Image Captioning with Visual Object Representations Groundedin the Textual Modality1

Dušan Variš1∗,∗ Katsuhito Sudoh2,3 , Satoshi Nakamura2,3Charles University, Faculty of Mathematics and Physics, Czechia
2
Nara Institute of Science and Technology, Japan
3
RIKEN AIP, Japanvaris@ufal.mff.cuni.com{sudoh,s-nakamura}@is.naist.jp
Abstract

We present our work in progress exploring thepossibilities of a shared embedding space between textual and visual modality. Leveraging
the textual nature of object detection labels andthe hypothetical expressiveness of extracted visual object representations, we propose an approach opposite to the current trend, grounding of the representations in the word embedding space of the captioning system instead ofgrounding words or sentences in their associated images.
Based on the previous work, we apply additional grounding losses to the image captioning training objective aiming to force visualobject representations to create more heterogeneous clusters based on their class label andcopy a semantic structure of the word embedding space. In addition, we provide an analysisof the learned object vector space projection
and its impact on the IC system performance.
With only slight change in performance,
grounded models reach the stopping criterion
during training faster than the unconstrainedmodel, needing about two to three times lesstraining updates. Additionally, an improvement in structural correlation between theword embeddings and both original and projected object vectors suggests that the grounding is actually mutual.

1

Introduction

The task of image captioning (IC) or image description generation focuses on generating a sentence
describing a given input image. Combining thetasks from computer vision and natural language
generation, IC has become an important task forthe currently dominant deep learning research. The
emergence of image captioning datasets such as
MSCOCO (Lin et al., 2014) or Flick30k (Plummer∗* Contribution during the student internship at RIKEN
AIP.

et al., 2017) has also greatly helped the research
progress in this area.
Throughout the years, several captioning approaches have been suggested. In the early daysof neural IC, the systems were trained mainly inthe end-to-end fashion using an output of a convolutional neural network (CNN) for conditioning asequential language model to generate an output
caption. The CNN output consisting of some midlevel image features was either used to initialize
the language model decoding (Vinyals et al., 2017)or fed directly into each decoding step via attention
mechanism (Xu et al., 2015). Despite its good performance, these models are hard to interpret andit has been shown that they mainly exploit distributional similarity in the multimodal feature space(Madhyastha et al., 2018).
Recently, several methods that suggest using information about the objects present in the image
were revisited. Object information is usually extracted via an object detection network (Girshicket al., 2014; Ren et al., 2015; Redmon et al., 2016)
and consists of a set of bounding boxes locating
the objects and labels that describe them. Treatingsuch labels as a simple bag-of-objects (BOO) representations Wang et al. (2018) or using a contextdependent encoding of the detected objects Yin
and Ordonez (2017) and using them to initialize
the language model can outperform IC systems
using CNN features as their input. Also, using explicitly detected objects makes these models easier
to interpret.
Even though the detection-based IC systems
show promising results, by design, they are boundto fail at distinguishing images with identical BOO
representations as shown in Figure 1. The image contains the same number of detected animals(dogs and cows) with different location distribution.
Even though we can alleviate the BOO identity byincorporating the location (Yin and Ordonez, 2017)

the learned VOR projections and their relationshipwith the IC captioning performance. The paper is
structured as follows: In section 2, we provide abrief overview of the previous work on multimodal
grounding and shared embedding space representational learning. In section 3, we describe ourmethod. In section 4, we describe the settings andresults of our experiments and the learned projection space analysis. Section 5 concludes the paper.

2

Figure 1: Example of two images where an identicalsets of objects were detected even though the actions
related to the objects differ (e.g. upper: dog jumping,
lower: dog lying down).

and size (Wang et al., 2018) of the objects into themodel, it should be impossible for the model tocapture details about the actions tied to the objects(dog jumping vs dog lying down). Using a morerobust (detailed) object detector might help to solve
this problem, but we argue that it is often too costly
and task-specific.
Instead of using embedded object detection labels, Anderson et al. (2018) proposed a method
using intermediate object representations (called
bottom-up attention) extracted from the CNN features via their object bounding boxes. According
to their results, these object features are more effective than the “raw” CNN features. We also hypothesise that these features are more descriptive
than the combination of object labels and boundingboxes. The potential problem of this approach isthat the parts of the network (CNN, object detection network) are trained independently which canlead to different representational spaces for each
modality. This can be partially fixed by end-to-end
fine-tuning of the whole captioning pipeline, but
again this can be quite costly.
In this work we propose a novel method ofgrounding of the pretrained intermediate visualobject representations (VOR) in the textual embedding space. We also focus on the analysis ofRelated Work

