The PhD thesis of my 15th PhD student, Shibhansh Dohare (@ShibhanshD), is now available. Title: Learning Forever using Artificial Neural Networks Url: incompleteideas.net/papers/D… Abstract: In this dissertation, I study the ability of artificial neural networks to learn new things. I refer to this ability as plasticity. Plasticity is a desirable property of intelligent systems in the real world. Systems that interact with the real world can become outdated if they do not learn from the new information revealed to them by the extremely large and partially observable real world. All naturally intelligent systems continually learn new things to keep up with the changes in the world. Many applications like predicting markets or weather, learning human preferences, controlling factories or prosthetics, etc require AI systems that continually learn new things. I provide direct and thorough demonstrations of loss of plasticity in deep learning systems. The demonstrations range from simple feed-forward networks to deep residual networks and cover a wide range of algorithmic choices. While seeking an understanding of the phenomenon, I found that loss of plasticity occurred simultaneously with hidden units becoming dormant and becoming similar to each other, hampering the ability of the networks to learn new things. Next, I describe the continual backpropagation algorithm. I designed this algorithm, in collaboration with others, to maintain plasticity in artificial neural networks. Continual backpropagation is a simple extension of the conventional backpropagation algorithm. The conventional backpropagation has two main parts: initialization with small random weights and gradient descent at each step. In addition to gradient descent, continual backpropagation reinitializes a small fraction of units at each step. Continual backpropagation indefinitely maintained plasticity in many continual supervised learning problems. Finally, I study plasticity of deep reinforcement learning systems. First, I show that plasticity loss has an effect even in stationary reinforcement learning problems. It is unsurprising that plasticity loss affects stationary reinforcement learning because it requires continual learning due to changing policies and bootstrapping. This effect is visible under long-term learning. I found that some commonly used RL algorithms do not scale with data. Their performance drops as they continue interacting with the environment. Plasticity loss hampers the ability of these algorithms to recover and relearn when the performance drops, resulting in agents that do not get better with experience. Continual backpropagation allows these algorithms to recover when performance drops, leading to agents whose performance keeps improving with experience. Second, I show that loss of plasticity is a significant problem in continual reinforcement learning problems, and continual backpropagation maintains plasticity in these problems. Shibhansh is now a research scientist at Keen Technologies.