  Catastrophic forgetting (CF) happens whenever a neural network overwrites
past knowledge while being trained on new tasks. Common techniques to handle CF
include regularization of the weights (using, e.g., their importance on past
tasks), and rehearsal strategies, where the network is constantly re-trained on
past data. Generative models have also been applied for the latter, in order to
have endless sources of data. In this paper, we propose a novel method that
combines the strengths of regularization and generative-based rehearsal
approaches. Our generative model consists of a normalizing flow (NF), a
probabilistic and invertible neural network, trained on the internal embeddings
of the network. By keeping a single NF throughout the training process, we show
that our memory overhead remains constant. In addition, exploiting the
invertibility of the NF, we propose a simple approach to regularize the
network's embeddings with respect to past tasks. We show that our method
performs favorably with respect to state-of-the-art approaches in the
literature, with bounded computational power and memory overheads.
