Metaphor generation attempts to replicate human creativity with language, which is an attractive but challengeable text generation task. Previous efforts mainly focus on template-based or rule-based methods and result in a lack of linguistic subtlety. In order to create novel metaphors, we propose a neural approach to metaphor generation and explore the shared inferential structure of a metaphorical usage and a literal usage of a verb. Our approach does not require any manually annotated metaphors for training. We extract the metaphorically used verbs with their metaphorical senses in an unsupervised way and train a neural language model from wiki corpus. Then we generate metaphors conveying the assigned metaphorical senses with an improved decoding algorithm. Automatic metrics and human evaluations demonstrate that our approach can generate metaphors with good readability and creativity. Metaphor is a kind of language filled with vitality and elasticity. It employs words in a way that deviates from their normal meaning to represent another concept (Li and Sporleder, 2010). Using metaphor allows us to express not just information but also real feelings and complex attitudes. There is a clear need in computational metaphor generation whose insightful results can be used in many applications from entertainment to educati-on (Veale, 2016). Besides, a unified metaphor annotation procedure and creation of a large publicly available metaphor corpus are in great demands. Such resources make it possible to interpretate the identified metaphorical expressions and enhance the performance on other Natural Language Processing (NLP) applications (Shutova, 2010). Although metaphor has a long history of academic studies in both philosophy and linguistics Figure 1: The Metaphor Generation Process (Genesereth, 1980; G. and M., 1985), it still remains a tough problem for the NLP community. As the metaphor is hardly to be well-defined and modeled, little work focuses on the metaphor generation. Most of the previous efforts rely on hand-coded knowledge (Martin, 1990; Feldman and Narayanan, 2004; Agerri et al., 2007) , which heavily constrains the diversity of generated metaphors. The end-to-end approach presented to sequence learning has been proved effective on the generation tasks like machine translation (Sutskever et al., 2014), abstractive summarization (Tan et al., 2017), product review generation (Zang and Wan, 2017) and multi-label classification (Yang et al., 2018). The approach is able to train a language model which can generate fluent and creative sentences with sufficient corpus. Unfortunately, in spite of the industrious exploration of the metaphor corpus, the annotated data available now is still far from training a good language model. To the best of our knowledge, there has been no work combining metaphor generation with the end-to-end approach. In this paper, we propose a neural approach for metaphor generation trained with Wiki corpus rather than the limited annotated metaphor corpus, which assures the quality of the language model. The approach is shown in Figure 1. Relevant statistics demonstrate that the most frequent type of metaphor is expressed by a verb (Martin, 2006; Steen, 2010). In this paper, we focus on generating verb-oriented metaphors. We use an unsupervised method to extract the metaphorically used verbs in Wiki corpus. A metaphorical pair consists of a target verb (e.g. devoured) and a fit word (e.g. en-joyed). And it is used to model the metaphorical usage of the target verb. According to Narayanan (1997), a metaphorical usage and a literal usage share inferential structure. We follow this intuition to find an intersection between the metaphorical usage and the literal usage of a word. For example, in she devoured (enjoyed) his novels, the literal sense of enjoyed represents the contextual sense of devoured in such contexts. But the similarity between enjoyed and devoured is low, which means the target verb devoured is merely used in such sense and can be seen as a metaphorically used verb. For metaphor generation, we first propose a POS constrained language model to generate a sentence containing a given verb and use an elaborately designed algorithm to consider its fit word simultaneously while decoding. For a profound exploration, we introduce automatic metrics as well as manual ways to evaluate the generation results. Experimental results demonstrate that our approach is capable of generating fluent and seemly metaphors. All the generated metaphors are novel and do not exist in the corpus. To summarise, the contributions of our work are as follows 1 :  As far as we know, our work is the first en-deavor to adopt the end-to-end framework on metaphor generation. Besides, the proposed method does not require any manually labeled metaphor corpus. We automatically extract the verbs and their fit words in the corpus in an unsupervised way and use them (e.g. devoured, enjoyed) to model the metaphorical senses of the verbs for the generation process. We further