Currently, there is an ongoing research studying
shared multimodal representation learning. Inspired by human perception, the aim is to find a way
to comprehensively represent information from various sensory inputs. In the case of deep learning,these sensory inputs are usually represented by different parts of the neural network model. This isoften difficult to accomplish either due to a factthat we usually train these parts of the network separately on different tasks or by a lack of constraints
that would force the network to learn better semantically correlated representations between themodalities, the latter being referred to as a heterogeneity gap (Guo et al., 2019).
In the case of combining a visual and textual
modality, current research focuses mainly on thenatural language grounding in its visual counterpart.
This is motivated by a difference in occurrence
between the textual representation of objects andtheir relations and their actual real-world frequency(Gordon and Van Durme, 2013). Majority of thecurrent work therefore focus on reducing this bias
between text and reality.
There are generally two approaches towards the
textual grounding: a word-level and a sentencelevel approach. The word-level methods either
learn grounded representations separately for each
modality first and combine them later (Silberer andLapata, 2014; Collell et al., 2017) or learn themjointly from multiple sources (Hill et al., 2016).
This can be done by taking a standard word embedding objective, e.g. continuous bag-of-words(Mikolov et al., 2013a) or skip-gram (Mikolov
et al., 2013b) and introducing an additional training objective to force the word vectors to imitate
the structure of the related visual features (Lazaridou et al., 2015) or visual context (Zablocki et al.,2018).
The sentence-level methods usually try to learn
grounded sentence representations by adding an ad-ditional training objective for predicting visual features associated with the sentence (Chrupała et al.,2015; Kiela et al., 2018). They accomplish thisvia cross-modal projection, however, according toCollell and Moens (2018), the learned cross-modal
mapping does not properly preserve the structureof the original representation space. To address thisissue, Bordes et al. (2019) suggest using an intermediate grounded space as it weakens the imposedconstraint on the textual representations.
The currently most common approach towards
IC is to condition a language model on the intermediate visual features extracted by an image processing network. These features can be either generalfeature map or bounded to objects detected inside
the picture. Ideally, we would like the language
model to extract important details about the input
from these features, however, Madhyastha et al.
(2018) show that these models mostly exploit a
distributional similarity in the multimodal feature
space, finding the closest training examples in themultimodal space and combining their captions to
describe the test image.
In spite of that, we think that intermediate VORs
extracted by an object detection network can contain more useful information than we are currently
able to exploit. Since word embeddings are a crucial part of the IC decoder, we assume that bygrounding object features in the word embedding
space using the embeddings of their labels can help
the IC decoder to better extract structural information about the input image. This idea seems
even more plausible with the attention-based IC
decoders (Xu et al., 2015; Anderson et al., 2018),
where decoder learns to shift its focus to different
parts of an image during the caption generation
process.

3

Proposed Method

In this section, we describe our proposed method
starting from its model architecture, the motivation
behind grounding of VORs in a word embedding
space and the investigated grounding methods.
3.1

Model

Our model implementation is based on the one proposed by Anderson et al. (2018). For a given image,the model takes an input set of k feature vectors
v = {v1 ..vk }, vi ∈ Rdin , where din is the vector
dimension. These vectors can be extracted either
directly from the object detection network, such as

Figure 2: Overview of the the IC decoder architecture used during our experiments, similar to Anderson
et al. (2018). Instead of using pretrained feature vectors
v = {v1 ..vk } directly we use their trainable projectionz = {z1 ..zk }.

Faster RCNN (Ren et al., 2015) or by cropping out
the relevant object regions from the CNN feature
map using methods such as region-of-interest (RoI)
pooling (Girshick et al., 2014).
Since we use only fixed image/object feature
vectors in this work, we want to learn a following
transformation f (·) of the input:
zi = f (vi )(1)where zi ∈ Rd , d being a dimension of the projected vector space, is a feature representation specific for our image captioning task. The transformation f (·) can be either linear transformation ora MLP.
The transformed features z = {z1 ..zk } arethen passed to the LSTM decoder (Hochreiter andSchmidhuber, 1997). The decoder consists of two
stacked LSTM layers; a top-down attention layer
and a language model layer. Figure 2 shows aschematic overview of the model.
At time step t ∈ {1..T }, we encode a previously
generated output symbol yi−1 using an embedding
matrix We ∈ Rd×|V | , where V is the captioning
symbol vocabulary, producing an input embedding
vector xt :xt = We φ(yt−1 )(2)where φ converts the symbol yi−1 to its corresponding one-hot encoding. We then pass a concatenation of the input vector
P xt , mean-pooled transformed features z = k1 i zi , and a previous hid-den state from the second layer h2t−1 to the topdown attention LSTM layer:
h1t = LST M 1 ([xt , z, h2t−1 ], h1t−1 )(3)

