Fig. 3. Demonstration of triplet-STDP and its related BCM learning rules in Pt/WO_3−x/W memristors.

a Schematic of the typical ‘post-pre-post’ and ‘pre-post-pre’ triplets, which can be simplified as a superposition of the LTP and LTD processes. Each pre- or postsynaptic spike comprises two pulses with amplitude V⁺/V⁻ = 2 V/−2 V and duration 50 ms. Taking the former as an example, the LTD process activated in the first ‘post-pre’ pair with spike timing of Δt₁ < 0 is followed by the LTP process induced by the second ‘pre-post’ pair with spike timing of Δt₂ > 0. b Synaptic modification of triplet-STDP in the ‘post-pre-post’ and ‘pre-post-pre’ sequences using symmetrical spike timing |Δt₁| = |Δt₂|, with three levels of the initial G₀ considered as the learning experiences (i.e., 0.1, 0.5, and 3.0 µS). c Triplet-STDP with asymmetrical spike timing. Red column: Δt₁ = −70 ms, Δt₂ is from 10 to 120 ms in the ‘post-pre-post’ sequence; Blue column: Δt₁ = +70 ms, Δt₂ is from −10 to −120 ms in the ‘pre-post-pre’ sequence. d, e Summaries of triplet-STDP results in our experiments, where potentiation or depression with different synaptic weights is obtained using different spike sequences and different timing intervals. The insets show the schematic of ‘post-pre-post’ and ‘pre-post-pre’ sequences. Here, both the size of symbols and the background color indicate the magnitude of ΔG_c. A relatively high G₀ of 3.0 µS was adopted for the measurements of Fig. 3 (d, e) to highlight the history-dependent characteristics. f The dependence of ΔG_c on both the presynaptic spike rate ρ_x and postsynaptic spike rate ρ_y. The schematic of the operation signal is shown in the inset, in which three pairs of presynaptic spike and postsynaptic spikes were used. g Triplet-STDP-based BCM learning rules with the EDE in the low-frequency region and the threshold sliding effect, which is highly similar to the biological BCM curve.