explore the semantic shift of a verb by changing the adjustable factors. Both automatic metrics and human evaluation results demonstrate the efficacy of our mo-1 https://github.com/ArleneYuZhiwei/Metaphor-Generation del. Metaphor is highly frequent in language and its computational processing is indispensable for real-world NLP applications addressing semantic tasks. Automatic processing of metaphor can be clearly divided into two subtasks: metaphor identification and metaphor interpretation (Shutova, 2010), little research has been devoted to the metaphor generation. In this subsection, we briefly review some prior work on metaphor generation. Jones (1992) aims to generate a specific class of metaphors: Transparently-Motivated Metaphor, which is based on universal groundings that are often linked to bodily experience. Abe et al. (2006), Terai and Nakagawa (2010) generate metaphors in the form of A (target) like B (vehicle). They firstly compute the probabilistic relationship between concepts and words with a statistical analysis of language data and then select candidates to fill in the template. From a given mapping between the concepts of two domains, Hervas et al. (2007) present an approach to the application of metaphors for referring to concepts in an automatically generated text. Mason (2004) obtains domain-specific selectional preferences of verbs from large corpora and maps their common nominal arguments in other domains. The corresponding metaphorical mappings are achieved by such systematic variations and can generate simple conceptual metaphors in the form of: A is (a) B. Ov-chinnikova et al. (2014) also rely on characteristic predicate but use general propositions instead of the verb and adjective phrases. As some me-taphors' target domain is lexically divorced from the source, Lederer (2016) identifies constellations of source-domain triggers in limited source domains. To make the metaphor generators more comprehensible and forceful, Veale (2016) presents a knowledge-base to generate XYZ metaphors such as Bruce Wayne is the Donald Trump of Gotham City. Previous methods make groundbreaking explorations on metaphor generation. However, these approaches mainly focus on modeling the phrase-level metaphor expressions and the generation process depends on the templates, which causes the lack of linguistic subtlety to some extent and 2 Based on two-tailed paired t-test with p < 0.05. they are not able to build a large publicly available metaphor corpus for further study. Our approach focuses on generating sentence-level metaphor in a template-free way. Metaphors are ubiquitous in the normal corpus but lack of annotation. First, we extract the metaphorical pair which consists of a target word (e.g. devoured) and its fit word (e.g. enjoyed) automatically. Then we adopt an end-to-end neural framework to train a POS constrained language model which can generate a sentence containing the assigned verb. For metaphor inference, we apply an adjustable joint beam search algorithm to the decoding phase. In this way, the target verb is metaphorically used in the generated sentence. The proposed model is named Adjustable Joint Model and is shown in Figure 2. 3.1 Automatic Extraction of Metaphor Pairs Our automatic extraction method is based on the hypotheses as follows: H1 . A metaphorical word is employed in the sentence to represent another concept and deviates from its normal meaning (Wilks, 1978; Li and Sporleder, 2010; Mao et al., 2018). H2 . The metaphorical senses of words occur with relatively lower frequency in the corpus than their literal senses do (Cameron, 2003; Martin, 2006; Mao et al., 2018). Algorithm 1 Automatic Extraction Require: S : the corpus, a set of sentences containing the target words Require: T : the set of target words in S Require: M : a trained CBOW model. v ia denotes the input vector of word a . v oa denotes the output vector of word a . v icontext denotes the average input vectors of the words in the context . return P As metaphors begin their lives with marked rhetorical effects, whose comprehension requires a special imaginative leap (Nunberg, 1987), it is intuitive to assume that a metaphorical word can be distinguished from the literal one in the corpus with the violation of semantic constraints within a context. It has been proved that the dissimilarity between neural embeddings of the two words in a phrase is indicative of identifying the metaphoricity of the phrase (Shutova et al., 2016; Rei et al., 2017). Thus we find the semantic violation in the corpus based on word embeddings. The word embeddings are obtained by using Continuous Bag-of-Words Model(CBOW) (Mikolov et al., 2013). Inspired by Mao et al. (2018), we use a fit word to model the contextual sense of the target word. To find a fit word for the target word t , we construct a candidate word set candidates which consists of the target word as well as its synonyms and direct hypernyms extracted from WordNet(Miller, 1998). The target word may have several senses. Each of these senses has a set of corresponding synonyms and hypernyms. We extract the synonyms and hypernyms of all the verb senses. We augment the set with the inflections of the items in it. The word in the candidates which has the highest similarity with the given context represents the contextual sense of the target verb in the sentence. For example,  i am afraid this spells trouble  is a sentence in the corpus and the target word t is  spells . The word (e.g.  means ) in candidates has the highest similarity with the given context (e.g.  i am afraid this [ ] trouble. ), and is the fit word of t . We then compute the similarity between the target verb (e.g.  spells ) and its fit word (e.g.  means ) . If the similarity is less than or equal to a threshold  = 0 . 