Figure 2.

Stimulation protocols. A, Two groups (S1 and S2) of 2000 input units project onto 10 AIF neurons (only one is shown) with a connection probability of 10%. An extracellular stimulation pulse is modeled by a Gaussian packet of action potentials with a SD of 3 ms, with 1 spike per axonal projection. For three or fewer consecutive packets separated by 50 ms (as in a sLFS protocol, see D), a postsynaptic neuron generates a single spike, whereas several spikes are emitted if stimulation occurs at 100 Hz (as in a wTET, see B). B, Detailed view of a wTET protocol. Spike arrivals at 20 synapses (black bars) are shown together with spikes of all 10 postsynaptic units (red bars). The effect on individual weights can be seen in the bottom (gray lines). Note that potentiation happens only after the second postsynaptic spike of a neuron. Only those synapses crossing the long-term stability barrier (dashed horizontal line) will undergo a long-lasting change. The mean weight is shown in black. C, Tetanic stimulation protocols. A wTET (21 pulses at 100 Hz) or a sTET (3 × 100 pulses at 100 Hz) is strong enough to overcome neuronal adaptation so that the postsynaptic neuron fires several spikes (red bars, schematic). Because potentiation needs at least two postsynaptic spikes (post-pre-post triplet), a high postsynaptic firing rate gives rise to LTP (Pfister and Gerstner, 2006; Clopath et al., 2010), indicated as a shift to the metastable state on the right. D, With a sLFS protocol (3 pulses at 20 Hz repeated every second), the postsynaptic neuron fires only once per second (top, interrupted time axis). Synapses initially in the high state (gray traces) become weaker on average (black). Note that the first and second volley of spike arrivals, ∼50 ms after the postsynaptic spike, lead to synaptic depression. E, Low-frequency stimulation protocols. During a sLFS, we have one postsynaptic spike (red bar, schematic) for three presynaptic spikes (black) at the same synapse arising from three subsequent stimulation volleys, which gives rise to a strong post-before-pre LTD. During a wLFS (900 pulses at 1 Hz, top), the postsynaptic neuron fires once per volley, but because spikes arrive approximately in the middle of the packet (compare first stimulus in D), approximately half the synapses perceive a post-before-pre pattern leading to LTD, whereas the other half perceives an isolated pre-before-post causing no effect. The accumulation of the 900 pulses makes the LTD stimulus strong enough to have a long-lasting effect. In the case of an sLFS, the larger fraction of post-before-pre spikes gives rise to a larger effect. F, Resetting effect. A reset protocol (250 at 1 Hz) is not enough to produce any LTD on a synapse in the high state (left) because the number of pulses is not enough to push the weight variable over the potential barrier. For a synapse in the e-LTP state (right) the barrier for depression is lower because the meta-stable position of the tag is somewhat shifted to the left. This enables softer protocols to have a clear depotentiation effect.