The top-down LSTM output h1t is then passed
to an attention layer together with the transformedfeature vectors producing a context vector ct :ct = Att(h1t , z)(4)where Att(·) is an attention mechanism (Bahdanau
et al., 2015). Following the previous work, we
use the concat attention described in Luong et al.
(2015).
A concatenated output of the attention layer ct
and the top-down LSTM layer h1t is the passed to
the language LSTM layer producing a hidden state
h2t :h2t = LST M 2 ([ct , h1t ], h2t−1 )(5)Using the second hidden state, we produce a conditional probability distribution over the symbol
vocabulary using a following transformation:p(yt |y1:t−1 ) = Sof tmax(Wo h2t + bo )(6)

where Wo ∈ R|V |×d is output projection matrix
and bo ∈ R|V | is bias. Getting the output probability distribution for each decoding time step, we
can compute the distribution over the entire caption
space as their product over the time steps:p(y1:T ) =T
Yp(y|y1:t−1 )

(7)

t=1

We initialize y0 with special beginning of sentence symbol and h10 , h20 with zero values.
3.2

Grounding methods

The main idea behind the VOR grounding is totransform the object features in such a way that the
IC decoder can extract necessary information inan easier way. This approach improves the performance on downstream tasks when the learned projection is between two spaces with a same modality(Artetxe et al., 2016, 2018), however to our knowledge, it is still unclear how well this can be applied
when different modalities are involved.
We decided to perform an initial experimentwith a simple IC system. We used MSCOCO (Linet al., 2014) with Karpathy splits (Karpathy andLi, 2015)1 , using training dataset for training and1
http://cs.stanford.edu/people/
karpathy/deepimagesent

development dataset for training validation. For
each example, we extracted a feature map using
Oxford VGG16 (Simonyan and Zisserman, 2015)
pretrained on ImageNet (Russakovsky et al., 2015)
with fixed weights. Then we applied ground truth
bounding boxes from the MSCOCO dataset to poolthe object features out of the feature map.
The IC system used a following function for
input projection:f (vi ) = Win vi(8)

The size of the projection, embeddings and LSTM
were 512. We used batch size 100, learning rate
2e-3 and applied early stopping when the CIDEr(Vedantam et al., 2014) score on the development
dataset did not improve for 10 epochs.
We then looked at the learned embeddings andthe input projection. To analyze the embeddings,
we extracted a subset of 65 words that overlap with
the MSCOCO object detection labels. We analyzed object vectors extracted from the instancesin the development dataset. Since each object classcontains a various number of object vectors, we
decided to compute object centroids for each classas the class representation. We used t-SNE (van der
Maaten and Hinton, 2008) for vector visualization.
Figure 3 shows a proximity comparison between
the object centroid space to the word embedding
space (left) and a cluster overlap of the class vectors
for a chosen subset of classes (right). We compare
the VORs before the linear projection (top) andafter its application.
We can see, that without any implicit constraints,
the IC system tries to learn a mapping that closes
the distance between the object representations andword embeddings. However, both representations
differ in the way they model relationships between
the classes (e.g. carrot being close to the animals
in the centroid cluster and distant in the word embedding cluster when projected by t-SNE). Furthermore, we can see that the learned input projectionis unable to explicitly separate clusters of objects
with a same class label. We hypothesise that by
explicitly learning to separate these clusters we can
improve the model performance.
Based on these observations, we decided to apply an idea presented by Bordes et al. (2019) andground the projected VOR space in the word embedding space using their grounding objectives.
Similarly, we constrain our input projection function f (vi , θf ) by an additional grounding objective:

Figure 3: A visualization of the distance between the object centroids and word embeddings (two figures in the left
column) and overlap of the object vector clusters from various class (right column). The comparison is between
the object representations/centroids before linear projection (top half) and after applying learned linear projection(bottom half). The projection and the word embeddings were extracted by our initial IC model.

Lg (θg ) = αC LC (θf ) + αP LP (θf )(9)where LC and LP are the losses introducedin the original paper, αC and αP are the lossweighting parameters and θf ⊂ θ, θ being trainable
parameters of the whole network.
The cluster information loss LC is a max-margin
ranking loss (Karpathy and Li, 2015; Carvalho
et al., 2018) with the following formulation:
LC =

1 X
bγ − cos(zi , zj ) + cos(zi , zk )c+N(zi ,zj ,zk )c(zi )=c(zj )c(zi )6=c(zk )(10)where c(·) denotes a class of the object vector.
Due to the memory constraints, in practice we sample a subset of triplets (zi , zj , zk ) of size N from a
training batch and normalize the loss sum by the
number of valid triplets per sample. Using this loss
we aim to explicitly separate clusters of object features according to their classes and examine what
effect it has on the learning abilities of the image
captioning system.

The cluster loss alone does not provide any direct basis for the grounding in the textual modality.
Our hypothesis is that a separation of VORs withdifferent class labels without any structure can hurt
the performance since it disregards actual relations
between the classes. For example in Figure 3, eventhough we want the representations of dog, cat andtrain to be in separate cluster, we still want most of
the time to have dog closer to cat rather then the
train because they are semantically closer.
The perceptual information loss LP aims tosolve this problem by modeling relations between
the object representations based on the relationsof the word embeddings of their labels. Bordes
et al. (2019) express these relations through vector
similarities, formulating the following loss:
text
LP = −ρ({simobj
i,j }, {simi,j })(11)where ρ is Pearson correlation. The similarity functext
tions simobj
i,j and simi,j are defined as follows:
simobji,j = cos(zi , zj )simtext
i,j = cos(We φ(c(zi )), We φ(c(zj )))(12)

Similar to cluster loss, we sample a subset of object vectors from the batch. Note that we compute
similarity correlation only using vector pairs thatbelong to different classes to avoid further clustering this process might lead to.