6 3 , we extract the metaphorical word together with the fit word to form a metaphorical pair. The extraction process is described in Algorithm 1. Follow the previous work (Nalisnick et al., 2016; Mao et al., 2018), we use OUT-IN vectors to measure the similarity between a fit word and its given context, use IN-IN vectors to measure the similarity between two words in a metaphorical pair 4 . Our goal is to generate a sentence containing a metaphorically used verb. However, the vanilla end-to-end model cannot guarantee the target word appearing in the generated sequence all the time, letting alone the appearance of a word with a specific part-of-speech. To solve this problem, we present a neural language model which can ensure an assigned verb to appear in the generated sentence. Our design is inspired by the asynchronous forward/backward generation model proposed by (Mou et al., 2016). The POS constrained language model is trained end-to-end. Given a target verb as input, the model first generates the backward sequence starting from the target word w t at position t of the sentence and ending up with  < /s >  at position 0 of the sentence. n is the position of the last word in the sentence. p ( w 1 t ) denotes the probability of the backward sequence. Then we reverse the output of the backward sequence as the input to the forward model. And it generates the rest part of the sentence accordingly. p ( w nt ) denotes the probability 3 Keep in line with Mao et al. (2018) 4 IN vectors are input vectors of a trained CBOW model. of the forward sequence. The generated sentence is a connection of the input and output of the forward model whose probability can be decomposed as: p (  w 1 t ||  w nt )= p ( w t ) t  1 (cid:89) i =1 p ( bw ) (  w t  i | ) n  t (cid:89) i =1 p ( fw ) (  w t + i | ) , (1) where p ( || ) means the probability of a particular backward/forward sequence. p ( bw ) (  w t | ) or p ( fw ) (  w t | ) indicates the probability of w t given previous sequence  in the backward or forward model respectively. So far, the model is able to generate a sentence containing an assigned word. But the target word is not used as a verb in the generated sequence all the time. We regard a word with various parts-of-speech as a polyseme. If the word is used with a specific part-of-speech which we are concerned about, we label it with the specific part-of-speech tag, otherwise, the word remains unchanged. In our case, we use a POS tagger (Bird et al., 2009) to label all the words in the metaphorical pairs we extracted into two categories: verb and the other. For instance,  spells  is labeled as  spells.v  in the corpus if it is used as a verb otherwise  spells . We train the POS constrained model with the labeled corpus, and we generate sentences like  she spells.v her husband at the wheel  rather than  he whispered spells as he moved his hands.  when given  spells.v  as input. In the end-to-end model, the goal of the decoder is to find a sequence  y which maximizes the conditional probability given by a specific model  and an input sequence x : where Y is a set of all the possible sequences in the output space. It is impracticable to explore the whole space. Models decompose this problem into a sequence of time steps. The original beam search algorithm produces a probability distribution at each time step over the vocabulary V . And the log function is applied on the probability distribution to get the score distribution. Instead of simply choosing the token with the highest score, the beam search algorithm keeps top k candidates from a large matrix of dimensions k  | V | which expands the search space, where k denotes Algorithm 2 Adjustable Joint Beam Search Require: k : beam size Require: ,  : adjustment factors Require: L : maximum sequence length Require: w 1 , w 2 : input words Decoder  Init ( w i , n ) copies the initial state of the decoder n times when inputting w i to the decoder. the beam size. When  < /s >  is selected among the highest scoring candidates the beam is reduced by one. When the beam is zero, the beam search algorithm stops (Lowerre, 1976; Post and Vilar, 2018). As the metaphorical usage of the verb is represented by the metaphorical pair. We need to generate a sentence for the target verb where the contextual sense