Temporal word embeddings in the study of metaphor change over time and across genres: a proof-of-concept study on English
Authors: Veronica Mangiaterra, Chiara Barattieri di San Pietro, Valentina Bambini (IUSS, 2024)
Topic: Lexical and Semantic Resources and Analysis

Abstract
Temporal word embeddings have been successfully employed in semantic change research to identify and trace shifts in the meaning of words. In a previous work, we developed an approach to study the diachrony of complex expressions, namely literary metaphors. Capitalizing on the evidence that measures of semantic similarity between the two terms of a metaphor approximate human judgments of the difficulty of the expression, we used time-locked measures of similarity to reconstruct the evolution of processing costs of literary metaphors over the past two centuries. In this work, we extend this approach previously used on Italian literary metaphors and we present a proof-of-concept study testing its crosslinguistic applicability on a set of 19th-century English literary metaphors. Our results show that the processing costs of metaphors changed as a function of textual genre but not of epoch: cosine similarity between the two terms of literary metaphors is higher in literary compared to nonliterary texts, and this difference is stable across epochs. Furthermore, we show that, depending on the metaphor structure, the difference between genres is affected by word-level variables, such as the frequency of the metaphor’s vehicle and the stability of the meaning of both topic and vehicle. In a broader perspective, general considerations can be drawn about the history of literary and nonliterary English language and the semantic change of words.

Introduction
Does the metaphor “The wind is a wrestler” convey the same feeling today, as it did in the 1888 when Gerard Manley Hopkins used it in the poem “That nature is a Heraclitean Fire and of the comfort of the Resurrection” AUTHOR? The answer to this question is not trivial: human languages evolve constantly, alongside with the society in which they are used, so much so that the concepts associated with each word, as well as their semantic as- sociations with other words, have changed to different degrees AUTHOR. Studies on lexical semantic change have a long tradition AUTHOR but, with the increasing availability of historical language data and the development of new digital tools, they radically opened up to new approaches coming from computational linguistics and distributional semantics AUTHOR. In the diachronic declination of the Distributional Hypothesis AUTHOR, it is said that changes in the contexts in which a word occurs over time may reveal a change in meaning AUTHOR. Operatively, this means that by training vector space models on historical text corpora from different epochs, it is possible to create time-locked representations of words: if the meaning of a word changed over time, its vectorial representation at t_{1} will be different from its vectorial representation at time t_{2}; conversely, if the two vectors of the same word at t_{1} and t_{2} are in close proximity, the meaning of the word has remained stable. Comparing words vectors diachronically, however, is not effortless and requires the temporal vector space models to be aligned. Alignment is a crucial step in diachronic distributional semantics and it has been tackled by different approaches AUTHOR. Previous studies employing temporal embeddings have found that more frequent words change slower than less frequent words, and that polysemous words change faster than monosemous words AUTHOR, while synonyms tend to change meaning comparably AUTHOR. However, temporal word embeddings have been mostly applied to the study of the semantic change of single words and only marginally to complex linguistic expressions leaving the field with a knowledge gap on the evolution of meaning of a widespread linguistic and textual phenomenon such as, for instance, metaphors. Within the theoretical framework of Relevance Theory AUTHOR, metaphors are non-literal uses of language involving a conceptual adjustment described as context-driven broadening of lexically denoted meaning of words. In terms of linguistic structure, metaphors normally involve two terms, the topic and the vehicle: for example, in the metaphor ‘Sally is a chameleon’, the topic Sally is described by the broadened vehicle chameleon, to indicate a person who changes attitude/behavior to fit their surroundings. While metaphors are broadly used in everyday communication, they are certainly a distinctive feature of literary texts, as long evidenced in stylistics AUTHOR. Past studies on literary metaphors, however, report mixed results. The rating study by Katz et al. AUTHOR found no difference between literary and everyday metaphors, while other studies showed that the former type is less familiar and more open-ended than the latter AUTHOR, but literary metaphors are rated as less difficult and more familiar when presented together with their original context AUTHOR. Moreover, the processing of literay metaphors seems to be particularly effortful, given the multitude of possible meanings they evoke AUTHOR. Therefore, open questions remain regarding how literary metaphors are processed. It must be also underlined that the literary metaphors used in previous studies were written tens or hundreds of years ago. Yet, the effect of this diachronic dimension on their processing costs, as well as its interplay with textual genre in which metaphors are embedded, remains an open question. In addition to its diachronic application, the use of vector space models can help characterize metaphors thanks to the ability of these models to approximate human performance in psycholinguistic tasks. Measures derived from vector space models were shown to be able to approximate how humans process word meaning AUTHOR and, more specifically to correlate with how humans perceive metaphorical expressions in terms of metaphoricity, difficulty, and other psycholinguistic dimensions AUTHOR. In particular, semantic similarity, operationalized in vector space models as cosine similarity (CS) between topic and vehicle, has long been considered relevant for metaphor studies AUTHOR and, more recently, for automatic metaphor identification AUTHOR. In a previous study on Italian AUTHOR, we developed a novel method, employing the Temporal Word Embeddings with a Compass (TWEC) model AUTHOR as training procedure, to capture the temporal dynamics of literary metaphors. This method combines the computational models’ abilities to approximate human judgments and their diachronic applications, allowing to track the diachronic evolution of how literary metaphors are perceived by readers over the course of 200 years. In the present proof-of-concept study, we apply this approach to English, to test its crosslinguistic applicability and whether it can provide language-specific insights into the evolution of metaphors. We take the similarity between the topic and vehicle of a metaphor as a proxy for its difficulty and we analyze how it varies across time and textual genres. We also consider the role of word frequency (WF) and vector coherence (VC), two widely used measures in the study of semantic change AUTHOR, as well as semantic neighborhood density (SND) in shaping the difficulty of the expression. WF and VC were considered to assess the effect of the semantic change of the single word on the evolution of whole metaphor understanding, while SND was considered to analyze the impact of a measure known to synchronically impacts metaphor understanding AUTHOR on its diachronic unfolding.

