Detection and Aptness: A study in metaphor detection and aptness assessment through neural networks and distributional semantic spaces

Sammanfattning: Metaphor is one of the most prominent, and most studied, figures of speech. While it is considered an element of great interest in several branches of linguistics, such as semantics, pragmatics and stylistics, its automatic processing remains an open challenge. First of all, the semantic complexity of the concept of metaphor itself creates a range of theoretical complications. Secondly, the practical lack of large scale resources for machine learning approaches forces researchers to work under conditions of data scarcity. This compilation thesis provides a set of experiments to (i) automatically detect metaphors and (ii) assess a metaphor's aptness with respect to a given literal equivalent. The first task has already been tackled by a number of studies. We approach it as a way to assess the potentialities and limitations of our approach, before dealing with the second task. For metaphor detection we were able to use existing resources, while we created our own dataset to explore metaphor aptness assessment, which constitutes the most innovative part of this research. In all of the studies presented here, I have used a combination of word embeddings and neural networks. This combination appears particularly effective since pre-trained word embeddings can provide the networks with information necessary to deal with metaphors under conditions of data scarcity. To deal with metaphor aptness assessment, we frame the problem as a case of paraphrase identification. Given a sentence containing a metaphor, the task is to find the best literal paraphrase from a set of candidates. We build a dataset designed for this task, that allows a gradient scoring of various paraphrases with respect to a reference sentence, so that paraphrases are ordered according to their degree of aptness. Therefore, we can use it for both binary classification and ordering tasks. This dataset is annotated through crowd sourcing by an average of 20 annotators for each pair. We then design a deep neural network to be trained on this dataset. We show that its architecture is able achieve encouraging levels of performance, despite the serious limitations of data scarcity in which it is applied. In the final experiment of this compilation, more context is added to a sub-section of the dataset in order to study the effect of extended context on metaphor aptness rating. We show that extended context changes human perception of metaphor aptness and that this effect is reproduced by our neural classifier. The conclusion of the last study is that extended context compresses aptness scores towards the center of the scale, raising low ratings and decreasing high ratings given to paraphrase candidates outside of any context.