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First Align, then Predict: Understanding the Cross-Lingual Ability of Multilingual BERT

Multilingual pretrained language models have demonstrated remarkable zero-shot cross-lingual transfer capabilities. Such transfer emerges by fine-tuning on a task of interest in one language and evaluating on a distinct language, not seen during the fine-tuning. Despite promising results, we still lack a proper understanding of the source of this transfer. Using a novel layer ablation technique and analyses of the model’s internal representations, we show that multilingual BERT, a popular multilingual language model, can be viewed as the stacking of two sub-networks: a multilingual encoder followed by a task-specific language-agnostic predictor. While the encoder is crucial for cross-lingual transfer and remains mostly unchanged during fine-tuning, the task predictor has little importance on the transfer and can be reinitialized during fine-tuning. We present extensive experiments with three distinct tasks, seventeen typologically diverse languages and multiple domains to support our hypothesis.


Introduction
Zero-shot Cross-Lingual transfer aims at building models for a target language by reusing knowledge acquired from a source language. Historically, it has been tackled with a two-step standard crosslingual pipeline (Ruder et al., 2019): (1) Building a shared multilingual representation of text, typically by aligning textual representations across languages. This step can be done using feature extraction (Aone and McKee, 1993;Schultz and Waibel, 2001) as with the delexicalized approach (Zeman and Resnik, 2008;Søgaard, 2011) or using word embedding techniques (Mikolov et al., 2013;Smith et al., 2017) by projecting monolingual embeddings onto a shared multilingual embedding space, this step requiring explicit supervision signal in the target language in the form of features or parallel data. (2) Training a task-specific model using supervision on a source language on top of the shared representation.
Recently, the rise of multilingual language models entailed a paradigm shift in this field. Multilingual pretrained language models (Devlin et al., 2019;Conneau and Lample, 2019) have been shown to perform efficient zero-shot cross-lingual transfer for many tasks and languages (Pires et al., 2019;Wu and Dredze, 2019). Such transfer relies on three-steps: (i) pretraining a mask-language model (e.g. Devlin et al. (2019)) on the concatenation of monolingual corpora across multiple languages, (ii) fine-tuning the model on a specific task in the source language, and (iii) using the finetuned model on a target language. The success of this approach is remarkable, and in contrast to the standard cross-lingual pipeline, the model sees neither aligned data nor task-specific annotated data in the target language at any training stage.
The source of such a successful transfer is still largely unexplained. Pires et al. (2019) hypothesize that these models learn shared multilingual representations during pretraining. Focusing on syntax, Chi et al. (2020) recently showed that the multilingual version of BERT (mBERT) (Devlin et al., 2019), encodes linguistic properties in shared multilingual sub-spaces. Recently, Gonen et al. (2020) suggest that mBERT learns a language encoding component and an abstract cross-lingual component. In this work, we are interested in understanding the mechanism that leads mBERT to perform zero-shot cross-lingual transfer. More specifically, we ask what parts of the model and what mechanisms support cross-lingual transfer?
By combining behavioral and structural analyses (Belinkov et al., 2020), we show that mBERT operates as the stacking of two modules: (1) A multilingual encoder, located in the lower part of the model, critical for cross-lingual transfer, is in charge of aligning multilingual representations; and (2) a task-specific, language-agnostic predictor which has little importance for cross-lingual transfer and is dedicated to performing the downstream task. This mechanism that emerges out-of-the-box, without any explicit supervision, suggests that mBERT behaves like the standard cross-lingual pipeline. Our contributions advance the understanding of multilingual language models and as such have the potential to support the development of better pretraining processes.

Analysis Techniques
We study mBERT with a novel behavioral test that disentangles the task fine-tuning influence from the pretraining step ( §2.1), and a structural analysis on the intermediate representations ( §2.2). Combining the results from these analyses allows us to locate the cross-lingual transfer and gain insights into the mechanisms that enable it.

Locating Transfer with RANDOM-INIT
In order to disentangle the impact of the pretraining step from the fine-tuning, we propose a new behavioral technique: RANDOM-INIT. First, we randomly initialize a set of parameters (e.g. all the parameters of a given layer) instead of using the parameters learned during the pretraining step. Then, we fine-tune the modified pretrained model and measure the downstream performance. 1 By replacing a given set of pretrained parameters and fine-tuning the model, all other factors being equal, RANDOM-INIT enables us to quantify the contribution of a given set of pretrained parameters on downstream performance and therefore to locate which pretrained parameters contribute to the cross-lingual transfer.
If the cross-language performance is significantly lower than same-language performance, we conclude that these layers are more important to cross-language performance than they are for samelanguage performance. If the cross-language score does not change, it indicates that cross-language transfer does not rely on these layers.
This technique is reminiscent of the recent Amnesic Probing method (Elazar et al., 2020), that removes from the representation a specific feature, e.g. Part-of-Speech, and then measures the outcome on the downstream task. In contrast, RANDOM-INIT allows to study a specific architecture component, instead of specific features.

