Figure S8. a) For the device that only has gradual conductance incremental behavior (PCM device in Fig. 5e), potentiation and depression operation are performed in the positive branch and negative branch of the synaptic cell consisting of a differential pair of the memristive devices, respectively. b) Illustration of typical synaptic evolution of the differential pair. The two memristive devices are initialized to low conductance status. Increment of the conductance of the positive branch results in the gradual increase of the overall synaptic weight. Vice-versa increment the conductance of the negative branch results in the gradual decrease of the overall synaptic weight. However, after multiple weight update operations, the differential pair might reach a status that both the devices are in high conductance states, which prevents further weight update operations and degrades the performance of the neural network. In the training case in this work, no such issues were detected, since the number of weight update operations on each memristive synaptic cell is highly reduced. c) For the device only has gradual conductance decrement behavior (OxRRAM device in Fig. 5d), potentiation and depression operation are performed in the negative branch and positive branch of the synaptic cell consisting of a differential pair of the memristive devices, respectively. b) Illustration of typical synaptic evolution of the differential pair.