4

Experiments

4.1

Experiment setup

Similar to our initial run, we use MSCOCO dataset
in our experiments with the Karpathy splits. During
training, we use CIDEr score on the validation split(5,000 images) as the early stopping criterion and
we report results measured on the test split (another
5,000 images). We use a default caption preprocessing available at the NeuralTalk2 (Karpathy andLi, 2015) codebase2 , resulting in a captioning vocabulary of size around 9570 tokens.
In all our following experiments, we use thefollowing network parameters:• batch size: 100• optimizer: Adam (Kingma and Ba, 2015)• initial learning rate: 2e-3
• exponential decay: 0.8, every 6000 steps• gradient clipping (by norm): 1.0• dropout: 0.2• maximum sentence length: 16
When we compute the LC and LP , we sample
N = 500 object vector instances from a current
batch. During training, we use teacher forcing(Sutskever et al., 2014) where at each time step we
use a ground truth token as an input instead of aprevious network output. We use greedy decoding during inference, outputting a token with thehighest probability at each time step. This is toremove any other variables from our experiments.
In practice, sampling techniques like beam search
are commonly used to avoid phenomena such as
exposure bias (Bengio et al., 2015; Ranzato et al.,2016).
In our experiments, we investigate two scenarios
based on the process of the object feature extraction, CNN+SSD and Bottom Up+SSD.
2
https://github.com/karpathy/
neuraltalk2

CNN+SSD. We use a Single Shot Multibox
Detector (SSD) network available at Tensorflow
Object Detection API (Huang et al., 2017).3 We
decided to use the ssd mobilenet v2 coco model
pretrained on the MSCOCO dataset mainly becauseof its fast performance. The object detector output
consists of object bounding boxes and their labels,
however, it does not provide us with any explicit
object representation vectors. Therefore, we use
the bounding boxes to extract object representations from the output of VGG16 (Simonyan andZisserman, 2015) similar to our initial experiment.
We use the 14x14x512 feature map extracted fromthe last convolutional layer before applying max
pooling. Then, we apply RoI pooling on the feature
map using the bounding boxes to extract the VORs.
Bottom Up+SSD. We use pretrained object
features provided by Anderson et al. (2018).4 They
extract the features using Faster RCNN network
initialized with ResNet-101 (He et al., 2016) pretrained on ImageNet (Russakovsky et al., 2015)
which they fine tune on the Visual Genome (Krishna et al., 2017) dataset. They further modify theobject classifier to also predict attribute classes forevery detected object. The Visual Genome dataset
contains a very large number of object and attributeclasses, however, the authors use only a subset of1600 object classes and 400 attribute classes for themodel training.
The downside of using these pretrained features
is a fact that the authors provide only the VORs andtheir bounding box information, however, they do
not extract any object nor attribute labels. To assign
the MSCOCO labels to the extracted VORs we
compute an intersection-over-union (IoU) between
the pretrained representation bounding boxes andthe bounding boxes of the objects extracted by the
SSD network we used in the CNN+SSD scenario.
Each object representation then gets assigned alabel with the highest IoU value. If there is an IoU
overlap of 0 with every SSD bounding box, we
assign an UNK label to the VOR vector.
In both scenarios, we provide a comparison witha baseline setting which is only optimized for the
standard captioning cross-entropy objective. It optimizes the trainable parameters θ using the ground3
https://github.com/tensorflow/models/tree/master/research/object_detection
4
https://github.com/peteanderson80/
Up-Down-Captioner

BLEU-1

BLEU-2

BLEU-3

BLEU-4

ROUGE-L

CIDEr

CN N + SSD

64.6

46.0

31.8

22.0

47.1

68.9

+LC
+LP
+LC + LP

65.0
63.2
64.5

46.1
44.1
45.4

32.0
29.8
31.1

22.3
20.1
21.4

47.0
45.6
46.6

69.4
65.5
68.5

Bottom U p + SSD

65.0

46.4

32.3

22.4

47.1

70.5

+LC
+LP
+LC + LP

64.6
64.5
64.6

46.0
45.9
46.2

31.9
31.8
32.2

22.1
22.0
22.3

46.9
46.6
47.2

68.7
68.5
68.8

One-Hot(SSD)

63.3

43.9

29.5

20.3

45.6

66.2

Table 1: Comparison of the performance between the suggested model baselines (CNN-SSD, Bottom Up+SSD)
and their combination with the cluster loss (LC ), perceptual loss (LP ) or their combination (LC + LP ). We
include One-Hot model for comparison. Results outperforming their respective baseline are in bold.

