Symmetric-threshold ReLU for Fast and Nearly Lossless ANN-SNN Conversion

The artificial neural network-spiking neural network (ANN-SNN) conversion, as an efficient algorithm for deep SNNs training, promotes the performance of shallow SNNs, and expands the application in various tasks. However, the existing conversion methods still face the problem of large conversion error within low conversion time steps. In this paper, a heuristic symmetric-threshold rectified linear unit (stReLU) activation function for ANNs is proposed, based on the intrinsically different responses between the integrate-and-fire (IF) neurons in SNNs and the activation functions in ANNs. The negative threshold in stReLU can guarantee the conversion of negative activations, and the symmetric thresholds enable positive error to offset negative error between activation value and spike firing rate, thus reducing the conversion error from ANNs to SNNs. The lossless conversion from ANNs with stReLU to SNNs is demonstrated by theoretical formulation. By contrasting stReLU with asymmetric-threshold LeakyReLU and threshold ReLU, the effectiveness of symmetric thresholds is further explored. The results show that ANNs with stReLU can decrease the conversion error and achieve nearly lossless conversion based on the MNIST, Fashion-MNIST, and CIFAR10 datasets, with 6× to 250 speedup compared with other methods. Moreover, the comparison of energy consumption between ANNs and SNNs indicates that this novel conversion algorithm can also significantly reduce energy consumption.