Figure 1.

Metaplasticity of short-term plasticity. (A) Shift Problem. The goal is for the post-synaptic unit (red) to fire to the shift pattern (left), but not to the synchronous pattern (right). (B) If both input synapses exhibit the same type of STP the shift problem cannot be solved. The traces depict the voltage contribution of each input (light and dark blue) to the total post-synaptic voltage (red). PPF or paired-pulse depression in both inputs cannot solve the problem because the neuron's peak response (red trace) will always be to the second or first pulse of the Synch pattern, respectively. Each input exhibits PPF or PPD depending on whether the inputs have a low or high U, respectively. (C) A simple learning rule that adjusts the variable U (“Pr”) at each synaptic terminal can solve the shift problem. $\Delta U_i=\{\begin{array}{r}\hfill \alpha \cdot S_i(t)S_i(t)\le 1\\ \hfill -\beta \cdot (S_i(t)-1)S_i(t)>1\end{array}$. S is a variable that reflects the number of presynaptic spikes (see Materials and Methods). Training: Pairing post-synaptic depolarization (I) – which generates a spike and acts as the “supervisor” – with the coincident presynaptic spikes of the Shift pattern results in PPF at synapse 1 and PPD at synapse 2, in addition to conventional post-synaptic LTP at both synapses (driven by STDP). The rationale is that the time of the post-synaptic spike in relation to a presynaptic spike train determines whether those synapses will show PPD (early pairing) or PPF (late pairing). By pairing post-synaptic depolarization with either the first or second spikes of the synchronous pattern the post-synaptic neuron will also learn to respond selectively to the synch pattern.