of the target word equals to the literal sense of its fit word, which means both target word and its fit word should be suitable in the generated sentence. Given two different verbs in a metaphorical pair as inputs (e.g.  devoured .v,  enjoyed .v), we hope to generate a same context for them. However, the original beam search algorithm can hardly choose the same candidates at each time step for them. Yu et al. (2018) propose the joint beam search algorithm for pun generation. The algorithm selects candidates for the two inputs according to the joint score distribution on all beam while decoding. Nevertheless, the semantic distance between the metaphorical sense and the literal sense of the target verb is not the same with the distance between the metaphorical sense and contextual sense of the target word. In addition, the frequencies of the words in each metaphorical pair differ, and there is no reason to believe that two words in a metaphorical pair have the same influence on generating metaphors. Therefore, we use the adjustment factors ,  to modify the weights of the score distributions for two inputs. The proposed adjustable joint beam search algorithm is described in Algorithm 2. Different from the original beam search algorithm, to start the adjustable joint beam search algorithm, the initial states of the decoder are copied k/ 2 times for each input. At each time step, candidates are chosen from a weighed summation of the score distributions from two sets of the beam. The beam for the next time step is filled by taking the states corresponding to the selected beam ids and word ids. Half of the chosen words are duplicate but come from distinct beam, which means the outputs are one-to-one correspondent for the two distinct inputs. Although the corresponding two sequences select the same word at each time step, they have distinct hidden states, and thus their score distributions are different. The decoder finally outputs k sentences in parallel and the corresponding two sentences are the same except for the input words. We find an intersection between a metaphorical usage and a literal usage of a verb by this means. To avoid that the generated sentence is semantically inclined to the word in a metaphorical pair whose frequency is relatively higher and take the second hypothesis ( H2 ) into consideration, we calculate the adjustment factors as follows:  =  (1  wf ( word 1) wf ( word 1) + wf ( word 2) ) , (3)  =  (1  wf ( word 2) wf ( word 1) + wf ( word 2)) , (4) where wf ( a ) denotes the frequency of the word a in the corpus. As metaphorical senses of words occur with relatively lower frequency in the corpus, we adjust the weights negatively correlated to the word frequency. And we use  , the sigmoid function which is differentiable and widely used in the neural network models, to smooth the adjustment factors. Adequate data is a prerequisite condition for training a good language model. It is unrealistic to achieve the goal based on the limited annotated metaphor corpus. In this paper, we use the English Wikipedia corpus to train the POS constrained language model. The corpus is split into sentences whose maximum length is 50 words. We lowercase and tokenize the sentences. All the numeric tokens are replaced with #. We automatically extract 2812 metaphorical pairs from the corpus, and label words in them into two forms:  word.v  and  word . We use 461,685 sentences as the training set. We keep 120,000 most frequently occurring words and replace other words with the  < unk >  token. We call this processed corpus a normal corpus . For comparison, we extract the metaphors in the normal corpus using an unsupervised metaphor identification approach (Mao et al., 2018) and build a metaphor corpus with 310,908 sentences. We keep the same vocabulary size. To explore the influence of a fine-grained sense in the metaphor generation task, we use a Word Sense Disambiguation (WSD) tool 5 to label the verbs in the normal corpus. As the polysemes are regarded as different words which obviously increases the vocabulary size, we keep 165,000 most frequently occurring words. And this corpus is named sense corpus . Since there has been no neural model applied on metaphor generation and the previous template-based models can not generate such verb-oriented metaphors, we implement six neural models for comparison and explore the intrinsic characteristics of metaphor generation. Models are trained on the normal corpus , unless otherwise specified. Normal Model : This model is a basic end-to-end framework whose inputs are verbs and outputs are sentences. We use the teacher forcing algorithm (Williams and Zipser, 1989) while training. The input of the encoder is a verb, and the reference for the decoder is a sentence containing the verb. POS constrained Model: This model combines the basic end-to-end framework with the POS constrained language model. The model can gene-5 https://github.com/alvations/pywsd rate a sentence containing a given verb. Fit Word Model: The model is trained in the same