Hidden State Similarities across Languages
To strengthen the behavioral evidence brought by RANDOM-INIT, and provide finer analyses that focus on individual layers, we study how the textual representations differ between parallel sentences in different languages. We hypothesize that an efficient fine-tuned model should be able to represent similar sentences in the source and target languages similarly, even-though it was fine-tuned only on the source language.
To measure the similarities of the representation across languages, we use the Central Kernel Alignment metric (CKA), introduced by Kornblith et al. (2019). We follow Conneau et al. (2020) who use the CKA as a similarity metric to compare the representations of monolingual and bilingual pretrained models across languages. In our work, we use the CKA to study the representation difference between source and target languages in pretrained and fine-tuned multilingual models. For every layer, we average all contextualized tokens in a sentence to get a single vector. 2 Then we compute the similarity between target and source representations and compare it across layers in the pretrained and fine-tuned models. We call this metric the cross-lingual similarity.

Experimental Setup
Tasks, Datasets and Evaluation We consider three tasks covering both syntactic and semantic aspects of language: Part-Of-Speech Tagging (POS), dependency parsing, and Named-Entity Recognition (NER). For POS tagging and parsing we use the Universal Dependency (Nivre et al., 2018) treebanks, and for NER, we use the WikiANN dataset (Pan et al., 2017). We evaluate our systems with the standard metrics per task; word-level accuracy for POS tagging, F1 for NER and labeled attachment score (LAS) for parsing. All the reported scores are computed on the test set of each dataset.
We experiment with English, Russian and Arabic as source languages, and fourteen typologically diverse target languages, including Chinese, Czech, German and Hindi. The complete list can be found in the Appendix A. The results of a model that is fine-tuned and evaluated on the same language are referred to as same-language and those evaluated on distinct languages are referred to as cross-language.

Multilingual Model
We focus on mBERT (Devlin et al., 2019), a 12-layer model trained on the concatenation of 104 monolingual Wikipedia corpora, including our languages of study.

Fine-Tuning
We fine-tune the model for each task following the standard methodology of Devlin et al. (2019). The exact details for reproducing our results can be found in the Appendix. All reported scores are averaged on 5 runs with different seeds.

Disentangling the Pretraining Effect
For each experiment, we measure the impact of randomly-initializing specific layers as the difference between the model performance without any random-initialization (REF) and with randominitialization (RANDOM-INIT). Results for two consecutive layers are shown in Table 1. The rest of the results, which exhibit similar trends, can be found in the Appendix (Table 5).
For all tasks, we observe sharp drops in the cross-language performance at the lower layers of the model but only moderate drops in the samelanguage performance. For instance, the parsing experiment with English as the source language, results in a performance drop on English of only 0.96 points (EN-EN), when randomly-initializing layers 1 and 2. However, it leads to an average drop of 15.77 points on other languages (EN-X).
Furthermore, we show that applying RANDOM-INIT to the upper layers does not harm samelanguage and cross-language performances (e.g. when training on parsing for English, the performance slightly decreases by 0.09 points in the same-language while it increases by 1.00 in the cross-language case). This suggests that the upper layers are task-specific and language-agnostic, since re-initializing them have minimal change on performance. We conclude that mBERT's upper layers do not contribute to cross-language transfer.

Does the Target Domain Matter?
In order to test whether this behavior is specific to the crosslanguage setting and is not general to out-ofdistribution (OOD) transfer, we repeat the same RANDOM-INIT experiment by evaluating on samelanguage setting while varying the evaluated domain. 3 If the drop is similar to cross-language performance, it means that lower layers are important for out-of-distribution transfer in general. Otherwise, it would confirm that these layers play a specific role for cross-language transfer.
We report the results in Table 2. For all analyzed domains (Web, News, Literature, etc.) applying RANDOM-INIT to the two first layers of the models leads to very moderate drops (e.g. -0.91 when the target domain is English Literature for parsing), while it leads to large drops when the evaluation is done on a distinct language (e.g. -5.82 when evaluated on French). The trends are similar for all the domains and tasks we tested on. We conclude that the pretrained parameters at the lower layers are consistently more critical for cross-language transfer than for same-language transfer, and cannot be explained by the possibly different domain of the evaluated datasets.

