Soft-bound synaptic plasticity outperforms hard-bound plasticity

Accurate models of synaptic plasticity are essential to understand the adaptive properties of the nervous system and for realistic models of learning and memory. Experiments have shown that synaptic plasticity depends not only on preand post-synaptic activity patterns, but also on the strength of the connection itself. Namely, weaker synapses are more easily strengthened than already strong ones. This so called soft-bound plasticity automatically constrains the synaptic strengths. It is known that this has important consequences for the dynamics of plasticity, however, its impact on information storage is unknown. In this modelling study we show that soft-bound plasticity increases the storage capacity of synapses for a variety of performance criteria. Author Summary It is generally believed that our memories are stored in the synaptic connections between neurons. Numerous experimental studies have therefore examined when and how the synaptic connections change. In parallel many computational studies have examined the properties of memory and synaptic plasticity, with as a goal to better understand human memory and allow for neural network models of the brain. However, the plasticity rules used in most studies are highly simpli ed and do not take into account the rich behaviour found in experiments. For instance, it has been observed in experiments that it is hard to make strong synapses even stronger. Here we show that this saturation of plasticity enhances the number of memories that can be stored.