  The background activity of a cortical neural network is modeled by a
homogeneous integrate-and-fire network with unreliable inhibitory synapses.
Numerical and analytical calculations show that the network relaxes into a
stationary state of high attention. The majority of the neurons has a membrane
potential just below the threshold; as a consequence the network can react
immediately - on the time scale of synaptic transmission- on external pulses.
The neurons fire with a low rate and with a broad distribution of interspike
intervals. Firing events of the total network are correlated over short time
periods. The firing rate increases linearly with external stimuli. In the limit
of infinitely large networks, the synaptic noise decreases to zero.
Nevertheless, the distribution of interspike intervals remains broad.