Cross-Lingual Similarity in mBERT
The results from the previous sections suggest that the lower layers of the model are responsible for the cross lingual transfer, whereas the upper layers are language-agnostic. In this section, we assess the transfer by directly analyzing the intermediate representations and measuring the similarities of the hidden state representations between source and target languages. We compute the CKA metric (cf. §2.2) between the source and the target representations for pretrained and fine-tuned models using parallel sentences from the PUD dataset (Zeman et al., 2017). In Figure 1, we present the similarities between Russian and English with mBERT pretrained and fine-tuned on the three tasks. 4 The cross-lingual similarity between the representations constantly increases up to layer 5 for all the three tasks (reaching 78.1%, 78.1% and 78.2% for parsing, POS tagging and NER respectively). From these layers forward, the similarity decreases. We observe the same trends across all languages (cf. Figure 5). This demonstrates that the finetuned model creates similar representations regard- less of the language and task, and hints on an alignment that occurs in the lower part of the model. Interestingly, the same trend is also observed in the pretrained model, suggesting that the fine-tuning step preserves the multilingual alignment. These results do not match the findings of Singh et al. (2019), who found no language alignment across layers, although they inspected Natural Language Inference, a more "high-level task" (Dagan et al., 2005;Bowman et al., 2015). We leave the inspection of this mismatch to future work.

Better Alignment Leads to Better Cross-Lingual Transfer
In the previous section we showed that fine-tuned models align the representations between parallel sentences, across languages. Moreover, we demonstrated that the lower part of the model is critical for cross-language transfer but hardly impacts the same-language performance. In this section, we show that the alignment measured plays a critical role in cross-lingual transfer. As seen in Figure 2 in the case of English to Russian (and in Figures 6-8 in the Appendix for other languages), when we randomly-initialize the lower part of the model, there is no alignment: the similarity between the source and target languages decreases. We observe the same trend for all other languages and tasks and report it in the Appendix in Figures 6-8. This result matches the drop in crosslingual performance that occurs when we apply RANDOM-INIT to the lower part of the model while impacting moderately same-language performance.
For a more systematic view of the link between the cross-lingual similarities and the crosslanguage transfer, we measure the Spearman correlation between the cross-lang gap (i.e the difference between the same-language perfromance and the cross-language performance) (Hu et al., 2020) and the cross-lingual similarity averaged over all the layers. We measure it with the cross-lingual similarity computed on the pretrained and fine-tuned models (without random-initialization) on all the languages. We find that the cross-lingual similarity correlates significantly with the cross-lang gap for all three tasks, both on the fine-tuned and pretrained models. The spearman correlation for the fine-tuned models are 0.76, 0.75 and 0.47 for parsing, POS and NER, respectively. 5 In summary, our results show that the cross-lingual alignment is highly correlated with the cross-lingual transfer.

Discussion
Understanding the behavior of pretrained language models is currently a fundamental challenge in NLP. A popular approach consists of probing the intermediate representations with external classifiers (Alain and Bengio, 2017;Adi et al., 2017;Conneau et al., 2018) to measure if a specific layer captures a given property. Using this technique, Tenney et al. (2019) showed that BERT encodes linguistic properties in the same order as the "classical NLP pipeline". However, probing techniques only indirectly explain the behavior of a model and do not explain the relationship between the information captured in the representations and its effect on the task (Elazar et al., 2020). Moreover, recent works have questioned the usage of probing as an interpretation tool (Hewitt and Liang, 2019;Ravichander et al., 2020). This motivates our approach to combine a structural analysis based on representation similarity with behavioral analysis. In this regard, our findings extend recent work from Merchant et al. (2020) in the multilingual setting, who show that fine-tuning impacts mainly the up-per layers of the model and preserves the linguistic features learned during pretraining. In our case, we show that the lower layers are in charge of aligning representations across languages and that this cross-lingual alignment learned during pretraining is preserved after fine-tuning.

Conclusion
The remarkable performance of multilingual languages models in zero-shot cross-lingual transfer is still not well understood. In this work, we combine a structural analysis of the similarities between hidden representation across languages with a novel behavioral analysis that randomlyinitialize the models' parameters to understand it. By combining those experiments on 17 languages and 3 tasks, we show that mBERT is constructed from: (1) a multilingual encoder in the lower layers, which aligns hidden representations across languages and is critical for cross-language transfer, and (2) a task-specific, language-agnostic predictor that has little effect to cross-language transfer, in the upper layers. Additionally, we demonstrate that hidden cross-lingual similarity strongly correlates with downstream cross-lingual performance suggesting that this alignment is at the root of these cross-lingual transfer abilities. This shows that mBERT reproduces the standard cross-lingual pipeline described by Ruder et al. (2019) without any explicit supervision signal for it. Practically speaking, our findings provide a concrete tool to measure cross-lingual representation similarity that could be used to design better multilingual pretraining processes.

