Illustration of synaptic competition induced by hetero-synaptic plasticity.

(A) Competition induced by post-synaptic hetero-synaptic plasticity. Excitatory pre-synaptic neurons i target a post-synaptic neuron j. We assume that the resources required for increasing the post-synaptic components b_ij of the weights w_ji = a_ijb_ij are limited and therefore distributed in a competitive manner. That is, we assume hetero-synaptic plasticity where afferent synapses that receive large eligibility signals ε_ij increase their efficacy while synapses that receive weaker (but for excitatory synapses always positive) signals will instead weaken. (B) Pre-synaptically induced competition is assumed to follow the same principle. The signals for the changes of the pre-synaptic components a_ij, however, are assumed to depend on the realized amount of potentiation at the post-synaptic side, i.e., to take the post-synaptic hetero-synaptic competition into account. We found that this choice is more parameter tolerant and yields more robust memory than a symmetric version where pre- and post-synaptic hetero-synaptic plasticity are based on the same eligibility signal which, however, can also realize self-organized spike pattern detection (not shown).