The invasion of a synaptic terminal by an action potential often fails to induce a postsynaptic response both in the hipppcampus [Allen and Stevens, 1994, Dobrunz and Stevens, 1997] and in the cortex [Stratford et al., 1996]. While the release probability varies across synapses onto the same neuron [Rosenmund et al., 1993, Hessler et al., 1993, Castro-Alamancos and Connors, 1997] and as a function of history of use [Dobrunz and Stevens, 1997, Markram and Tsodyks, 1996, Abbott et al., 1997, Varela et al., 1997, Zador and Dobrunz, 1997], for simplicity we make the assumption here that the release probability is the same at all terminals.
Fig. 3A shows the dependence of information on firing rate for several values of . The top curve shows , and is the same as the solid curve in Fig. 2B. The lower three curves show that as is decreased (to 0.9, 0.6 and 0.3), the form of the dependence is largely preserved, but the curves are shifted down. Thus as expected, synaptic unreliability lowers the information rate. In these simulations, the input Poisson rate was held constant as described in Sec. 2.2, so the decrease in the information rate was due solely to an increase in the conditional entropy per spike and not to a change in firing rate.
Fig. 3B illustrates the dependence on in more detail. For this curve, the firing rate was held constant at 40 Hz, and was varied from 0 to 1. The information is a monotonically increasing function of . This is reasonable, since as the synaptic reliability increases, so should the reliability with which information is transmitted. No sharp transition is observed from an unreliable to a reliable mode.