Abstract: We introduce a lifelong language learning setup where a model needs to learn from a stream of text examples without any dataset identifier. We propose an episodic memory model that performs sparse experience replay and local adaptation to mitigate catastrophic forgetting in this setup. Experiments on text classification and question answering demonstrate the complementary benefits of sparse experience replay and local adaptation to allow the model to continuously learn from new datasets. We also show that the space complexity of the episodic memory module can be reduced significantly (~50-90%) by randomly choosing which examples to store in memory with a minimal decrease in performance. We consider an episodic memory component as a crucial building block of general linguistic intelligence and see our model as a first step in that direction.

Introduction: The ability to continuously learn and accumulate knowledge throughout a lifetime and reuse it effectively to adapt to a new problem quickly is a hallmark of general intelligence. State-of-the-art machine learning models work well on a single dataset given enough training examples, but they often fail to isolate and reuse previously acquired knowledge when the data distribution shifts (e.g., when presented with a new dataset)—a phenomenon known as catastrophic forgetting BIBREF0 , BIBREF1 . The three main approaches to address catastrophic forgetting are based on: (i) augmenting the loss function that is being minimized during training with extra terms (e.g., a regularization term, an optimization constraint) to prevent model parameters learned on a new dataset from significantly deviating from parameters learned on previously seen datasets BIBREF2 , BIBREF3 , BIBREF4 , (ii) adding extra learning phases such as a knowledge distillation phase, an experience replay BIBREF5 , BIBREF6 , and (iii) augmenting the model with an episodic memory module BIBREF7 . Recent methods have shown that these approaches can be combined—e.g., by defining optimization constraints using samples from the episodic memory BIBREF8 , BIBREF9 . In language learning, progress in unsupervised pretraining BIBREF10 , BIBREF11 , BIBREF12 has driven advances in many language understanding tasks BIBREF13 , BIBREF14 . However, these models have been shown to require a lot of in-domain training examples, rapidly overfit to particular datasets, and are prone to catastrophic forgetting BIBREF15 , making them unsuitable as a model of general linguistic intelligence. In this paper, we investigate the role of episodic memory for learning a model of language in a lifelong setup. We propose to use such a component for sparse experience replay and local adaptation to allow the model to continually learn from examples drawn from different data distributions. In experience replay, we randomly select examples from memory to retrain on. Our model only performs experience replay very sparsely to consolidate newly acquired knowledge with existing knowledge in the memory into the model. We show that a 1% experience replay to learning new examples ratio is sufficient. Such a process bears some similarity to memory consolidation in human learning BIBREF16 . In local adaptation, we follow Memory-based Parameter Adaptation BIBREF7 and use examples retrieved from memory to update model parameters used to make a prediction of a particular test example. Our setup is different than a typical lifelong learning setup. We assume that the model only makes one pass over the training examples, similar to BIBREF9 . However, we also assume neither our training nor test examples have dataset identifying information (e.g., a dataset identity, a dataset descriptor). Our experiments focus on lifelong language learning on two tasks—text classification and question answering. BIBREF17 show that many language processing tasks (e.g., classification, summarization, natural language inference, etc.) can be formulated as a question answering problem. We argue that our lifelong language learning setup—where a model is presented with question-answer examples without an explicit identifier about which dataset (distribution) the examples come from—is a more realistic setup to learn a general linguistic intelligence model. Our main contributions in this paper are:

Question: What is episodic memory?
Answer: It is a memory that stores previously seen examples throughout its lifetime