Multilingual Bert sentence vector captures language used more than meaning - working as interned?

Question

Playing around with BERT, I downloaded the Huggingface Multilingual Bert and entered three sentences, saving their sentence vectors (the embedding of [CLS]), then translated them via Google Translate, passed them through the model and saved their sentence vectors.

I then compared the results using cosine similarity.

I was surprised to see that each sentence vector was pretty far from the one generated from the sentence translated from it (0.15-0.27 cosine distance) while different sentences from the same language were quite close indeed (0.02-0.04 cosine distance).

So instead of having sentences of similar meaning (but different languages) grouped together (in 768 dimensional space ;) ), dissimilar sentences of the same language are closer.

To my understanding the whole point of Multilingual Bert is inter-language transfer learning - for example training a model (say, and FC net) on representations in one language and having that model be readily used in other languages.

How can that work if sentences (of different languages) of the exact meaning are mapped to be more apart than dissimilar sentences of the same language?

My code:

import torch

import transformers
from transformers import AutoModel,AutoTokenizer

bert_name="bert-base-multilingual-cased"
tokenizer = AutoTokenizer.from_pretrained(bert_name)
MBERT = AutoModel.from_pretrained(bert_name)

#Some silly sentences
eng1='A cat jumped from the trees and startled the tourists'
e=tokenizer.encode(eng1, add_special_tokens=True)
ans_eng1=MBERT(torch.tensor([e]))

eng2='A small snake whispered secrets to large cats'
t=tokenizer.tokenize(eng2)
e=tokenizer.encode(eng2, add_special_tokens=True)
ans_eng2=MBERT(torch.tensor([e]))

eng3='A tiger sprinted from the bushes and frightened the guests'
e=tokenizer.encode(eng3, add_special_tokens=True)
ans_eng3=MBERT(torch.tensor([e]))

# Translated to Hebrew with Google Translate
heb1='חתול קפץ מהעץ והבהיל את התיירים'
e=tokenizer.encode(heb1, add_special_tokens=True)
ans_heb1=MBERT(torch.tensor([e]))

heb2='נחש קטן לחש סודות לחתולים גדולים'
e=tokenizer.encode(heb2, add_special_tokens=True)
ans_heb2=MBERT(torch.tensor([e]))

heb3='נמר רץ מהשיחים והפחיד את האורחים'
e=tokenizer.encode(heb3, add_special_tokens=True)
ans_heb3=MBERT(torch.tensor([e]))


from scipy import spatial
import numpy as np

# Compare Sentence Embeddings

result = spatial.distance.cosine(ans_eng1[1].data.numpy(), ans_heb1[1].data.numpy())

print ('Eng1-Heb1 - Translated sentences',result)


result = spatial.distance.cosine(ans_eng2[1].data.numpy(), ans_heb2[1].data.numpy())

print ('Eng2-Heb2 - Translated sentences',result)

result = spatial.distance.cosine(ans_eng3[1].data.numpy(), ans_heb3[1].data.numpy())

print ('Eng3-Heb3 - Translated sentences',result)

print ("\n---\n")

result = spatial.distance.cosine(ans_heb1[1].data.numpy(), ans_heb2[1].data.numpy())

print ('Heb1-Heb2 - Different sentences',result)

result = spatial.distance.cosine(ans_eng1[1].data.numpy(), ans_eng2[1].data.numpy())

print ('Heb1-Heb3 - Similiar sentences',result)

print ("\n---\n")

result = spatial.distance.cosine(ans_eng1[1].data.numpy(), ans_eng2[1].data.numpy())

print ('Eng1-Eng2 - Different sentences',result)

result = spatial.distance.cosine(ans_eng1[1].data.numpy(), ans_eng3[1].data.numpy())

print ('Eng1-Eng3 - Similiar sentences',result)

#Output:
"""
Eng1-Heb1 - Translated sentences 0.2074061632156372
Eng2-Heb2 - Translated sentences 0.15557605028152466
Eng3-Heb3 - Translated sentences 0.275478720664978

---

Heb1-Heb2 - Different sentences 0.044616520404815674
Heb1-Heb3 - Similar sentences 0.027982771396636963

---

Eng1-Eng2 - Different sentences 0.027982771396636963
Eng1-Eng3 - Similar sentences 0.024596810340881348
"""

P.S.

At least the Heb1 was closer to Heb3 than to Heb2. This was also observed for the English equivalents, but less so.

N.B. Originally asked on Stack Overflow, here

Answer 1

Sadly, I don't think that Multilingual BERT is the magic bullet that you hoped for. As you can see in Beto, Bentz, Becas: The Surprising Cross-Lingual Effectiveness of BERT (Wu and Dredze, 2019), the mBERT was not trained with any explicit cross-lingual task (for example, predicting a sentence from one language given a sentence from another language).

Rather, it was trained using sentences from Wikipedia in multiple languages, forcing the network to account for multiple languages but not to make the connections between them.

In other words, the model is trained with predicting a masked instance of 'cat' as 'cat' given the rest of the (unmasked) sentence, and predicting a foreign word meaning 'cat' in a masked space given a sentence in that language. This setup is does not push the model towards making the connection.

You might want to have a look in Facebook's LASER, which was explicitly trained to match sentences from different languages.

P.S

The fact that the sentences do not have the same representation does not mean that the mBERT cannot be used for zero-shot transfer learning across languages. Again, please see Wu and Dredze