Paper:
EMERGENCE OF GRID-LIKE REPRESENTATIONS BY TRAINING RECURRENT NEURAL NETWORKS TO PERFORM SPATIAL LOCALIZATION
Decades of research on the neural code underlying spatial navigation have revealed a diverse set of neural response properties. The Entorhinal Cortex (EC) of the mammalian brain contains a rich set of spatial correlates, including grid cells which encode space using tessellating patterns. However, the mechanisms and functional significance of these spatial representations remain largely mysterious. As a new way to understand these neural representations, we trained recurrent neural networks (RNNs) to perform navigation tasks in 2D arenas based on velocity inputs. Surprisingly, we find that grid-like spatial response patterns emerge in trained networks, along with units that exhibit other spatial correlates, including border cells and band-like cells. All these different functional types of neurons have been observed experimentally. The order of the emergence of grid-like and border cells is also consistent with observations from developmental studies. Together, our results suggest that grid cells, border cells and others as observed in EC may be a natural solution for representing space efficiently given the predominant recurrent connections in the neural circuits.

Review:
This paper aims at better understanding the functional role of grid cells found in the entorhinal cortex by training an RNN to perform a navigation task.
On the positive side: 
This is the first paper to my knowledge that has shown that grid cells arise as a product of a navigation task demand. I enjoyed reading the paper which is in general clearly written. I have a few, mostly cosmetic, complaints but this can easily be addressed in a revision.
On the negative side: 
The manuscript is not written in a way that is suitable for the target ICLR audience which will include, for the most part, readers that are not expert on the entorhinal cortex and/or spatial navigation. 
First, the contributions need to be more clearly spelled out. In particular, the authors tend to take shortcuts for some of their statements. For instance, in the introduction, it is stated that previous attractor network type of models (which are also recurrent networks) “[...] require hand-crafted and fined tuned connectivity patterns, and the evidence of such specific 2D connectivity patterns has been largely absent.” This statement is problematic for two reasons: 
(i) It is rather standard in the field of computational neuroscience to start from reasonable assumptions regarding patterns of neural connectivity then proceed to show that the resulting network behaves in a sensible way and reproduces neuroscience data. This is not to say that demonstrating that these patterns can arise as a byproduct is not important, on the contrary. These are just two complementary lines of work. In the same vein, it would be silly to dismiss the present work simply because it lacks spikes. 
(ii) the authors do not seem to address one of the main criticisms they make about previous work and in particular "[a lack of evidence] of such specific 2D connectivity patterns". My understanding is that one of the main assumptions made in previous work is that of a center-surround pattern of lateral connectivity. I would argue that there is a lot of evidence for local inhibitory connection in the cortex. Somewhat related to this point, it would be insightful to show the pattern of local connections learned in the RNN to see how it differs from the aforementioned pattern of connectivity.
Second, the navigation task used needs to be better justified. Why training a network to predict 2D spatial location from velocity inputs? Why is this a reasonable starting point to study the emergence of grid cells? It might be obvious to the authors but it will not be to the ICLR audience. Dead-reckoning (i.e., spatial localization from velocity inputs) is of critical ecological relevance for many animals. This needs to be spelled out and a reference needs to be added.  As a side note, I would have expected the authors to use actual behavioral data but instead, the network is trained using artificial trajectories based on "modified Brownian motion”. This seems like an important assumption of the manuscript but the issue is brushed off and not discussed. Why is this a reasonable assumption to make? Is there any reference demonstrating that rodent locomotory behavior in a 2D arena is random?
Figure 4 seems kind of strange. I do not understand how the “representative units” are selected and where the “late” selectivity on the far right side in panel a arises if not from “early” units that would have to travel “far” from the left side… Apologies if I am missing something obvious.
I found the study of the effect of regularization to be potentially the most informative for neuroscience but it is only superficially treated. It would have been nice to see a more systematic treatment of the specifics of the regularization needed to get grid cells.