Original: I NTERACTIVE G ROUNDED L ANGUAGE A CQUISITION AND G ENERALIZATION IN A 2D W ORLD
Revised: I NTERACTIVE G ROUNDED L ANGUAGE A CQUISITION AND G ENERALIZATION IN A 2D W ORLD

Original: A BSTRACT
Revised: A BSTRACT

Original: We build a virtual agent for learning language in a 2D maze-like world.
Revised: We build a virtual agent for learning language in a 2D maze-like world.

Original: The agent sees images of the surrounding environment, listens to a virtual teacher, and takes actions to receive rewards.
Revised: The agent sees images of the surrounding environment, listens to a virtual teacher, and takes actions to receive rewards.

Original: It interactively learns the teacher’s language from scratch based on two language use cases: sentence-directed navigation and question answering.
Revised: It interactively learns the teacher’s language from scratch based on two language use cases: sentence-directed navigation and question answering.

Original: It learns simultaneously the visual representations of the world, the language, and the action control.
Revised: It learns simultaneously the visual representations of the world, the language, and the action control.

Original: By disentangling language grounding from other computational routines and sharing a concept detection function between language grounding and prediction, the agent reliably interpolates and extrapolates to interpret sentences that contain new word combinations or new words missing from training sentences.
Revised: By disentangling language grounding from other computational routines and sharing a concept detection function between language grounding and prediction, the agent reliably interpolates and extrapolates to interpret sentences that contain new word combinations or new words missing from training sentences.

Original: The new words are transferred from the answers of language prediction.
Revised: The new words are transferred from the answers of language prediction.

Original: Such a language ability is trained and evaluated on a population of over 1.6 million distinct sentences consisting of 119 object words, 8 color words, 9 spatial-relation words, and 50 grammatical words.
Revised: Such a language ability is trained and evaluated on a population of over 1.6 million distinct sentences consisting of 119 object words, 8 color words, 9 spatial-relation words, and 50 grammatical words.

Original: The proposed model signiﬁcantly outperforms ﬁve comparison methods for interpreting zero-shot sentences.
Revised: The proposed model signiﬁcantly outperforms ﬁve comparison methods for interpreting zero-shot sentences.

Original: In addition, we demonstrate human-interpretable intermediate outputs of the model in the appendix.
Revised: In addition, we demonstrate human-interpretable intermediate outputs of the model in the appendix.

Original:  I NTRODUCTION
Revised:  I NTRODUCTION

Original: Some empiricists argue that language may be learned based on its usage (Tomasello, 2003).
Revised: Some empiricists argue that language may be learned based on its usage (Tomasello, 2003).

Original: Skinner (1957) suggests that the successful use of a word reinforces the understanding of its meaning as well as the probability of it being used again in the future.
Revised: Skinner (1957) suggests that the successful use of a word reinforces the understanding of its meaning as well as the probability of it being used again in the future.

Original: Bruner (1985) emphasizes the role of social interaction in helping a child develop the language and posits the importance of the feedback and reinforcement from the parents during the learning process.
Revised: Bruner (1985) emphasizes the role of social interaction in helping a child develop the language and posits the importance of the feedback and reinforcement from the parents during the learning process.

Original: This paper takes a positive view of the above behaviorism and tries to explore some of the ideas by instantiating them in a 2D virtual world where interactive language acquisition happens.
Revised: This paper takes a positive view of the above behaviorism and tries to explore some of the ideas by instantiating them in a 2D virtual world where interactive language acquisition happens.

Original: This interactive setting contrasts with a common learning setting in that language is learned from dynamic interactions with environments instead of from static labeled data.
Revised: This interactive setting contrasts with a common learning setting in that language is learned from dynamic interactions with environments instead of from static labeled data.

Original: Language acquisition can go beyond mapping language as input patterns to output labels for merely obtaining high rewards or accomplishing tasks.
Revised: Language acquisition can go beyond mapping language as input patterns to output labels for merely obtaining high rewards or accomplishing tasks.

Original: We take a step further to require the language to be grounded (Harnad, 1990).
Revised: We take a step further to require the language to be grounded (Harnad, 1990).

Original: Speciﬁcally, we consult the paradigm of procedural semantics (Woods, 2007) which posits that words, as abstract procedures, should be able to pick out referents.
Revised: Speciﬁcally, we consult the paradigm of procedural semantics (Woods, 2007) which posits that words, as abstract procedures, should be able to pick out referents.

