Aiding Intra-Text Representations with Visual Context for Multimodal Named Entity Recognition

04/02/2019
by   Omer Arshad, et al.
0

With massive explosion of social media such as Twitter and Instagram, people daily share billions of multimedia posts, containing images and text. Typically, text in these posts is short, informal and noisy, leading to ambiguities which can be resolved using images. In this paper we explore text-centric Named Entity Recognition task on these multimedia posts. We propose an end to end model which learns a joint representation of a text and an image. Our model extends multi-dimensional self attention technique, where now image helps to enhance relationship between words. Experiments show that our model is capable of capturing both textual and visual contexts with greater accuracy, achieving state-of-the-art results on Twitter multimodal Named Entity Recognition dataset.

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I Introduction

Recent years have seen a surge in multimodal data containing various media types. Typically, users combine image, text, audio or video data to express views on a social media platform. The combination of these media types has been extensively studied to solve various tasks including classification [1, 2, 3], cross-modal retrieval [4, 5] semantic relatedness [6, 7] and Visual Question Answering (VQA) [8, 9]. Recently, works in [10, 11] and [12] combined text and image in a multimodal approach for text Named Entity Recognition (NER) problem [13]. Typically, the text component of a NER multmodal problem is challenging due to informal language, slang and typos etc. [13]. These attributes make the task more challenging, compared to traditional NER. Moreover there are some ambiguous cases that can only be resolved with visual context, shown in Fig. 1. If we consider only the text in the first example “My daughter got place in Apple valley Tags gymnastics”, Apple is recognized as the name of an Organization, but, in this tweet, Apple should be labeled as Location. Similarly, the text in the second example “Apple’s latest iOS update is bad for advertisers”, Apple is wrongly recognized as the name of an Organization. In both cases, the disambiguation of the text is a non-trivial task, without considering the visual context.

In this work, we propose a novel neural network architecture which leverages on visual context to recognize named entities. We combine character and word embeddings to handle characteristics of NER textual component. In addition, self attention mechanism is extended to capture relationships between two words and image regions, unlike previous works 

[10, 12] which used only single words to capture visual attention. Finally, we introduce a gated multimodal fusion module to select information dynamically from textual and visual features. Intuitively, our model captures two forms of interactions: intra-modal and cross-modal interactions.111Intra-modal interactions deal within same modality, whereas cross-modal captures interaction between modalities. We achieved state-of-the-art results on NER multimodal dataset [10]. In addition, we performed extensive experiments to show the effectiveness of the proposed model. Our main contributions are:

  • introduction of an end-to-end model based on attention only that jointly learns intra and cross-modal dependencies, enhancing relationship of two words;

  • state-of-the-art results on NER multimodal dataset [10].

My daughter got 1 place in [Apple valley LOC] Tags gymnastics [Apple ORG] ’s latest [iOS OTHERS] update is bad for advertisers
Fig. 1: Two NER multimodal examples show how some entities in the text can be correctly tagged in combination with visual information. Looking only at the text, the word Apple is ambiguous in the text description on the left, because it can be interpreted as Location (LOC) or as Organization (ORG).

Ii Related work

In this section, we summarize relevant background on previous works on attention techniques and multimodal NER.
Attention Techniques.

Attention techniques allow models to focus on parts of visual or textual inputs of a task. Visual attention models selectively pay attention to small regions of an image to extract features. On the other hand, textual attention techniques find semantic or syntactic alignments in handling long-term dependencies. Attention techniques have been extensively employed to vision and text related tasks, such as Image Captioning 

[14], VQA [9], Cross-Modal Retrieval [15] NER  [10, 11, 12].
Multimodal Named Entity Recognition. NER task for short and noisy texts has been extensively studied in the literature [16, 17]. Recent years have seen an interest in capturing visual context from social media posts to improve this task [10, 11, 12]

. These works used bi-directional Long Short Term Memory (LSTM) networks to extract features from a sequence of words. The work in 

[10] captures interactions between words and an image in a bi-directional way. However, it represents this interaction in a uni-directional manner. In our work, we extended multi-dimensional attention to jointly learn intra and cross-modal dependencies.