way as the POS constrained model. The input is a fit word, and we directly replace the fit word in the generated sentence with the corresponding target word. Metaphor Based Model: The model is the same as the POS constrained model but trained on the metaphor corpus . Uncommon Sense Model: Based on the second hypothesis( H2 ), metaphorical sense appears less common in the corpus than literal sense. It is intuitive to associate the metaphorical sense of the target word with a low-frequency sense. We keep the senses of a target word which appear in the corpus more than 9 times in a sense list. Then we choose the sense with the lowest frequency in the sense list as the metaphorical sense of the target verb. This model generates sentences similarly to the POS constrained model, except that it is trained on the sense corpus. Adjustable Joint Model: The training process is exactly the same as the POS constrained mo-del's, but we use the adjustable joint beam search algorithm while decoding. We extract the metaphorical pairs automatically in the normal corpus and conduct experiments. The automatic evaluation results are shown in Table 1. Model l.ave w.num ppl. Each target verb may form distinct metaphorical pairs with different fit words. Both the normal model and the POS constrained model generate the same sentences for the same target verbs while the fit word model generates the same sentences for the same fit words. Taking the metaphorical pairs into consideration, adjustable joint model can generate various sentences when the target words are the same but used in diffe-Figure 3: Results of human evaluation. rent senses. For a fair quantitative analysis, each model generates 1555 sentences for distinct target verbs. Length-average (l.ave) is the average length of the generated sentences of each model. Word.number(w.num) is the total number of the distinct words in the generated sentences, and the adjustable joint model decodes words with the highest diversity compared to the other models. We use the language modeling toolkit SRILM 6 to evaluate the perplexity scores (ppl.) . Theoretically, the normal model should generate more fluent sentences without considering the constraint that a given word must appear in the outputs, however the ppl. is calculated as Eq 5 shows: ppl = 10  (  logP ( T ) s.num + w.num  OOV s ) , (5) where P ( T ) denotes the probabilities of all the sentences, s.num denotes the total number of the sentences. OOV s denotes the number of out-of-vocabulary words. The outputs of the metaphor based models tend to be shorter which results in a higher P ( T ) and thus a low ppl. Although the adjustable joint model generates sentences with lexical constraints, it chooses the candidates with the highest joint score at each time step and uses more distinct words, and thus it obtains a high P ( T ) as well as a large w.num . It achieves a relatively low ppl. What's more, all the sentences generated by the adjustable joint model are novel and do not exist in the corpus according to the duplicate checking . The fit word model decodes according 6 http://www.speech.sri.com/projects/srilm/ to the fit words and then the fit words in the sentences are replaced by their target words directly, which results in a higher ppl. Since the amount of the training data corresponding to the inputs of the uncommon sense model is the least, the generated sentences are not so fluent. For a thorough comparison, we select 80 gold metaphors with high confidence ( > 0.6) from the data set proposed by (Mohammad et al., 2016). Each verb in the data set was annotated for metaphoricity by 10 annotators and we use the verbs in the selected metaphors as the target words. As metaphor is such a creative and delicate language that automatic evaluation is not adequate. We ask native English speakers on Amazon Mechanical Turk to evaluate all the sentences generated by the neural models and corresponding gold metaphors in four aspects. Each sentence is scored from 1 to 5 by 5 judges. Readability(read.) indicates whether the sentence is fluent and consistent with the rules of grammar. Creativity(crea.) indicates whether the sentence is distinct and creative. Metaphorical or Literal Usage of the Verb(meta.-v) indicates whether the target word is used literally or metaphorically. 1 denotes that the usage of the verb is definitely literal and 5 denotes the verb is obviously metaphorically used. We display typical properties of metaphorical and literal senses as follows: literal usages tend to be more basic, straightforward meaning, more physical and closely tied to our senses; Metaphorical usages tend to be more abstract, more vague and often surprising, someti-Model Generated Sentences Target Verb: absorbed Fit Word: learn Normal it is absorbed by the united states . mes bring in imagery from a different domain. The aspects above are rated according to comparison. Metaphoricity of the sentence(meta.