Original: We will attempt to explicitly link words to environment concepts instead of treating the whole model as a black box.
Revised: We will attempt to explicitly link words to environment concepts instead of treating the whole model as a black box.

Original: Such a capability also implies that, depending on the interactions with the world, words would have particular meanings in a particular context and some content words in the usual sense might not even have meanings in our case.
Revised: Such a capability also implies that, depending on the interactions with the world, words would have particular meanings in a particular context and some content words in the usual sense might not even have meanings in our case.

Original: As a result, the goal of this paper is to acquire “in-context” word meanings regardless of their suitability in all scenarios.
Revised: As a result, the goal of this paper is to acquire “in-context” word meanings regardless of their suitability in all scenarios.

Original: On the other hand, it has been argued that a child’s exposure to adult language provides inadequate evidence for language learning (Chomsky, 1991), but some induction mechanism should exist to bridge this gap (Landauer & Dumais, 1997).
Revised: On the other hand, it has been argued that a child’s exposure to adult language provides inadequate evidence for language learning (Chomsky, 1991), but some induction mechanism should exist to bridge this gap (Landauer & Dumais, 1997).

Original: This property is critical for any AI system to learn inﬁnite sentences from ﬁnite training data.
Revised: This property is critical for any AI system to learn inﬁnite sentences from ﬁnite training data.

Original: This type of generalization problem is specially addressed in our problem setting.
Revised: This type of generalization problem is specially addressed in our problem setting.

Original: After training, we want the agent to generalize to interpret zero-shot sentences of two types:
Revised: After training, we want the agent to generalize to interpret zero-shot sentences of two types:

Original: [Figure]
Revised: [Figure]

Original: 1) interpolation , new combinations of previously seen words for the same use case, or 2) extrapolation , new words transferred from other use cases and models.
Revised: 1) interpolation , new combinations of previously seen words for the same use case, or 2) extrapolation , new words transferred from other use cases and models.

Original: In the following, we will call the ﬁrst type ZS1 sentences and the second type ZS2 sentences.
Revised: In the following, we will call the ﬁrst type ZS1 sentences and the second type ZS2 sentences.

Original: Note that so far the zero-shot problems, addressed by most recent work (Hermann et al., 2017; Chaplot et al., 2018) of interactive language learning, belong to the category of ZS1.
Revised: Note that so far the zero-shot problems, addressed by most recent work (Hermann et al., 2017; Chaplot et al., 2018) of interactive language learning, belong to the category of ZS1.

Original: In contrast, a reliable interpretation of ZS2 sentences, which is essentially a transfer learning (Pan & Yang, 2010) problem, will be a major contribution of this work.
Revised: In contrast, a reliable interpretation of ZS2 sentences, which is essentially a transfer learning (Pan & Yang, 2010) problem, will be a major contribution of this work.

Original: We created a 2D maze-like world called XWORLD (Figure 1), as a testbed for interactive grounded language acquisition and generalization.
Revised: We created a 2D maze-like world called XWORLD (Figure 1), as a testbed for interactive grounded language acquisition and generalization.

Original: In this world, a virtual agent has two language use cases: navigation (NAV) and question answering (QA).
Revised: In this world, a virtual agent has two language use cases: navigation (NAV) and question answering (QA).

Original: For NAV, the agent needs to navigate to correct places indicated by language commands from a virtual teacher.
Revised: For NAV, the agent needs to navigate to correct places indicated by language commands from a virtual teacher.

Original: For QA, the agent must correctly generate single-word answers to the teacher’s questions.
Revised: For QA, the agent must correctly generate single-word answers to the teacher’s questions.

Original: NAV tests language comprehension while
Revised: NAV tests language comprehension while

Original: QA additionally tests language prediction.
Revised: QA additionally tests language prediction.

Original: They happen simultaneously: When the agent is navigating, the teacher might ask questions regarding its current interaction with the environment.
Revised: They happen simultaneously: When the agent is navigating, the teacher might ask questions regarding its current interaction with the environment.

Original: Once the agent reaches the target or the time is up, the current session ends and a new one is randomly generated according to our conﬁguration (Appendix B).
Revised: Once the agent reaches the target or the time is up, the current session ends and a new one is randomly generated according to our conﬁguration (Appendix B).

Original: The ZS2 sentences deﬁned in our setting require word meanings to be transferred from single-word answers to sentences, or more precisely, from language prediction to grounding .
Revised: The ZS2 sentences deﬁned in our setting require word meanings to be transferred from single-word answers to sentences, or more precisely, from language prediction to grounding .

