This paper discusses a probabilistic model-based approach to clus(cid:173) tering sequences, using hidden Markov models (HMMs) . The prob(cid:173) lem can be framed as a generalization of the standard mixture model approach to clustering in feature space. Two primary issues are addressed. First, a novel parameter initialization procedure is proposed, and second, the more difficult problem of determining the number of clusters K, from the data, is investigated. Experi(cid:173) mental results indicate that the proposed techniques are useful for revealing hidden cluster structure in data sets of sequences.

It is shown that conventional computers can be exponentiallx faster than planar Hopfield networks: although there are planar Hopfield networks that take exponential time to converge, a stable state of an arbitrary planar Hopfield network can be found by a conventional computer in polynomial time.

A central theme of computational vision research has been the re(cid:173) alization that reliable estimation of local scene properties requires propagating measurements across the image. Many authors have therefore suggested solving vision problems using architectures of locally connected units updating their activity in parallel. Unfor(cid:173) tunately, the convergence of traditional relaxation methods on such architectures has proven to be excruciatingly slow and in general they do not guarantee that the stable point will be a global mini(cid:173) mum. In this paper we show that an architecture in which Bayesian Be(cid:173) liefs about image properties are propagated between neighboring units yields convergence times which are several orders of magni(cid:173) tude faster than traditional methods and avoids local minima. In particular our architecture is non-iterative in the sense of Marr [5]: at every time step, the local estimates at a given location are op(cid:173) timal given the information which has already been propagated to that location.

In this paper we propose a method for learning Bayesian belief networks from data. The method uses artificial neural networks as probability estimators, thus avoiding the need for making prior assumptions on the nature of the probability distributions govern(cid:173) ing the relationships among the participating variables. This new method has the potential for being applied to domains containing both discrete and continuous variables arbitrarily distributed. We compare the learning performance of this new method with the performance of the method proposed by Cooper and Herskovits in [7]. The experimental results show that, although the learning scheme based on the use of ANN estimators is slower, the learning accuracy of the two methods is comparable.

Real-valued random hidden variables can be useful for modelling latent structure that explains correlations among observed vari(cid:173) ables. I propose a simple unit that adds zero-mean Gaussian noise to its input before passing it through a sigmoidal squashing func(cid:173) tion. Such units can produce a variety of useful behaviors, ranging from deterministic to binary stochastic to continuous stochastic. I show how "slice sampling" can be used for inference and learning in top-down networks of these units and demonstrate learning on two simple problems.

We describe the implementation of a hidden Markov model state decoding system, a component for a wordspotting speech recogni(cid:173) tion system. The key specification for this state decoder design is microwatt power dissipation; this requirement led to a continuous(cid:173) time, analog circuit implementation. We characterize the operation of a 10-word (81 state) state decoder test chip.

Hidden Markov models (HMMs) for automatic speech recognition rely on high dimensional feature vectors to summarize the short(cid:173) time properties of speech. Correlations between features can arise when the speech signal is non-stationary or corrupted by noise. We investigate how to model these correlations using factor analysis, a statistical method for dimensionality reduction . Factor analysis uses a small number of parameters to model the covariance struc(cid:173) ture of high dimensional data. These parameters are estimated by an Expectation-Maximization (EM) algorithm that can be em(cid:173) bedded in the training procedures for HMMs.

A new policy iteration algorithm for partially observable Markov decision processes is presented that is simpler and more efficient than an earlier policy iteration algorithm of Sondik (1971,1978). The key simplification is representation of a policy as a finite-state controller. This representation makes policy evaluation straightforward. The pa(cid:173) per's contribution is to show that the dynamic-programming update used in the policy improvement step can be interpreted as the trans(cid:173) formation of a finite-state controller into an improved finite-state con(cid:173) troller. The new algorithm consistently outperforms value iteration as an approach to solving infinite-horizon problems.

Exact inference in densely connected Bayesian networks is computation(cid:173) ally intractable, and so there is considerable interest in developing effec(cid:173) tive approximation schemes. One approach which has been adopted is to bound the log likelihood using a mean-field approximating distribution. While this leads to a tractable algorithm, the mean field distribution is as(cid:173) sumed to be factorial and hence unimodal. In this paper we demonstrate the feasibility of using a richer class of approximating distributions based on mixtures of mean field distributions. We derive an efficient algorithm for updating the mixture parameters and apply it to the problem of learn(cid:173) ing in sigmoid belief networks.

This paper reports about an application of Bayes' inferred neu(cid:173) ral network classifiers in the field of automatic sleep staging. The reason for using Bayesian learning for this task is two-fold. First, Bayesian inference is known to embody regularization automati(cid:173) cally. Second, a side effect of Bayesian learning leads to larger variance of network outputs in regions without training data. This results in well known moderation effects, which can be used to detect outliers.