Panta rei: Tracking Semantic Change with Distributional Semantics in Ancient Greek
Authors: Martina A. Rodda, Marco S.G. Senaldi, Alessandro Lenci (Scuola Normale Superiore, Università di Pisa, 2016)
Topic: Distributional Semantics

Abstract
We present a method to explore semantic change as a function of variation in distributional semantic spaces. In this paper we apply this approach to automatically identify the areas of semantic change in the lexicon of Ancient Greek between the pre-Christian and Christian era. Distributional Semantic Models are used to identify meaningful clusters and patterns of semantic shift within a set of target words, defined through a purely data-driven approach. The results emphasize the role played by the diffusion of Christianity and by technical languages in determining semantic change in Ancient Greek and show the potentialities of distributional models in diachronic semantics.

Introduction
Distributional Semantics is grounded on the assumption that the meaning of a word can be described as a function of its collocates in a corpus. This suggests that diachronic meaning shifts can be traced through changes in the distribution of these collocates over time AUTHOR. While some studies focused on testing the explanatory power of this method over frequency- and syntax-based approaches AUTHOR, more advanced contributions to the field explored how distributional models can be used to test competing hypotheses about semantic change AUTHOR, or to investigate the productivity of constructions in diachrony AUTHOR. The results attest the explanatory power of distributional methods in modeling diachronic shifts in meaning. In this paper, we propose a method to identify semantic change through the Representational Similarity Analysis AUTHOR of distributional vector spaces built from diachronic corpora. RSA is a method extensively used in neuroscience to test cognitive and computational models by comparing the geometry of their representation spaces AUTHOR. Stimuli are represented with a representational dissimilarity matrix that contains a measure of the dissimilarity relations of the stimuli with each other. Different matrices are compared to evaluate the correspondence of the representational spaces built from different sources (e.g., behavioral and neuroimaging data). We argue that this method can be applied to compare distributional representations of the lexicon at different temporal stages. The hypothesis is that the elements in the lexical spaces showing larger geometrical variations in time correspond to the lexical areas that have undergone major semantic changes. To the best of our knowledge, this is the first time RSA is used in diachronic distributional semantics. Here we present a case study that applies RSA to track patterns of semantic change within the lexicon of Ancient Greek. We focus on the first few centuries AD, when the rise of Christianity caused a deep and widespread cultural shift within the Hellenic world. We predict that this shift will be reflected in the Greek lexicon of the time. In addition to past studies AUTHOR, we apply a bottom-up approach to the detection of semantic change, with no prior definition of a list of lemmas to be analyzed. The goal is to develop a quantitative “discovery procedure” to detect lexical semantic changes. From a methodological standpoint, this study aims to show how Distributional Semantics can be applied fruitfully to such a small and literary corpus as the collection of Ancient Greek texts. The results will also highlight the ways in which Distributional Semantics can complement the intuition of the researcher in analyzing semantic change in Ancient Greek, providing a useful tool for future studies in Classics.

Conclusion
This paper shows how Distributional Semantics can be used as an exploratory tool to detect semantic change. In this case study on Ancient Greek, the proposed method based on distributional RSA not only confirms the hypothesis that the diffusion of Christianity was a crucial cause of semantic change in the Greek lexicon, but also allows for the identification of unexpected patterns of evolution, such as the apparent specialization in the usage of technical terms. This last phenomenon could also be influenced by the fact that the AD-corpus is richer in philosophical and technical treatises; however, a documented change in the proportion of different possible usages of a word is in itself a very informative result, especially in a field such as Classics, where the analysis of (literary) texts is paramount. Further research should undoubtedly highlight the effect of corpus composition. A focus on shorter periods of time might be of interest, since, for instance, the rise of technical prose writing is a characteristic of the Hellenistic Age AUTHOR. From a methodological standpoint, the fact that the results obtained from such a small corpus of purely literary texts are both meaningful and informative is of great relevance. Furthermore, the choice to adopt a data-driven approach proved fruitful, in that it brought to light directions of change that were not expected a priori. For traditional research in Classics, a computational approach to the lexicon of Ancient Greek is compelling because it provides new information about a language for which the judgments of native speakers are unavailable AUTHOR. The results of this study show how Distributional Semantics can complement the assertions of the philologist, as well as help discover patterns of lexical change that would otherwise be impossible to grasp beyond an intuitive level.