Paper:
In this work, we focus on the problem of grounding language by training an agent to follow a set of natural language instructions and navigate to a target object in a 2D grid environment. The agent receives visual information through raw pixels and a natural language instruction telling what task needs to be achieved. Other than these two sources of information, our model does not have any prior information of both the visual and textual modalities and is end-to-end trainable. We develop an attention mechanism for multi-modal fusion of visual and textual modalities that allows the agent to learn to complete the navigation tasks and also achieve language grounding. Our experimental results show that our attention mechanism outperforms the existing multi-modal fusion mechanisms proposed in order to solve the above mentioned navigation task. We demonstrate through the visualization of attention weights that our model learns to correlate attributes of the object referred in the instruction with visual representations and also show that the learnt textual representations are semantically meaningful as they follow vector arithmetic and are also consistent enough to induce translation between instructions in different natural languages. We also show that our model generalizes effectively to unseen scenarios and exhibit zero-shot generalization capabilities. In order to simulate the above described challenges, we introduce a new 2D environment for an agent to jointly learn visual and textual modalities.

Review:
**Paper Summary**
The paper studies the problem of navigating to a target object in a 2D grid environment by following given natural language description as well as receiving visual information as raw pixels. The proposed architecture consists of a convoutional neural network encoding visual input,  gated recurrent unit encoding natural language descriptions, an attention mechanism fusing multimodal input, and a policy learning network. To verify the effectiveness of the proposed framework, a new environment is proposed. The environment is 2-D grid based and it consists of an agent, a list of objects with different attributes, and a list of obstacles. Agents perceive the environment throught raw pixels with a limited visible region, and they can perform actions to move in the environment to reach target objects.
The problem has been studied for a while and therefore it is not novel. The proposed framework is incremental. The proposed environment is trivial and therefore it is unclear if the proposed framework is able to scale up to a more complicated environment. The experiemental results do not support several claims stated in the paper. Overall, I would vote for rejection.
    - This paper solves the problem of navigating to the target object specified by language instruction in a 2D grid environment. It requires understanding of language, language grounding for visual features, and navigating to the target object while avoiding non-target objects. An attention mechanism is used to map a language instruction into a set of 1x1 convolutional filters which are intended to distinguish visual features described in the instruction from others. The experimental results show that the proposed method performs better than other methods.
    - This paper presents an end-to-end trainable model to navigate an agent through visual sources and natural language instructions. The model utilizes a proposed attention mechanism to draw correlation between the objects mentioned in the instructions with deep visual representations, without requiring any prior knowledge about these inputs. The experimental results demonstrate the effectiveness of the learnt textual representation and the zero-shot generalization capabilities to unseen scenarios. 
**Paper Strengths**
- The paper proposes an interesting task which is a navigation task with language instructions. This is important yet relatively unxplored.
- The implementation details are included, including optimizers, learning rates with weight decayed, numbers of training epochs, the discount factor, etc. 
- The attention mechanism used in the paper is reasonable and the learned language embedding clearly shows meaningful relationships between instructions.
- The learnt textual representation follows vector arithmetic, which enables the agent to perceive unseen instructions as a new combination of the attributes and perform zero-shot generalization.
**Paper Weaknesses**
- The problem of following natural language descriptions together with visual representations of environments is not completely novel. For example, Both the problem and the proposed method are similar to those already introduced in the Gated Attention method (Chaplot et al., 2017). Although the proposed method performs better than the prior work, the approach is incremental. 
- The proposed environment is simple. The vocabulary size is 40 and the longest instruciton only consists of 9 words. Whether the proposed framework is able to deal with more complicated environments is not clear. The experimental results shown in Figure 5 is not convincing that the proposed method only took less than 20k iterations to perform almost perfectly. The proposed environment is small and simple compared to the related work. It would be better to test the proposed method in a similar scale with the existing 3D navigation environments (Chaplot et al., 2017 and Hermann et al., 2017).
- The novelty of the proposed framework is unclear. This work is not the first  one which proposes the multimodal fusion network incorporating a CNN achitecture dealing with visual information and a GRU architecture encoding language instructions. Also, the proposed attention mechanism is an obvious choice.
- The shown visualized attention maps are not enough to support the contribution of proposing the attention mechanism. It is difficult to tell whether the model learns to attend to correct objects. Also, the effectiveness of incorporating the attention mechanism is unclear.
- The paper claims that the proposed framework is flexible and is able to handle a rich set of natural language descriptions. However, the experiemental results are not enough to support the claim.
- The presentaiton of the experiment is not space efficient at all.
- The reference of the related papers which fuse multimodal data (vision and language) are missing.
- Comapred to 8 pages was the suggested page limit, 13 pages is a bit too long.
- Stating captions of figures above figures is not recommended.
- It would be better to show where each 1x1 filter for multimodal fusion attends on the input image.  Ideally, one filter should attend on the target object and others should attend on non-target objects. However, I wonder how RNN can generate filters to detect non-target objects given an instruction. Although Figure 6 and Figure 7 try to show insights about the proposed attention model, they don’t tell which kernel is in charge of which visual feature. Blurred attention maps in Figure 6 and 7 make it hard to interpret the behavior of the model. 
- The graphs shown in the Figure 5 are hard to interpret because of their large variance. It would be better to smoothing curves, so that comparing methods clearly.
- For zero-shot generalization evaluation, there is no detail about the training steps and comparisons to other methods.
- A highly related paper (Hermann et al., 2017) is missing in the references.
- Since the instructions are simple, the model does not require attention mechanism on the textual sources. If the framework can take more complex language, might be worthwhile to try visual-text co-attention mechanism. Such demonstration will be more convincing.
- The attention maps of different attribute is not as clear as the paper stated. Why do we need several “non-target” objects highlight if one can learn to consolidate all of them?
- The interpretation of n in the paper is vague, the authors should also show qualitatively why n=5 is better than that of n=1,10. If the attention maps learnt are really focusing on different attributes, given more and more objects, shouldn’t n=10 have more information for the policy learning?
- The unseen scenario generalization should also include texture change on the grid environment and/or new attribute combinations on non-target objects to be more convincing.
- The contribution in the visual part is marginal.
** Preliminary Evaluation**
- The modality fusion technique which leads to the attention maps is an effective and seem to work well approach, however, the author should present more thorough ablated analysis. The overall architecture is elegant, but the capability of it to be extended to more complex environment is in doubt. The vector arithmetic of the learnt textual embedding is the key component to enable zero-shot generalization, while the effectiveness of this method is not convincing if more complex instructions such that it contains object-object relations or interactions are perceived by the agent.