Modeling Irregular Time Series with Continuous Recurrent Units

Recurrent neural networks (RNNs) like long short-term memory networks (LSTMs) and gated recurrent units (GRUs) are a popular choice for modeling sequential data. Their gating mechanism permits weighting previous history encoded in a hidden state with new information from incoming observations. In many applications, such as medical records, observations times are irregular and carry important information. However, LSTMs and GRUs assume constant time intervals between observations. To address this challenge, we propose continuous recurrent units (CRUs) -a neural architecture that can naturally handle irregular time intervals between observations. The gating mechanism of the CRU employs the continuous formulation of a Kalman filter and alternates between (1) continuous latent state propagation according to a linear stochastic differential equation (SDE) and (2) latent state updates whenever a new observation comes in. In an empirical study, we show that the CRU can better interpolate irregular time series than neural ordinary differential equation (neural ODE)-based models. We also show that our model can infer dynamics from im-ages and that the Kalman gain efficiently singles out candidates for valuable state updates from noisy observations.