Paper:
Continual learning is the problem of sequentially learning new tasks or knowledge while protecting previously acquired knowledge. However, catastrophic forgetting poses a grand challenge for neural networks performing such learning process. Thus, neural networks that are deployed in the real world often struggle in scenarios where the data distribution is non-stationary (concept drift), imbalanced, or not always fully available, i.e., rare edge cases. We propose a Differentiable Hebbian Consolidation model which is composed of a Differentiable Hebbian Plasticity (DHP) Softmax layer that adds a rapid learning plastic component (compressed episodic memory) to the fixed (slow changing) parameters of the softmax output layer; enabling learned representations to be retained for a longer timescale. We demonstrate the flexibility of our method by integrating well-known task-specific synaptic consolidation methods to penalize changes in the slow weights that are important for each target task. We evaluate our approach on the Permuted MNIST, Split MNIST and Vision Datasets Mixture benchmarks, and introduce an imbalanced variant of Permuted MNIST -a dataset that combines the challenges of class imbalance and concept drift. Our proposed model requires no additional hyperparameters and outperforms comparable baselines by reducing forgetting.

Review:
The authors introduce DIFFERENTIABLE HEBBIAN CONSOLIDATION,a new framework for continual learning that can be implemented in the usual differentiable programming setups. This framework is motivated in terms of complementary learning system (CLS) theory which features an episodic memory module. The method is shown to be easily implemented as seen in their pytorch pseudocode (authors also suggest code will be released). Additionally, authors show the method leads to significant  improvements over simple baselines, and can complement other task-specific hebbian-based learning paradigms
As an alien to the continual learning literature, my judgment is mostly positive. First, the paper is well-written, and the method is well-motivated in terms of cognitive principles with a nice commentary about underlying neurobiological substrates. The description of alternative methods seems exhaustive (but see below). The resulting algorithm is simple and doesn't seem to entail a significant computational burden. Importantly, it is differentially programing-friendly. Results show improvements over a baseline. However, I have two significant criticisms regarding the experimental section that I hope the authors will address.
1)Authors comment on two paradigms to hebbian-based continual learning: the task-specific and theirs, based on CLS. Authors show their method complement improvements due to task-specific approaches (which is great), but proper comparisons to other CLS-based approaches seem missing. In other words, I would hope a much better statement than "
However, all of these methods were focused on rapid learning on simple tasks or metalearning over a distribution of tasks or datasets. Furthermore, they did not examine learning a large
number of new tasks while, alleviating catastrophic forgetting in continual learning" Perhaps the authors may try find experimental cases where comparison to such approaches are possible, and show the empirical results? I am concerned the finetuned MLP baseline could be a bit weak.
2)The authors should try more experiments. They showed many MNIST variants and a 5-vision dataset. However, results in other papers in the literature are shown in much more sophisticated contexts (e.g. learning to play games). I understand this could exceed the computational capabilities of authors, but  why, for example, not to try on a different dataset, as CIFAR? (as the Zenke et al paper did).
MInor: On section 3, page 4 authors mention the number of labels d but that number appears undefined