Iii Proposed Model

In this paper we propose a novel architecture, inspired from Disan [18], to learn a joint representation of text and image for multimodal NER. Our model improves the intra-text attention to learn enhanced representations exploiting the relevancy with images. Instead of learning text representations separately from textual context and then leveraging on image information [10] and [12], our model jointly learns shared semantics between intra-text representation and visual features. In next subsections, we first explain each module of our network and then, the proposed end to end model, shown in Fig. 3.

Iii-a Attention

The purpose of the attention module is to compute an alignment score

between elements coming from different sources. In Natural Language Processing, given a sequence of word embeddings

and the embedding of a query , with , the alignment score between and can be calculated using the common additive attention:

(1)

where

is an activation function,

is a vector of weights and

, are weights matrices. A graphical representation of the additive attention is available in Fig. 2(a). Furthermore, we use a special case of attention called self attention in which both elements and come from the same source.

Fig. 2: (a) A neural representation of the additive attention described in Eq. (1), for two word embeddings and . (b) The additive multi-dimensional attention described in Eq. (5).

is a scalar score, which determines how important is to a query . Alignment score between and all tokens becomes:

(2)

Then, a probability distribution

is calculated over by applying softmax. This gives a measure on how much the token is important to a query .

(3)

The final output we obtain from attention vector is the weighted sum of all tokens in x

(4)

that is the context vector for a query .

Iii-B Multi-Dimensional Attention

Multi-dimensional attention [18] proposed for self attention, is an additional attention technique which computes the feature-wise score vector described in Eq. (5) instead of computing the scalar score shown in Eq. (1).

(5)

where is a vector with the same length as , and , , are the weight matrices.

Softmax is applied to the output function to compute categorical distribution over all tokens.

To find the importance of each feature in a word embedding , becomes and the categorical distribution is calculated as:

(6)

Thus the final context becomes:

(7)

The multi-dimensional attention defined in Eq. (5) is known as ”token2token” self attention. It explores dependency between elements of the same source, i.e. query and word from a single source x.

Iii-C Image Feature extraction

In order to obtain features from an image we use a pretrained VGG-19 model. We extracted features of different image regions from the last pooling layer which has a shape of . To simplify the calculations, we resized it to . We have regions with as dimension for each feature vector with . Regional features of a given image are represented by the matrix .

Iii-D Character-based representation

Text extracted from social media is usually informal. In addition, it contain many out of vocabulary words. Character level features can play a crucial role in handling such text. We use a 2D Convolutional Neural Network to learn character embeddings. A word

is transformed into a sequence of characters where is the word length. A convolutional operation of filter size is applied to the matrix , where and is the character embedding size. Then, we compute column-wise maximum operation to get the embedding for word .

Iii-E Attention Guided Visual Attention

Our proposed model uses alignment score between two textual tokens to compute cross-modal attention, show in Fig. 4. Given a word and query , their alignment score is calculated using Eq. (5).

(8)

where is a feature-wise score vector with same length of . We calculate attention between and image feature matrix .

(9)

where is a vector representing a single row of the visual attention scores matrix between and , , , is number of image regions, , , are the weight matrices. To obtain the final visual attention matrix, we compute , such that

Then we normalize the scores by applying Eq. (6) to get a probability distribution (columns-wise) over all regions of image.

(10)
Fig. 3: End-to-end Multi Dimensional Attention based model improving intra-modal attention using visual context for multimodal NER.

The final output is a element-wise product between and .

(11)

where , is a vector containing context vector for .

Fig. 4: Attention Guided Visual Attention neural representation. The output of this model is a single vector that will be concatenated to the others to obtain the final matrix.

Iii-F Gated Fusion

In order to combine alignment score of a word and a query with its visual attention vector we use a gate function to dynamically combine alignment score and visual attention vectors.

(12)
(13)

Throughout our network, we used gated fusion to merge different modalities.

Iv End to End Model

Our end to end network jointly learns intra and cross-modal dependencies. The architecture is shown in Fig. 3.

Given a sequence of words , and image features , we first compute the alignment score between and a query , where ranges from to the length of a sentence. We use Eq. (8) to compute the alignment score . In order to aid intra-text representation , we compute alignment score between and using compatibility function given in Eq. (9).