Original: This is achieved by establishing an explicit link between grounding and prediction via a common concept detection function, which constitutes the major novelty of our model.
Revised: This is achieved by establishing an explicit link between grounding and prediction via a common concept detection function, which constitutes the major novelty of our model.

Original: With this transferring ability, the agent is able to comprehend a question containing a new object learned from an answer, without retraining the QA pipeline.
Revised: With this transferring ability, the agent is able to comprehend a question containing a new object learned from an answer, without retraining the QA pipeline.

Original: It is also able to navigate to a freshly taught object without retraining the NAV pipeline.
Revised: It is also able to navigate to a freshly taught object without retraining the NAV pipeline.

Original: It is worthwhile emphasizing that this seemingly “simple” world in fact poses great challenges for language acquisition and generalization, because:
Revised: It is worthwhile emphasizing that this seemingly “simple” world in fact poses great challenges for language acquisition and generalization, because:

Original: ˝
Revised: ˝

Original: The state space is huge.
Revised: The state space is huge.

Original: Even for a 7 ˆ 7 map with 15 wall blocks and 5 objects selected from 119 distinct classes, there are already octillions ( 10 27 ) of possible different conﬁgurations, not to mention the intra-class variance of object instances (see Figure 16 in the appendix).
Revised: Even for a 7 ˆ 7 map with 15 wall blocks and 5 objects selected from 119 distinct classes, there are already octillions ( 10 27 ) of possible different conﬁgurations, not to mention the intra-class variance of object instances (see Figure 16 in the appendix).

Original: For two conﬁgurations that only differ in one block, their successful navigation paths could be completely different.
Revised: For two conﬁgurations that only differ in one block, their successful navigation paths could be completely different.

Original: This requires an accurate perception of the environment.
Revised: This requires an accurate perception of the environment.

Original: Moreover, the conﬁguration constantly changes from session to session, and from training to testing.
Revised: Moreover, the conﬁguration constantly changes from session to session, and from training to testing.

Original: In particular, the target changes across sessions in both location and appearance.
Revised: In particular, the target changes across sessions in both location and appearance.

Original: ˝
Revised: ˝

Original: The goal space implied by the language for navigation is huge.
Revised: The goal space implied by the language for navigation is huge.

Original: For a vocabulary containing only 185 words, the total number of distinct commands that can be said by the teacher conforming to our deﬁned grammar is already over half a million.
Revised: For a vocabulary containing only 185 words, the total number of distinct commands that can be said by the teacher conforming to our deﬁned grammar is already over half a million.

Original: Two commands that differ by only one word could imply completely different goals.
Revised: Two commands that differ by only one word could imply completely different goals.

Original: This requires an accurate grounding of language.
Revised: This requires an accurate grounding of language.

Original: ˝
Revised: ˝

Original: The environment demands a strong language generalization ability from the agent.
Revised: The environment demands a strong language generalization ability from the agent.

Original: The agent has to learn to interpret zero-shot sentences that might be as long as 13 words.
Revised: The agent has to learn to interpret zero-shot sentences that might be as long as 13 words.

Original: It has to “plug” the meaning of a new word or word combination into a familiar sentential context while trying to still make sense of the unfamiliar whole.
Revised: It has to “plug” the meaning of a new word or word combination into a familiar sentential context while trying to still make sense of the unfamiliar whole.

Original: The recent work (Hermann et al., 2017; Chaplot et al., 2018) addresses ZS1 (for short sentences with several words) but not ZS2 sentences, which is a key difference between our learning problem and theirs.
Revised: The recent work (Hermann et al., 2017; Chaplot et al., 2018) addresses ZS1 (for short sentences with several words) but not ZS2 sentences, which is a key difference between our learning problem and theirs.

Original: We describe an end-to-end model for the agent to interactively acquire language from scratch and generalize to unfamiliar sentences.
Revised: We describe an end-to-end model for the agent to interactively acquire language from scratch and generalize to unfamiliar sentences.

Original: Here “scratch” means that the model does not hold any assumption of the language semantics or syntax.
Revised: Here “scratch” means that the model does not hold any assumption of the language semantics or syntax.

Original: Each sentence is simply a sequence of tokens with each token being equally meaningless in the beginning of learning.
Revised: Each sentence is simply a sequence of tokens with each token being equally meaningless in the beginning of learning.

Original: This is unlike some early pioneering systems (e.g., SHRDLU (Winograd, 1972)) that hard-coded the syntax or semantics to link language to a simulated world–an approach that presents scalability issues.
Revised: This is unlike some early pioneering systems (e.g., SHRDLU (Winograd, 1972) and A BIGAIL (Siskind, 1994)) that hard-coded the syntax or semantics to link language to a simulated world–an approach that presents scalability issues.

