for the proper maturation of feature selectivity and sensory learning in the visual neocortex? To this end, we will first generate and validate new genetic and optogenetic tools suited to manipulate the mode of GABA action in a time-, cell type- and/or location-specific manner. Two-photon Ca2+ imaging, in combination with electrophysiological techniques, will be applied in order to resolve spontaneous and sensory-evoked neuronal network activity at single cellular resolution in intact mice. We additionally aim at developing and implementing new methods of data analysis and data-driven modelling that are suited to reveal currently inaccessible mechanistic insights into the observed spatiotemporal patterns of network activity. In addition, behavioral testing procedures will be used to evaluate how alterations at the level of single cells or neuronal networks affect sensory-driven behaviors.