(14)

We then apply the method described in Eqs. (10)-(11) on to get a weighted sum . Now this is a visual context vector, which will contribute to . This step makes intra-modal relation between and stronger because it also includes image context. Relation between two words will remain the same if textual context is taken into account. But if we included visual context, this relation would become dynamic and more expressive. Thus helping intra-modal attention to learn improved relationships between words.

In order to make visual context vector helpful for , we combine them using gated fusion. This will dynamically select which features to select from various modalities. We applied Eqs. (12)-(13) to get a fused representation between and .

Now we have fused representation representing between query and . Note that, this is calculated for all tokens of a sequence x. We apply Eq. (6) to get a categorical distribution over all tokens of x. Then, element-wise product is computed between and each token of a sequence x to get context vector for query .

(15)

where , is total tokens of a sentence and is vector dimension of each token. Now is a context vector jointly learned from text and visual features. To handle attributes of text component of NER, we also used word representation with , exploiting gated fusion Eqs. (12)-(13) to obtain the final output . Furthermore, is passed through a fully connected layer.

(16)

where is the learnable parameter and

is the bias vector. For tag prediction, final output

is passed to Conditional Random Field (CRF) layer [19].

Iv-a Conditional Random Field

CRF [20] are useful in tasks where output labels have a strong dependency (e.g. I-PER cannot follow B-LOC). Predicting such outputs independently is challenging without correlation between labels and their neighborhood. Given as a text sequence and as a sequence of labels for x, possible labels sequences can be calculated using following equation.

(17)

Where and are potential functions. We use maximum conditional likelihood to learn best parameters that maximize the log-likelihood.

(18)

V Datasets

We used multimodal NER dataset [10]. It contains types of entities {Person, Location, Organization and Misc.} collected from tweets, containing samples for training/val/test sets respectively. Table I shows number of samples per entity. Fig. 1 and  5 show some ambiguous examples.

(a) Finding [California OTHER] [oil OTHER] [spill OTHER]‘s cause could take a month (b) Oil pipeline break dumps crude oil on California [LOC] beach
Fig. 5: Some examples showing how images can help to resolve ambiguities. The word ”California” in (a) and (b) is difficult to understand without looking at the images as it has different tags.
Train Val Test
Person 2217 552 1816
Location 2091 522 1697
Organization 928 247 839
Misc 940 225 726
TABLE I: Details of Dataset

Vi Experiments

We performed various experiments to evaluate the effectiveness of the proposed model on NER multimodal dataset [10]

. Standard Precision, Recall and F1 scores are used as evaluation metrics.

Vi-a Baselines

Vi-A1 Disan

Our model is inspired from Disan, thus we consider this approach as a baseline for the text only evaluation. We used multi-dimensional self attention method to extract context-aware representation of texts. For fair comparison, we keep the same architecture of Fig. 3, but we exclude the “Attention Guided Visual Attention” module. This to analyze differences between the proposed model and multi-dimensional self attention approaches.

Vi-A2 State-of-the-art models

We compare our model with previous state-of-the-art methods leveraging on LSTM networks to capture textual dependencies and visual attention to exploit cross-modal interactions.

Vi-B Word embeddings

We used 300D fasttext crawl embeddings. It contains million word vectors trained with subword information on Common Crawl (B tokens). However, we do not apply fine-tuning on these embeddings during the training stage.

Vi-C Character Embeddings

D character embeddings are trained from scratch using a single layer 2D CNN with a kernel size of .

Vi-D Optimization

We set Adam optimizer with different learning rate initialization: , and . We achieved the best score using the learning rate equal to . Batch size iand dropout keep probability are set and respectively.

(a)
(b)
(c)
Fig. 6: Examples of “attention guided visual attention”. In (a) a graphical representation of the visual attention matrix , for the the alignment score and in (b) a graphical representation of the visual attention matrix for the alignment score . In (c) the visual attention matrix obtained from a different image.

Vii Results

Table II shows comparison of our model with baseline and previous state-of-the-art methods. We achieved best score, outperforming previous approaches.

Vii-a Impact of word embeddings

Word embeddings with different sizes and trained on different corpora have a strong impact on performance. We evaluate the model on two word embeddings to analyze the effect.

  • Twitter Embeddings: we used embeddings trained on million tweets [10]. To compare our results with this work, we used same embeddings to evaluate the improvement that comes from our proposed architecture.