∗ :truth caption sequence y1:T
T

LXE =

1X
∗
log(pθ (y ∗ |y1:t−1
))
T

(13)

t=1

We normalize the sum by the caption length tosimplify its combination with the grounding losses.
We then compare a model variations trained using
LXE + LC , LXE + LP and LXE + LC + LPwith the baseline. For the loss parameters, we set
γ = 0.5 and αC = αP = 1.0.
For contrast, we also compare our results with a
One-Hot model which only uses one-hot label representations extracted by the SSD object detector.
4.2

Results

Beside CIDEr, we used BLEU (Papineni et al.,2002) and ROUGE-L (Lin, 2004) metrics that arecommonly used in the IC literature. We used thesame implementation as in NeuralTalk2.5
Table 1 shows a comparison between the described models. For CNN+SSD, we gained a small
improvement using the clustering objective LC .
That means that even objects cropped from a general feature map by a separately trained object detector can benefit from properly constrained input
projection. However, the perceptual loss seems tohave a negative effect on the overall system performance, both alone (LP ) and leads to small dropin performance when combined with the cluster
loss (LC + LP ). However, Figure 4 shows thatthe grounded models reach the early stopping cri5

https://github.com/tylin/coco-caption

terion about two to three times faster making theirtraining much efficient.
The results are worse for the Bottom Up+SSD
where an additional training losses did not have any
positive effect at all. We suspect that the main reason might be mislabeling of the pretrained object
vectors because we used an SSD with lower rate ofdetection together with a portion of VOR vectors
being assigned the generic UNK label.
Following Bordes et al. (2019), we also provide
results of a structural analysis in Table 2. Notethat we compute the mean nearest neighbor overlap(mNNO) (Collell and Moens, 2018) as a proportionneighbor overlap between the word embeddings
and object feature centroids of clusters for each
MSCOCO label. Other metrics are the same as theoriginal paper.
Surprisingly, it seems that cluster homogeneity
or a distinction between clusters is not crucial for
the IC task. The unconstrained model seems tolearn structure regardless of the vector clustering.
On the other hand, the structure of the projected
object vectors (mNNO) are always closer to thestructure of the word embeddings regardless ofwhether we use the grounding losses or not.
The cluster loss was able to efficiently separate
the clusters with different labels (Cinter = 23.8
and 25.3 in both scenarios) while still maintainingsome cluster homogeneity (Cintra = 43.4 and 48.9
vs the Cintra = 6.8 and 3.7 in the baselines).
The perceptual loss managed to achieve the best
results regarding the mNNO between the centroids
and word embeddings while disabling the default
unclustering behaviour of the baseline. This in-CIDErmNNO

ρvis

CinterCintraCN N + SSD

68.9

24.6 / 31.7

2.5 / 6.0

1.0 (85.8)

6.8 (87.5)+LC+LP+LC + LP

69.4
65.5
68.5

31.2 / 32.3
32.8 / 41.0
31.2 / 36.9

4.5 / -2.3
15.2 / 26.2
15.5 / 53.1

23.8
75.2
42.0

43.4
79.6
62.3

Bottom U p + SSD

70.5

28.7 / 35.8

0.7 / 5.3

0.7 (86.5)

3.7 (87.1)+LC+LP+LC + LP

68.7
68.5
68.8

26.1 / 31.2
32.8 / 37.9
31.2 / 34.8

0.7 / 8.0
6.1 / 10.6
6.4 / 32.1

25.3
86.5
67.0

48.9
87.6
77.2

Table 2: Analysis of the structure of the learned projection space. mNNO and ρvis compare the original/projected
object embedding space. Values in brackets (Cinter , Cintra ) are related to the original non-projected object spaces.
The best results outperforming the baseline are in bold. We include their respective CIDEr scores for comparison.

5

Conclusion

We presented a method for grounding of object
feature vectors extracted by an object detection network in a word embedding space learned by the
IC system and investigated a hypothesis whether
such grounding can improve said IC system performance. We used previously proposed grounding
losses to enforce clustering of the object features
based on their class label and a distance correlation
loss to enforce a structural similarity between thesefeatures and corresponding label embeddings.
The results indicate that while having similar performance measured by the automatic metrics, thegrounded models converge about two to three times
faster than the original baseline. Furthermore, thestructural analysis showed that the unconstrained

0.70

0.65
CIDEr

ability to separate the initial clusters could havebeen the reason for the lower CIDEr scores. The
combination of both losses then led to the best
structural similarity correlation (ρvis ). Interestingobservation is the overall increase in mNNO andvisual correlation between the original VOR vectors and the learned word embeddings when the
grounding losses are applied. This suggests thatthe grounding between the modalities is mutual:
not only the VOR vectors are being grounded in
the textual modality, the word embeddings are also
being grounded in the projected VOR space.
The results of the structural analysis are similar
between CNN-SSD and Bottom Up-SSD while being slightly worse for the latter in the mNNO andρvis . Again, this is probably due to a sub-optimallabel assignment for the pretrained bottom-up features.

