PhD thesis, Imperial College London (2023)

 

This thesis focuses on the study of training deep spiking neural networks (SNNs). In recent years there has been an increasing interest in using spiking neurons for deep learning with the aim of leveraging their unique properties and characteristics. These include their potential for very energy efficient training and inference due to their highly sparse activity and their suitability to model biological neurons. We first introduce the fundamental models that are used in the SNN training literature, including neuron models at different levels of abstraction, synapses and networks of neurons and neurons under noise. We also review the main SNN training methods that have been developed to this date. Then, we study the role of neural heterogeneity and study the performance and robustness of SNNs under different heterogeneity schemes on two different supervised learning methods. Next, we show how the sparse activity present in the forward pass on SNNs can also be achieved in the backward pass, leading to highly efficient implementations that can speed up the backward pass up to 150x and save 85% of the memory. Finally, we aim to solve the weight initialisation problem for SNNs to achieve a predictable network activity as well as prevent the gradient from vanishing or exploding. In this process, we identify and solve the firing rate collapse issue caused by the discretisation of SNNs for simulation. In addition, we obtain theoretical and empirical results for a general SNN initialisation strategy making use of variance propagation and diffusion/shot-noise/threshold integration methods, as well as the solution to the firing rate collapse problem we previously found. Besides the ideas and experiments discussed in this thesis, code for the methods described here can be found in https://github.com/npvoid.