-s) indicates whether the sentence is a metaphor. The score ratings are defined as: 1. not a metaphor at all; 2. pathetic metaphor; 3. not-so-bad metaphor; 4. interesting metaphor; 5. gold metaphor. Results are shown in Figure 3. The metaphor based model generates shorter sentences which results in a low ppl. , but it replicates some words at times, which makes it difficult for human to interpret. The fit word model replaces the fit words with the target words directly and may break the grammatical collocations. Although the adjustable joint model ensures the intersection sense of two words in the metaphorical pair to appear in one sentence, the generated utterance is still readable. As for the crea. , without any lexical constraint, the normal model always generates the sentences which are similar to the relatively high-frequency sentences in the corpus and results in the lack of novelty. In contrast, the target words are used in its less used literal senses in the uncommon sense model and the results seem to be kind of creative. The adjustable joint model performs closely to the gold metaphor on the crea. , which proves its ability in modeling the creative usage of the verbs. meta.-v directly reveals the capability of modeling the metaphorical senses of the target verb. The normal model is uncompetitive as it can not even ensure the appearance of the target verb. The difference between the POS constrained model and the uncommon sense model inspires us that although the metaphorical senses of the words occur with relatively lower frequency in the corpus than their literal senses, the metaphorical senses cannot be easily defined as the most uncommon senses of the target verbs, and thus the exploration on modeling the metaphorical senses is essential. As for the meta.-s , it reveals the comprehensiveness of the sentence-level metaphor generation and reminds us of modeling the metaphors in a more thoughtful perspective. The adjustable joint model outperforms the other models on the 3 aspects ( crea. , meta.-v , meta.-s ) significantly with paired t-test with p < 0.05. To illustrate concretely, we show some examples generated by different models in Table 2. Both the normal model and the POS constrained model generate sentences without considering the senses of the verbs and tend to generate monotonous sentences. And the fit word model generates sentences which are suitable for the fit word but inappropriate for the target word. For example,  they crowed for the first time  is fluent while  they poured for the first time  is strange. Other models decoding with a sense constraint generate sentences conveying the assigned senses to different degree. However, the uncommon sense model is fine-grained and the training corpus is annotated with WSD tools which results in not only the lack of corpora for some senses but also a sense-label error, as there are no WSD tools that could tag the senses of the verbs with a high precision ( > 0 . 6 ) (Luo et al., 2018b,a). To solve these problems, we use the extracted metaphorical pairs to depict a metaphorical sense and the generated metaphors are readable. We also explore the shared inferential structure of a metaphorical usage and a literal usage of a verb by changing the adjustable factors. Table 2 demonstrates the semantic shift of the verb. The POS constrained model can be seen as a special adjustable joint model whose adjustable factors are  = 1 ,  = 0 and it generates sentences only considering the target words. In this way the target words are literally used. For contrast, the fit word model with a special setting of the adjustable factors  = 0 ,  = 1 generates sentences completely dependent on the fit words, which may result in sentences that are not semantically appropriate. When ,  are caculated by Eq 3 and Eq 4, the generated sentences covey the metaphorical senses of verbs. We make an exploration on verb-oriented metaphor generation and propose a neural approach on automatic metaphor generation. The approach identifies metaphorically used verbs in the normal corpus and extracts the metaphorical pairs in the sentences. We propose a POS constrained model to ensure the appearance of the given verbs and decode with the adjustable joint beam search algorithm, which takes the metaphorical senses of the given verbs into consideration. We generate metaphors which are not only fluent and readable but also creative. However, we can only generate metaphors based on the verbs whose metaphorical senses can be found in the corpus. For future work, we will explore techniques to generate metaphors without extracting its fit words in the corpus and improve the quality of generated metaphors. This work was supported by National Natural Science Foundation of China (61772036) and Key Laboratory of Science, Technology and Standard in Press Industry (Key Laboratory of Intelligent Press Media Technology). We appreciate Hongyu Zang for the detailed suggestions and thoughts. We thank Fuli Luo, Pengcheng Yang, Rui Mao and the anonymous reviewers for their helpful comments. Xiaojun Wan is the corresponding author.
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