Learning Graphical Models
Reviews: Deep Generative Markov State Models
This paper proposes a novel learning frame-work for Markov State Models of real valued vectors. This model can handle metastable processes i.e. processes that evolve locally in short time-scales but switch between a few clusters after very long periods. The proposed framework is based on a nice idea to decompose the transition from x1 to x2 to the probability that x1 belongs to a long-lived state and a distribution of x2 given the state. The first conditional probability is modeled using a decoding deep network whereas the second one can be represented either using a network that assigns weights to x2 or using a generative neural network. This is a very interesting manuscript.
Reviews: rho-POMDPs have Lipschitz-Continuous epsilon-Optimal Value Functions
The paper addresses the problem of rho-POMDPs non-convex reward functions, proving that indeed under some cases they, and their resulting value functions, are Lipschitz-continuous (LC) for finite horizons. The paper also proposes and uses a more general vector form of LC, too. This result allows value function approximations of the optimal V * to be used, as well as upper and lower bounds (U and L) on value as in HSVI, and a wide array of new algorithms to be developed. This is analogous to the PWLC result for standard POMDPs, as LC is more general, allowing for similar contraction operators with Banach's fixed point theorem as in (PO)MDPs, and finite horizon approximations of the infinite horizon objective criteria. Once the paper establishes the main result, it discusses approximations of U and L using min or max, respectively, over sets of cones.
Reviews: Analyzing Hidden Representations in End-to-End Automatic Speech Recognition Systems
The authors conduct an analysis of CTC trained acoustic models to determine how information related to phonetic categories is preserved in CTC-based models which directly output graphemes. The work follows a long line of research that has analyzed neural network representations to determine how they model phonemic representations, although to the best of my knowledge this has not been done previously for CTC-based end-to-end architectures. The results and analysis presented by the authors is interesting, although there are some concerns I have with the conclusions that the authors draw that I would like to clarify these points. Please see my detailed comments below. In the paper, the authors conclude that (Line 159--164) "... after the 5th recurrent layer accuracy goes down again. One possible explanation to this may be that higher layers in the model are more sensitive to long distance information that is needed for the speech recognition task, whereas the local information which is needed for classifying phones is better captured in lower layers."
Reviews: On Learning Markov Chains
Summary: The paper's goal is to study the minimax rates for learning problems on Markovian data. The author(s) consider an interesting setting where the sequence of data observed follow a Markovian dependency pattern. They consider discrete state Markov chains with a state space [k] and study the minimax error rates for the following two tasks: Prediction: Given a trajectory X_1 - X_2 … - X_n from an unknown chain M, predict the probability distribution of the next state X_n 1, i.e., P(. X_1…n)) ] where the expectation is over the trajectory X_1…X_n. The loss function the paper focuses on is KL-divergence and presents a conjecture for how the L_1 loss should scale with respect to k and n.
Reviews: Multiscale Semi-Markov Dynamics for Intracortical Brain-Computer Interfaces
The paper describes a novel brain-computer-interface algorithm for controlling movement of a cursor to random locations on a screen using neuronal activity (power in the "spike-spectrum" of intra-cortically implanted selected electrodes). The algorithm uses a dynamic Bayesian network model that encodes possible target location (from a set of possible positions on a 40x40 grid, layed out on the screed). Target changes can only occur once a countdown timer reaches zero (time intervals are drawn at random) at which time the target has a chance of switching location. Observations (power in spike spectrum) are assumed to be drawn from a multi modal distribution (mixture of von Mises functions) as multiple neurons may affect the power recording on a single electrode and are dependent on the current movement direction. The position is simply the integration over time of the movement direction variable (with a bit of decay).
Reviews: Causal Effect Inference with Deep Latent-Variable Models
However, five other causal diagrams can also be considered with confounder proxies (see Figure 1 in [Miao et al., 2016]). It would be interesting to check whether the proposed method can be applied to all these causal diagrams or there are some limitations. For instance, why covariates of x_i are independent given z_i? Is it restricting the expressive power of this method? For instance, in Experimental Results section, it is assumed that z is a 20-dimensional variable.
Reviews: Information Theoretic Properties of Markov Random Fields, and their Algorithmic Applications
This paper is concerned with learning Markov random fields (MRF). It is a theoretical paper, which is ultimately focused with proving a particular statement: given a variable X in an MRF, and given some of its Markov blanket variables B, there exists another variable Y that is conditionally dependent on X given the subset of B. In general this statement is not true; so the goal here is to identify some conditions where this is true. Most of this paper is centered around this, from which the ability to learn an MRF follows. The paper is mostly technical; my main complaint is that I do not think it is very intuitive. It appears that central to the results is the assumption on non-degeneracy, which I believe should be explained in higher level terms.
Reviews: Deep Homogeneous Mixture Models: Representation, Separation, and Approximation
The paper discusses connections between multiple density models within the unifying framework of homogeneous mixture models: tensorial mixtures models [1], hidden Markov models, latent tree models and sum-product networks [2] are discussed. The authors argue that there is a hierarchy among these models by showing that a model lower in the hierarchy can be cast into a model higher in the hierarchy using linear size transformations. Furthermore, the paper gives new theoretical insights in depth efficiency in these models, by establishing a connection between properties of the represented mixture coefficient tensor (e.g. Finally, the paper gives positive and somewhat surprising approximation results using [3]. Strengths: connections between various models, which so far were somewhat folk wisdom, are illustrated a unifying tensor mixture framework.
Reviews: Lifted Weighted Mini-Bucket
The paper proposes a lifted version of the weighted mini-bucket inference algorithm. Weighted mini-bucket is a variant of Variable Elimination that can trade off computational cost (e.g., achieving runtime sub-exponential in the treewidth) for accuracy. It essentially uses Holder inequality and variational approximations to represent "messages" that do not fit in the available memory budget. The main idea is to extend the approach to relational models (e.g., Markov Logic Networks) to take advantage of the additional structure, specifically, the fact that ground factors are produced from first-order templates and therefore share significant structure. The work appears to be solid, but it is difficult to parse.