0.60
baseline+LC+LP+LC + LP

0.55

0.5

1
1.5
2
Training time (steps)

2.5
·105

Figure 4: Model performance on validation data during
CNN+SSD training showing faster convergence of thegrounded models.

IC system completely ignores cluster informationand is inclined to learn a projection that unclusters
the object features regardless of their class. Finally,
the proposed grounding methods enforce a mutual
grounding between the projected object features
and the word embedding space.
In future, we plan to confirm the results witha more descriptive set of object features properly
trained on a larger set of object labels such as Visual Genome, to rule-out a possibility that our results were biased by the coarseness of the relatively
small set of labels in MSCOCO dataset. We alsoplan to further investigate the mutual grounding
using different layers of textual representation, e.g.
intermediate or output states of the caption decoder.

References
Peter Anderson, Xiaodong He, Chris Buehler, Damien
Teney, Mark Johnson, Stephen Gould, and Lei
Zhang. 2018. Bottom-Up and Top-Down Attention
for Image Captioning and Visual Question Answering. In CVPR.
Mikel Artetxe, Gorka Labaka, and Eneko Agirre. 2016.
Learning principled bilingual mappings of word embeddings while preserving monolingual invariance.
In Proceedings of the 2016 Conference on Empirical Methods in Natural Language Processing, pages
2289–2294, Austin, Texas. Association for Computational Linguistics.
Mikel Artetxe, Gorka Labaka, and Eneko Agirre. 2018.
Unsupervised statistical machine translation. In Proceedings of the 2018 Conference on Empirical Methods in Natural Language Processing, pages 3632–
3642, Brussels, Belgium. Association for Computational Linguistics.
Dzmitry Bahdanau, Kyunghyun Cho, and Yoshua Bengio. 2015. Neural Machine Translation by Jointly
Learning to Align and Translate. In (Bengio and LeCun, 2015).
Samy Bengio, Oriol Vinyals, Navdeep Jaitly, andNoam Shazeer. 2015. Scheduled sampling for sequence prediction with recurrent neural networks.
In Proceedings of the 28th International Conference
on Neural Information Processing Systems - Volume 1, NIPS’15, pages 1171–1179, Cambridge, MA,
USA. MIT Press.
Yoshua Bengio and Yann LeCun, editors. 2015. 3rd
International Conference on Learning Representations, ICLR 2015, San Diego, CA, USA, May 7-9,
2015, Conference Track Proceedings.
Patrick Bordes, Eloi Zablocki, Laure Soulier, Benjamin
Piwowarski, and patrick Gallinari. 2019. Incorporating Visual Semantics into Sentence Representations within a Grounded Space. In Proceedingsof the 2019 Conference on Empirical Methods in
Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP), pages 696–707, Hong
Kong, China. Association for Computational Linguistics.
Micael Carvalho, Rémi Cadène, David Picard, Laure
Soulier, Nicolas Thome, and Matthieu Cord. 2018.
Cross-Modal Retrieval in the Cooking Context:
Learning Semantic Text-Image Embeddings. In The
41st International ACM SIGIR Conference on Research & Development in Information Retrieval, SIGIR ’18, pages 35–44, New York, NY, USA. ACM.
Grzegorz Chrupała, Ákos Kádár, and Afra Alishahi.
2015. Learning Language through Pictures. In Proceedings of the 53rd Annual Meeting of the Association for Computational Linguistics and the 7th International Joint Conference on Natural Language

