A single axon may sometimes make multiple synapses onto a postsynaptic target, or a single synapse (such as the neuromuscular junction) might have multiple release sites. To avoid ambiguity, we use functional contact to refer to any release site from a presynaptic axon to a postsynaptic target, whether it involves multiple synapses per axon, or multiple release sites per bouton. At the neuromuscular junction, functional contacts are counted by the thousands [Katz, 1966]. At excitatory synapses in the cortex, the number of functional contacts is much smaller, but still sometimes greater than one [Sorra and Harris, 1993, Markram and Tsodyks, 1996]. We have therefore explored the consequences of multiple functional contacts on the information rate.
Fig. 4A shows the dependence of information rate on release probability for three different values of , the number of functional contacts per axon. As in the previous simulations, the input Poisson rate was held constant as described in Sec. 2.2. The bottom curve is the same as that shown in Fig. 4B. As the number of functional contacts is increased, the information available in the output spike train is increased as well. Since multiple functional contacts can be seen as a form of redundancy, the increase in transmitted information is not unexpected. In these simulations, the input Poisson rate was held constant as described in Sec. 2.2. Although the firing rate was slightly () higher for large , the increase in the information rate was due primarily to both an increase in the total entropy per spike and a decrease in the conditional entropy per spike, and not to the increased firing rate.
Fig. 4B illustrates the dependence on the number of functional contacts in more detail. For this curve, release probability was held constant at , and was varied from 1 to 25. The information saturates at high , but no sharp transition is seen from a low to a high reliability mode.