A.1.1 Optimization
We fine-tune our models using the standard Adam optimizer (Kingma and Ba, 2015). We warmup the learning rate on the first 10% steps and use linear decay in the rest of the training. Using the validation set of the source language, we find the best combination of hyper-parameters with a grid search on batch size among {16, 32} and learning rate initialization among {1e-5, 2.5e-5, 5e-5} We select the model with the highest validation performance out of 15 epochs for parsing and out of 6 epochs for POS tagging and NER. Table 3, we report the best hyper-parameters set for each task, the bound of each hyperparameter, the estimated number of grid search trial for each task as well as the estimated run time. be/conll2003/ Languages For all our experiments, we use English, Russian and Arabic as source languages in addition to Chinese, Czech, Finish, French, Indonesian, Italian, Japanese, German, Hindi, Polish, Portuguese, Slovenian, Spanish, and Turkish as target languages.

Fine-tuning Data
For all the cross-lingual experiments, we use English, Russian and Arabic as source languages on which we fine-tune mBERT. For English, we take the English-EWT treebank for fine-tuning, for Russian the Russian-GSD treebank and for Arabic the Arabic-PADT treebank.
Evaluation Data For all our experiments, we perform the evaluation on all the 17 languages. For Parsing and POS tagging we use the test set from the PUD treebanks released for the CoNLL Shared Task 2017 (Zeman et al., 2017). For NER, we use the corresponding annotated datasets in the wikiner dataset.

Domain Analysis Datasets
We list here the datasets for completing our domain analysis experiment in Section 4.1 reported in Table 2. To have a full control on the source domains, we use for fine-tuning the English Partut treebank for POS tagging and parsing (Svizzera, 2014). It is a mix of legal, news and wikipedia text. For NER, we keep the WikiANN dataset (Pan et al., 2017). For the same-language and out-of-domain experiments, we use the English-EWT, English-Lines and English Lexnorm (van der Goot and van Noord, 2018) treebanks for Web Media data, Literature data and Noisy tweets respectively. For the cross-language French evaluation, we use the translation of the English test set, 6 as well as the French-GSD treebank. For NER, we take the CoNLL-2003 shared task English data as our out-of-domain evaluation extracted from the News domain. We note that the absolute performance on this dataset is not directly comparable to the one on the source wikiner. Indeed, the CoNLL-2003 dataset uses an extra MISC class. In our work, we only interpret the relative performance of different models on this test set.

Cross-Lingual Similarity Analysis
For a given source language l and a target language l , we collect a 1000 pairs of aligned sentences from the UD-PUD treebanks (Zeman et al., 2017). For a given model and for each layer, we get a single sentence embedding by averaging token-level embeddings (after excluding special tokens). We then concatenate the 1000 sentence embedding vectors and get the matrices X l and X l . Based on these two matrices, the CKA between the language l and the language l is defined as: with ||.|| F defining the Frobenius norm. We do so for each source-target language pairs using the representation of the pretrained mBERT model as well as for mBERT fine-tuned on each downstream task.
In addition to the results presented in §4.2, we report in Figure 4, a comparison of the cross-lingual similarity per hidden layer of mBERT fine-tuned on NER, across target languages. The trend is the same for all languages.

A.1.3 Computation
Infrastructure Our experiments were ran on a shared cluster on the equivalent of 15 Nvidia Tesla T4 GPUs. 7 Codebase All of our experiments are built using the Transformers library described in (Wolf et al., 2020).

A.1.4 Preprocessing
Our experiments are ran with word-level tokenization as provided in the datasets. We then tokenize each sequence of words at the sub-word level using the Wordpiece algorithm of BERT and provided by the Transformers library.

A.2.1 Correlation
We report here in Figure 4 the correlation between the hidden representation of each layer and the cross-lang gap between the source and the target averaged across all target languages and all layers. The correlation is strong and significant for all the tasks and for both the fine-tuned and the pretrained models. This shows that multilingual alignment that occurs within the models, learnt during pretraining is strongly related with crosslingual transfer. We report in Figure 3, the detail of this correlation per layer. For the pretrained model, we observe the same distribution for each task with layer 6 being the most correlated to cross-lingual transfer. We observe large variations in the finetuned cases, the most notable being NER. This illustrates the task-specific aspect of the relation between cross-lingual similarity and cross-lingual 7 https://www.nvidia.com/en-sg/data-center/tesla-t4/ transfer. More precisely, in the case of NER, the sharp increase and decrease in the upper part of the model provides new evidence that for this task, fine-tuning highly impacts cross-lingual similarity in the upper part of the model which correlates with cross-language transfer.       Figure 5: Cross-Lingual similarity (CKA) similarity ( §4.2) of hidden representations of a source language (English) sentences with a target language sentences on fine-tuned and pretrained mBERT. The higher the CKA value the greater the similarity.