Processing (Volume 2: Short Papers), pages 112–
118, Beijing, China. Association for Computational
Linguistics.
Guillem Collell and Marie-Francine Moens. 2018.
Do Neural Network Cross-Modal Mappings Really
Bridge Modalities? In Proceedings of the 56th Annual Meeting of the Association for Computational
Linguistics (Volume 2: Short Papers), pages 462–
468, Melbourne, Australia. Association for Computational Linguistics.
Guillem Collell, Ted Zhang, and Marie-Francine
Moens. 2017. Imagined Visual Representations As
Multimodal Embeddings. In Proceedings of theThirty-First AAAI Conference on Artificial Intelligence, AAAI’17, pages 4378–4384. AAAI Press.
Ross Girshick, Jeff Donahue, Trevor Darrell, and Jitendra Malik. 2014. Rich Feature Hierarchies for
Accurate Object Detection and Semantic Segmentation. In Proceedings of the 2014 IEEE Conference
on Computer Vision and Pattern Recognition, CVPR
’14, pages 580–587, Washington, DC, USA. IEEE
Computer Society.
Jonathan Gordon and Benjamin Van Durme. 2013.
Reporting Bias and Knowledge Acquisition. In
Proceedings of the 2013 Workshop on AutomatedKnowledge Base Construction, AKBC ’13, pages
25–30, New York, NY, USA. ACM.
Wenzhong Guo, Jianwen Wang, and Shiping Wang.
2019. Deep Multimodal Representation Learning:
A Survey. IEEE Access, 7:63373–63394.
K. He, X. Zhang, S. Ren, and J. Sun. 2016. Deep Residual Learning for Image Recognition. In 2016 IEEE
Conference on Computer Vision and Pattern Recognition (CVPR), pages 770–778.
Felix Hill, Kyunghyun Cho, and Anna Korhonen.
2016. Learning Distributed Representations of Sentences from Unlabelled Data. In Proceedings of the2016 Conference of the North American Chapter ofthe Association for Computational Linguistics: Human Language Technologies, pages 1367–1377, San
Diego, California. Association for Computational
Linguistics.
Sepp Hochreiter and Jürgen Schmidhuber. 1997. Long
Short-Term Memory. Neural Comput., 9(8):1735–
1780.
J. Huang, V. Rathod, C. Sun, M. Zhu, A. Korattikara,
A. Fathi, I. Fischer, Z. Wojna, Y. Song, S. Guadarrama, and K. Murphy. 2017. Speed/Accuracy TradeOffs for Modern Convolutional Object Detectors. In
2017 IEEE Conference on Computer Vision and Pattern Recognition (CVPR), pages 3296–3297.
Andrej Karpathy and Fei-Fei Li. 2015. Deep visualsemantic alignments for generating image descriptions. In IEEE Conference on Computer Vision and
Pattern Recognition, CVPR 2015, Boston, MA, USA,
June 7-12, 2015, pages 3128–3137. IEEE Computer
Society.

Douwe Kiela, Alexis Conneau, Allan Jabri, and Maximilian Nickel. 2018. Learning Visually Grounded
Sentence Representations. In Proceedings of the2018 Conference of the North American Chapter ofthe Association for Computational Linguistics: Human Language Technologies, Volume 1 (Long Papers), pages 408–418, New Orleans, Louisiana. Association for Computational Linguistics.
Diederik P. Kingma and Jimmy Ba. 2015. Adam: A
Method for Stochastic Optimization. In (Bengio and
LeCun, 2015).
Ranjay Krishna, Yuke Zhu, Oliver Groth, Justin Johnson, Kenji Hata, Joshua Kravitz, Stephanie Chen,
Yannis Kalantidis, Li-Jia Li, David A. Shamma,
Michael S. Bernstein, and Li Fei-Fei. 2017. Visual Genome: Connecting Language and Vision Using Crowdsourced Dense Image Annotations. Int. J.
Comput. Vision, 123(1):32–73.
Angeliki Lazaridou, Nghia The Pham, and Marco Baroni. 2015. Combining Language and Vision witha Multimodal Skip-gram Model. In Proceedings ofthe 2015 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, pages 153–
163, Denver, Colorado. Association for Computational Linguistics.
Chin-Yew Lin. 2004. ROUGE: A package for automatic evaluation of summaries. In Text Summarization Branches Out, pages 74–81, Barcelona, Spain.
Association for Computational Linguistics.
Tsung-Yi Lin, Michael Maire, Serge Belongie, James
Hays, Pietro Perona, Deva Ramanan, Piotr Dollár,
and C. Lawrence Zitnick. 2014. Microsoft COCO:
Common Objects in Context. In Computer Vision –ECCV 2014, pages 740–755, Cham. Springer International Publishing.
Thang Luong, Hieu Pham, and Christopher D. Manning. 2015. Effective Approaches to Attentionbased Neural Machine Translation. In Proceedings of the 2015 Conference on Empirical Methods in Natural Language Processing, pages 1412–
1421, Lisbon, Portugal. Association for Computational Linguistics.
Pranava Swaroop Madhyastha, Josiah Wang, and Lucia Specia. 2018. End-to-end Image Captioning
Exploits Distributional Similarity in Multimodal
Space. In British Machine Vision Conference 2018,
BMVC 2018, Northumbria University, Newcastle,
UK, September 3-6, 2018, page 306. BMVA Press.
Tomas Mikolov, Kai Chen, Greg Corrado, and Jeffrey
Dean. 2013a. Efficient Estimation of Word Representations in Vector Space. In 1st International Conference on Learning Representations, ICLR 2013,
Scottsdale, Arizona, USA, May 2-4, 2013, Workshop
Track Proceedings.

