Properties of synaptic transmission and the global stability of delayed activity states.

The influence of synaptic channel properties on the stability of delayed activity maintained by recurrent neural networks is studied. The duration of excitatory post-synaptic current (EPSC) is shown to be essential for the global stability of the delayed response. The NMDA receptor channel is a much more reliable mediator of the reverberating activity than the AMPA receptor, due to a longer EPSC. This allows one to interpret the deterioration of the working memory observed in NMDA channel blockade experiments. The key mechanism leading to the decay of the delayed activity originates in the unreliability of synaptic transmission. The optimum fluctuation of the synaptic currents leading to the decay is identified. The decay time is calculated analytically and the result is confirmed computationally.