Paper ID: | 9406 |
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Title: | Enabling hyperparameter optimization in sequential autoencoders for spiking neural data |

Authors provide novel approaches to calculate cross-validated reconstruction loss by using one of two proposed solutions: Sample validation and Coordinated dropout described above. The ideas are first described with the help of a synthetically generated dataset and experimental results on Monk This paper would be much stronger if the ideas were demonstrated on multiple real datasets. As well as demonstrating comparable performance using only x% of multiple datasets. In the current organization, the ideas are first demonstrated on synthetically generated data. It is not clear why the "Monkey J Maze" is not used right from the beginning, instead of spending significant portion of the data in describing the synthetic data generation process. Synthetic data is unconvincing especially in an unsupervised learning setting. These look like sound ideas and would be generally applicable for autoencoders in any domain. So if getting neurological data is challenging, data from other domains may be considered. It would be a good idea to use existing regularization methods for autoencoders as baselines e.g., Denoising autoencoders. While the proposed methods look sound, the motivation for using completely new techniques should be explained. While the proposed methods are promising, this paper appears to be research in progress, and would benefit from demonstration of more experimental results. ## Comments post rebuttal and review discussion Having reviewed the rebuttal and other reviews, I think its ok to accept. I am convinced with the need to demonstrate on synthetic data, it helps to explicitly make the case is made for readers unfamiliar with the domain for whom this is not obvious. Having further experimental results on real data helps to make the case as well. Good to see that the experimental results hold on the 2nd "Random Target" dataset. Ideally I would have liked to see experimental results on more than 2 datasets. It was also good to see baselines on dAE on synthetic dataset. Ideally, this should be demonstrated on the 2 real datasets as well. Partial CD (splitting data into input only, shared and output only) is a good suggestion. However, I don't understand why the experiments were conducted on a smaller subset of the data. As this was not part of the original paper, it is okay to ignore and leave it out. If a comparison of Partial CD will be included in the final version of the paper, we would need to see (i) complete results on Partial CD (ii) a clear description of exactly what splits were used for experiments. (iii) a better description of conditions under which partial CD does better than full CD [Fig. 3 in the rebuttal that shows improvement in 8 of 10 models.] (iv) description of when SV is also additionally needed.

A method to optimize hyperparameters (HPs) for sequential autoencoders (SAEs) are tested to prevent overfitting to infer structure from high-dimensional spiking data from biological neurons. The authors find that SAEs applied to spiking neural data are prone to a particular form of overfitting that cannot be detected using standard validation metrics, which prevents standard HP searches. Two potential solutions are explored: an alternate validation method (“sample validation”) and a novel dropout regularization method. Effectiveness of these regularizers is demonstrated on monkey motor cortex spike data. The biggest disadvantage of the model seems to be, that the method is tailored to spiking neuron model. It is therefore not clear if the proposed regularizers could be extended to other learning problems. Have the authors explored this possibility? A brief discussion would increase the utility of the paper.

The authors present a significant and original contribution to training autoencoder models in neuroscience that may be broadly applicable in machine learning applications. The issue of pathological overfitting and the proposed solutions are clearly described, although inclusion of some additional important details would improve clarity (see below). The training methods described in this paper will be useful to neuroscientists and machine learning practitioners training autoencoder models. The motivating example is a sequential autoencoding model LFADS. In LFADS (previously described elsewhere), the generative model is an RNN that generates low-D factors underlying higher-dimensional Poisson count data. At each time point, random inputs are fed into the RNN. The model is trained using amortized variational inference using an RNN encoding network to approximate the variational posterior over the initial condition of the generator RNN, and a “controller” RNN that approximates the posterior over the input perturbations. Fitting this model to different datasets with different dimensionalities and amounts of training data requires hyperparameter tuning. However, the authors show that when using random hyperparameter search, the “best” hyperparameters chosen via validation loss may correspond to a model that is pathologically overfit, where the controller RNN learns inferred inputs that simply reproduce spikes in the data. The authors propose two training schemes to overcome the pathological overfitting. In the first, called sample validation, random elements of the neuron by time by trial input array are dropped out at both the input to the approximate posterior (encoder network) and the output of the generative model, such that the gradients with respect to the dropped out elements are blocked. This scheme has been applied previously in cross validation for PCA. Next, the authors propose an original training scheme called coordinated dropout. In coordinated dropout, the data is split into two parts. The first split of the data (e.g. 70% of data) is used as input to the encoder network, while the rest of the data is used to compute the training objective and gradients. This method relies on the modeling assumption that the data can be described by low-d factors, and its utility is demonstrated on a linear autoencoder example. On a synthetic example, the authors show that when either method is used in training across random hyperparameters the models no longer exhibit pathological overfitting. Notably, the proposed coordinated dropout method produces a strong correlation between heldout loss and training performance. Accordingly, the validation loss can be used to select the proper hyperparameters. Finally, the authors show the power of using coordinated dropout when fitting to LFADS on real data with hyperparameter search. The models trained with coordinated dropout show improved decoding performance, especially on smaller dimensional datasets. The improved performance of the model on smaller dimensional datasets is notable. While the motivation for the new training methods, methods, and examples are clear, the paper could be improved with some additional details. - Can the authors state the training objective with the hyperparameters as an equation in the manuscript? As written, I think I can piece together the training objective - the ELBO with penalties on the two different KL terms, one for the inferred inputs and one for initial condition, plus penalties on the weights on the controller and generator RNNs. However, including the equation is critical to make this precise and clear to the general reader. - Can the authors also state the validation loss and how it is computed? - Additionally, it would be useful to fully describe the LFADS model, at least in an appendix. This will increase clarity to readers unfamiliar with LFADS. - Is there a prior on the input p(u)? In Pandarinath, Nature Methods 2018 an autoregressive prior was applied on p(u). Does the model still exhibit pathological overfitting with that autoregressive prior included? Comments -What settings of the random hyperparameters provided “good” fits? It would be interesting to include a discussion of this, including how this might vary across dataset size. -Is full-split coordinated dropout necessary, or could you also split the data into input only, shared, and output only splits? --------------------------- After reading the other reviewers and author feedback and conferring with reviewers, I have increased my score by a point. The experiments and details in the author feedback have improved the clarity and significance of the submission. I encourage the authors to include the comparison with dAEs and model details in the final version of the manuscript, and to follow Reviewer 1's guidance about whether or not to include Partial CD in the final version.