The subunit composition of the GABAA receptor determines the decay time of the GABAergic inhibitory postsynaptic current (IPSC). In mice in which the α1 subunit is deleted, the decay time is longer than in wild-type mice, while the spatial spread of activity in the visual cortex following local stimulation is reduced. Using a simple network model of the visual cortex, we show that this reduced spread of activity could be accounted for by the longer IPSC decay time. After local stimulation of the network, a patch of activity develops, the equilibrium size of which depends on the IPSC decay time. © 2004 Elsevier B.V. All rights reserved.