  • Crawl Embeddings: we used D Crawl embeddings [21] trained on B tokens to fully exploit the capability of our model,

Table II shows results with different word embeddings. Our model achieves better results using Twitter Embedding, but there is a clear performance boost with Crawl embeddings. It is interesting to note that entities “PER” and “LOC” have a minor improvement whereas there is an enhancement of 5-6% in “ORG” and “MISC”. With Crawl embeddings, our model performs best to predict the “ORG” category, with a 3% difference from baseline (text only using the Disan model). Our model achieves best F1 scores using both embeddings. This clearly shows that our model performs better regardless to the word embedding.

Vii-B Impact of cross-modal attention

Table II shows that “Attention guided visual attention” boosts Disan (intra-modal attention) performance. We present a qualitatively example in Fig 6 that shows the focus of visual attention. Given an annotated sentence:

S = [Air B-OTHER] [China I-OTHER] [747-8i I-OTHER] [departing O]… [Field O]

an alignment score (inter-modal attention) between two tokens “Air” and “China” is aided by visual context, see Fig. 6 (a). Our model can successfully focus on related image regions, strengthening the relation between two words. Whereas alignment score between “Air” and “Field” and visual context (Fig. 6 (b)) has less relation because word “Air” has higher dependency on “China” then on “Field” when labeling “Air” as “B-OTHER”. Similarly in order to verify that relation between two tokens varies according to image, we changed the image and kept same sentence. Relation between fake image and alignment score between two tokens “Air” and “China“ can be seen in Fig. 6(c) with no prominent relation, proving that our model learns better relationships between words given an image.

Similarly, Fig. 7 shows some examples in which our model pay attention to related image regions. It clearly shows that our model focuses on certain parts of image which are beneficial for named entity task. Fig. 7(a) shows that attention is payed only to car to predict correct tag for “Mercedes” and “Benz”. Similarly Fig. 7(b) shows that “Opera House” is a building and not a location, and our models correctly identifies it by paying attention to correct image regions.

PER
F1
LOC
F1
ORG
F1
MISC
F1
Overall
Prec. Recall F1
T-NER ([11) 83.64 76.18 50.26 34.56 69.54 68.65 69.09
Adaptive Co-Attention Network [10] 81.98 78.95 53.07 34.02 72.75 68.74 70.69
Disan [18] (Twitter Embedding) 82.07 76.87 55.34 32.29 71.00 70.53 70.77
Our Model (Twitter Embedding) 82.83 78.22 55.88 33.00 72.81 70.33 71.55
Disan [18] (Crawl Embedding) 83.03 77.96 56.66 39.54 71.65 71.89 71.77
Our Model (Crawl Embedding) 83.98 78.65 59.27 39.54 73.50 72.33 72.91
TABLE II: Comparison of our approach with baselines and previous state-of-the-art methods.

Vii-C Error Analysis

In Fig. 8, we show some examples where our approach fails because of the following reasons:

  • Unrelated image : Text information do not match with an image, as we can see in Fig. 8(a), ”Reddit” belongs to ”Other” but unrelated image caused wrong attention and it results to wrong prediction ”ORG”.

  • Wrong attention: Fig. 8(b) shows an example where text and image are aligned correctly, but wrong attention results in wrong tag prediction. Words ”Mount” and ”Sherman” were tagged as ”Person” as most of attention is on persons, whereas expected tag was ”LOC”.

(a) Mercedes OTHER Benz OTHER (b) photo of Sydney LOC Opera OTHER House OTHER
Fig. 7: Two example of correct visual attention. Our model successfully highlights related image regions required to predict correct tag.
(a) Reddit ORG needs to stop pretending (b) teachers on top of Mount PER Sherman PER
Fig. 8: Two examples of wrong visual attention: (a) shows an unrelated image and a wrong prediction, while (b) shows a related image with wrong attention and prediction.

Viii Conclusion

We introduced a novel model for multimodal NER. It extends multi-dimensional self attention approaches enhancing intra-text relationships using visual features. Qualitative examples show that our model successfully captures correct relations between words and images removing ambiguities caused by the text. Our model is flexible and can be further extended to other multimodal tasks. Experiments show that our model achieved state-of-the-art results on multimodal NER dataset.

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