Tomas Mikolov, Ilya Sutskever, Kai Chen, Greg Corrado, and Jeffrey Dean. 2013b. Distributed Representations of Words and Phrases and Their Compositionality. In Proceedings of the 26th International
Conference on Neural Information Processing Systems - Volume 2, NIPS’13, pages 3111–3119, USA.
Curran Associates Inc.
Kishore Papineni, Salim Roukos, Todd Ward, and WeiJing Zhu. 2002. Bleu: a method for automatic evaluation of machine translation. In Proceedings ofthe 40th Annual Meeting of the Association for Computational Linguistics, pages 311–318, Philadelphia,
Pennsylvania, USA. Association for Computational
Linguistics.
Bryan A. Plummer, Liwei Wang, Chris M. Cervantes,
Juan C. Caicedo, Julia Hockenmaier, and Svetlana Lazebnik. 2017. Flickr30K Entities: Collecting Region-to-Phrase Correspondences for Richer
Image-to-Sentence Models. Int. J. Comput. Vision,
123(1):74–93.
Marc’Aurelio Ranzato, Sumit Chopra, Michael Auli,
and Wojciech Zaremba. 2016. Sequence level training with recurrent neural networks. In 4th International Conference on Learning Representations,
ICLR 2016, San Juan, Puerto Rico, May 2-4, 2016,
Conference Track Proceedings.
Joseph Redmon, Santosh Kumar Divvala, Ross B. Girshick, and Ali Farhadi. 2016. You Only Look Once:
Unified, Real-Time Object Detection. In 2016 IEEE
Conference on Computer Vision and Pattern Recognition, CVPR 2016, Las Vegas, NV, USA, June 27-30,
2016, pages 779–788. IEEE Computer Society.
Shaoqing Ren, Kaiming He, Ross Girshick, and Jian
Sun. 2015. Faster R-CNN: Towards Real-time Object Detection with Region Proposal Networks. In
Proceedings of the 28th International Conference on
Neural Information Processing Systems - Volume 1,
NIPS’15, pages 91–99, Cambridge, MA, USA. MIT
Press.
Olga Russakovsky, Jia Deng, Hao Su, Jonathan Krause,
Sanjeev Satheesh, Sean Ma, Zhiheng Huang, Andrej Karpathy, Aditya Khosla, Michael Bernstein,Alexander C. Berg, and Li Fei-Fei. 2015. ImageNet
Large Scale Visual Recognition Challenge. Int. J.
Comput. Vision, 115(3):211–252.
Carina Silberer and Mirella Lapata. 2014. Learning
Grounded Meaning Representations with Autoencoders. In Proceedings of the 52nd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers), pages 721–732, Baltimore, Maryland. Association for Computational Linguistics.
Karen Simonyan and Andrew Zisserman. 2015. Very
Deep Convolutional Networks for Large-Scale Image Recognition. In International Conference on
Learning Representations.

Ilya Sutskever, Oriol Vinyals, and Quoc V. Le. 2014.
Sequence to sequence learning with neural networks.
In Proceedings of the 27th International Conference
on Neural Information Processing Systems - Volume 2, NIPS’14, pages 3104–3112, Cambridge, MA,
USA. MIT Press.
L.J.P. van der Maaten and G.E. Hinton. 2008. Visualizing High-Dimensional Data Using t-SNE. Journalof Machine Learning Research, 9:2579–2605.
Ramakrishna Vedantam, C. Lawrence Zitnick, andDevi Parikh. 2014. CIDEr: Consensus-based image
description evaluation. 2015 IEEE Conference on
Computer Vision and Pattern Recognition (CVPR),
pages 4566–4575.
Oriol Vinyals, Alexander Toshev, Samy Bengio, andDumitru Erhan. 2017. Show and Tell: Lessons
Learned from the 2015 MSCOCO Image Captioning
Challenge. IEEE Trans. Pattern Anal. Mach. Intell.,39(4):652–663.
Josiah Wang, Pranava Swaroop Madhyastha, and Lucia Specia. 2018. Object Counts! Bringing Explicit
Detections Back into Image Captioning. In Proceedings of the 2018 Conference of the North American
Chapter of the Association for Computational Linguistics: Human Language Technologies, Volume
1 (Long Papers), pages 2180–2193, New Orleans,Louisiana. Association for Computational Linguistics.
Kelvin Xu, Jimmy Ba, Ryan Kiros, Kyunghyun Cho,
Aaron Courville, Ruslan Salakhudinov, Rich Zemel,
and Yoshua Bengio. 2015. Show, Attend and Tell:
Neural Image Caption Generation with Visual Attention. In Proceedings of the 32nd International
Conference on Machine Learning, volume 37 ofProceedings of Machine Learning Research, pages
2048–2057, Lille, France. PMLR.
Xuwang Yin and Vicente Ordonez. 2017. Obj2Text:
Generating Visually Descriptive Language from Object Layouts. In Proceedings of the 2017 Conferenceon Empirical Methods in Natural Language Processing, pages 177–187, Copenhagen, Denmark. Association for Computational Linguistics.
Éloi Zablocki, Benjamin Piwowarski, Laure Soulier,and Patrick Gallinari. 2018. Learning Multi-Modal
Word Representation Grounded in Visual Context.
In Association for the Advancement of Artificial Intelligence (AAAI), New Orleans, United States.