"Grounded" language learning is the process of learning the semantics of natural language with respect to relevant perceptual inputs. Toward this goal, computational systems are trained with data in the form of natural language sentences paired with relevant but ambiguous perceptual contexts. With such ambiguous supervision, it is required to resolve the ambiguity between a natural language (NL) sentence and a corresponding set of possible logical meaning representations (MR). My research focuses on devising effective models for simultaneously disambiguating such supervision and learning the underlying semantics of language to map NL sentences into proper logical forms. Specifically, I will present two probabilistic generative models for learning such correspondences. The models are applied to two publicly available datasets in two different domains, sportscasting and navigation, and compared with previous work on the same data.
I will first present a probabilistic generative model that learns the mappings from NL sentences into logical forms where the true meaning of each NL sentence is one of a handful of candidate logical MRs. It simultaneously disambiguates the meaning of each sentence in the training data and learns to probabilistically map a NL sentence to its MR form depicted in a single tree structure. Evaluations are performed on the RoboCup sportscasting corpous, which show that it outperforms previous methods.
Next, I present a PCFG induction model for grounded language learning that extends the model of Borschinger, Jones, and Johnson (2011) by utilizing a semantic lexicon. Borschinger et al.'s approach works well when there is limited ambiguity such as in the sportscasting task, but it does not scale well to highly ambiguous situations when there are large sets of potential meaning possibilities for each sentence, such as in the navigation instruction following task studied by Chen and Mooney (2011). Our model overcomes such limitations by employing a semantic lexicon as the basic building block for PCFG rule generation. Our model also allows for novel combination of MR outputs when parsing novel test sentences.
For future work, I propose to extend our PCFG induction model in several ways: improving the lexicon learning algorithm, discriminative re-ranking of top-k parses, and integrating the meaning representation language (MRL) grammar for extra structural information. The longer-term agenda includes applying our approach to summarized machine translation, using real perception data such as robot sensorimeter and images/videos, and joint learning with other natural language processing tasks.
Technical Report, PhD proposal, Department of Computer Science, The University of Texas at Austin, June 2012.