Developmental upregulation of the GABAA receptor α1 subunit causes a faster decay of GABAergic inhibitory postsynaptic currents (IPSCs) in the visual cortex around the time of eye opening. In α1 deficient mice, a juvenile type of GABAA receptors is retained during maturation. As a result the decay time of the IPSCs is longer in α1-/- mice than in WT mice during the whole life span of the mice. Hence they form a valuable mouse model for studies on cellular aspects of neuronal network functioning. Using voltage sensitive dye imaging methods, we monitored the spatiotemporal excitation patterning in visual cortex slices upon local stimulation of the network. We found that in the α1-/- mice, the ability of the network to fire synchronously at γ-frequencies (20-50Hz) is diminished. This finding indicates that early onset of GABA synapse maturation is required for the normal neuronal network function in the maturating visual cortex.