Relationships between the three multi-spike STDP models.

If the second messengers activate instantaneously, the NMDAR-based model is qualitatively equivalent to the suppression model (top, right). If there exists an infinite reservoir of resting NMDARs and inactive second messengers, the NMDAR-base model reduces to the triplet model (bottom-left). If both assumptions are fulfilled, the NMDAR-based model reduces to the pair-based model (bottom- right).