Plasticity rules.

Each plot shows plasticity from spikes k and l from a visible and hidden neuron, respectively, which occur at times tk and tl. Red cross and blue triangle show learning from the two example dashed spikes in Fig 1b for feedforward and feedback connections, respectively. Note different x-axes on each plot. a: Standard STDP rule. Used for feedforward connections in the model, for which spike l is post-synaptic. x-axis shows time difference between post- and pre-synaptic spikes. The example spikes would strengthen the feedforward connection (red cross) but weaken the feedback connection, if feedback followed this rule (blue triangle). b: aSTDP rule, in which the time dependence is reversed. Used for feedback connections, for which spike k is post-synaptic. Learning rate is scaled by a constant ζ/η relative to STDP. c: Combined mSTDP rule. x-axis shows time difference between hidden and visible spikes, leading to identical profiles for STDP and aSTDP. Feedforward and feedback learning is symmetric (red cross and blue triangle).