Conclusion
In this proof-of-concept study, we characterized the temporal dynamics of a set of English literary metaphors to understand whether their processing costs changed over time. We also explored if this change was affected by the genre of the texts, as well as by the semantic properties of the constituting elements of the metaphors (topic and vehicle). By leveraging on the diachronic applications of distributional semantics and extending a method already applied to the study of Italian literary metaphors AUTHOR, we created a series of time-locked semantic representations of 139 English metaphors, from which we derived a measure of the cosine similarity between their terms (CS), taken as a proxy of their difficulty, together with semantic neighborhood density (SND), stability over time (VC), and, from four diachronic corpora, frequency (WF) of their topics and vehicles. Results showed no effect of epoch for either ‘A is B’ or ‘A of B’ literary metaphors. Thus, no noticeable change in CS over time was revealed, suggesting that these metaphors come with similar processing costs for contemporary readers and for readers of the epoch in which the metaphors were created. The absence of an effect of epoch can be better understood by considering the historical evolution of the English language, and specifically its early standardization. As stated by Wyld AUTHOR, literary writing as early as the 18th century was considered `English of our own age in all its essentials’. In line with this consideration, our results point to the stability of the main stylistic features of the English language in the last two centuries, including those related to metaphors. While literary metaphors are not processed differently based on the epoch, the influence of textual genre is noticeable. This factor emerged both as a main effect and in different interaction patterns with single-word variables, varying according to the type of metaphor. For `A of B’ metaphors, results revealed that the difficulty of these metaphors changed as a function of the genre. In particular, they are perceived as less difficult when found in literary contexts, compared to when encountered in nonliterary texts. Hence, the difficulty of these metaphors is sensitive to the style of the text in which metaphors are found: when read in a text that has a literary style and aesthetic intent, the metaphor is less striking than the same metaphor in a nonliterary text. Moreover, we found a strong effect of the stability of the meaning of the vehicle in interaction with epoch and genre. This suggests that `A of B’ metaphors with more unstable vehicles are perceived as less difficult than `A of B’ metaphors with vehicles whose meanings remained stable over time. We interpreted this result in light of Traugott's AUTHOR theory of metaphorization, according to which the metaphorical use of a word can become one of its stable meanings. In the context of the present study, words that changed the most could have done so by incorporating meanings derived from their metaphorical uses. As a result, when these unstable and broadened vehicles are used, metaphors appear less difficult. The reader does not need to broaden the concept expressed by the vehicle to interpret the metaphor, because the metaphorical nuances have entered the standard meaning of the word. From a qualitative observation of the data, we can notice, for instance, that a metaphor such as “Wave of horror”, where the vehicle wave incorporated the meaning of ‘sudden increase in a particular phenomenon’, is perceived as less metaphorical than “Clouds of doubt”, whose vehicle clouds has maintained its original meaning. For ‘A is B’ metaphors, the statistical model highlighted an effect of the frequency of the vehicle in interaction with epoch and genre. In nonliterary texts, the perceived difficulty of `A is B’ metaphors varied as a function of the word frequency (WF) of their vehicle, resulting in opposite patterns in the past and present. In the 19th century, metaphors with less frequent vehicles were perceived as more metaphorical. Conversely, in the 21st century, the less frequent the vehicle, the less metaphorical the metaphor. This 19th-century pattern aligns with prior studies \cite{doi:10.1080/10926488.2018.1434944} showing that metaphors with less frequent vehicles communicate novel information about the topic, thereby enhancing their perceived metaphorical nature. For instance, Hopkins’ metaphor “The wind is a wrestler” featured the vehicle wrestler, a low-frequency word in the 19th century, which conveyed novelty about the topic wind, rendering the metaphor more metaphorical and conceptually challenging. However, this same metaphor is perceived differently today as wrestler has become more frequent, reducing its novelty and metaphoricity for 21st-century readers. Overall, our results suggest that in English, metaphor processing costs are not influenced by the temporal distance between their creation (in the 19th century) and their processing by contemporary readers. Instead, the primary modulator of metaphor processing costs appears to be the textual genre in which they appear. This modulation varies depending on the syntactic structure of metaphors and interacts with single-word measures. Distinct patterns emerged for A of B' and A is B' structures in determining metaphor difficulty. For A of B' metaphors, the main effects of genre and vector coherence, interacting with epoch and genre, were observed. For A is B' metaphors, diachronic variations in processing costs were linked to the interaction of word frequency with epoch and genre. While these differences might reflect genuine syntactic effects on metaphorical predication \cite{bambini2013differentiating, carston2023metaphor, tonini2023relationship}, the imbalance in the size of the two metaphor sets might obscure some effects in the less represented `A is B' structure. Further studies are necessary to explore diachronic changes in processing across structural differences more comprehensively. In conclusion, this proof-of-concept study adapted a method using temporal word embeddings from Italian to English to investigate metaphor evolution. This approach demonstrated that English literary metaphor processing costs remain stable over time (unlike Italian) but are dynamically modulated by stylistic features and single-word measures. The method's sensitivity to language-specific characteristics underscores its crosslinguistic applicability.