Original: There are two aspects of the interaction: one is with the teacher (i.e., language and rewards) and the other is with the environment (e.g., stepping on objects or hitting walls).
Revised: There are two aspects of the interaction: one is with the teacher (i.e., language and rewards) and the other is with the environment (e.g., stepping on objects or hitting walls).

Original: The model takes as input RGB images, sentences, and rewards.
Revised: The model takes as input RGB images, sentences, and rewards.

Original: It learns simultaneously the visual representations of the world, the language, andthe action control.
Revised: It learns simultaneously the visual representations of the world, the language, and

Revised: the action control.

Original: We evaluate our model on randomly generated XWORLD maps with random agent positions, on a population of over 1.6 million distinct sentencesconsisting of 119 object words,color words, 9 spatial-relation words, and 50 grammatical words.
Revised: We evaluate our model on randomly generated XWORLD maps with random agent positions, on a population of over 1.6 million distinct sentences

Revised: consisting

Revised: of 119 object words, 8 color words, 9 spatial-relation words, and 50 grammatical words.

Original: Detailed analysis (Appendix A) of the trained model shows that the language is grounded in such a way that the words are capable to pick out referents in the environment.
Revised: Detailed analysis (Appendix A) of the trained model shows that the language is grounded in such a way that the words are capable to pick out referents in the environment.

Original: We specially test the generalization ability of  agent for zero shot sentences
Revised: We specially test the generalization ability of the agent for handling zero-shot sentences.

Original: ZS when the zero-shot portion is half, comparable to the rate of 90.5% in a normal language setting.
Revised: The average NAV success rates are 84.3% for ZS1 and 85.2% for ZS2 when the zero-shot portion is half, comparable to the rate of 90.5% in a normal language setting.

Original: The average QA accuracies are 97.8% for ZS1 and 97.7% for ZS2 when the zero-shot portion is half, almost as good as the accuracy of 99.7% in a normal language setting.
Revised: The average QA accuracies are 97.8% for ZS1 and 97.7% for ZS2 when the zero-shot portion is half, almost as good as the accuracy of 99.7% in a normal language setting.

Original:  M ODEL
Revised:  M ODEL

Original: Our model incorporates two objectives.
Revised: Our model incorporates two objectives.

Original: The ﬁrst is to maximize the cumulative reward of NAV and the second is to minimize the classiﬁcation cost of QA.
Revised: The ﬁrst is to maximize the cumulative reward of NAV and the second is to minimize the classiﬁcation cost of QA.

Original: For the former, we follow the standard reinforcement learning (RL) paradigm: the agent learns the action at every step from reward signals.
Revised: For the former, we follow the standard reinforcement learning (RL) paradigm: the agent learns the action at every step from reward signals.

Original: It employs the actor-critic (AC) algorithm (Sutton & Barto, 1998) to learn the control policy (Appendix E).
Revised: It employs the actor-critic (AC) algorithm (Sutton & Barto, 1998) to learn the control policy (Appendix E).

Original: For the latter, we adopt the standard supervised setting of Visual QA (Antol et al., 2015): the groundtruth answers are provided by the teacher during training.
Revised: For the latter, we adopt the standard supervised setting of Visual QA (Antol et al., 2015): the groundtruth answers are provided by the teacher during training.

Original: The training cost is formulated as the multiclass cross entropy.
Revised: The training cost is formulated as the multiclass cross entropy.

Original: 2. M OTIVATION
Revised: 2. M OTIVATION

Original: The model takes two streams of inputs: images and sentences.
Revised: The model takes two streams of inputs: images and sentences.

Original: The key is how to model the language grounding problem.
Revised: The key is how to model the language grounding problem.

Original: That is, the agent must link (either implicitly or explicitly) language concepts to environment entities to correctly take an action by understanding the instruction in the current visual context.
Revised: That is, the agent must link (either implicitly or explicitly) language concepts to environment entities to correctly take an action by understanding the instruction in the current visual context.

Original: A straightforward idea would be to encode the sentence s with an RNN and encode the perceived image e with a CNN, after which the two encoded representations are mixed together.
Revised: A straightforward idea would be to encode the sentence s with an RNN and encode the perceived image e with a CNN, after which the two encoded representations are mixed together.

Original: Speciﬁcally, let the multimodal module be M , the action module be A , and the prediction module be P , this idea can be formulated as:
Revised: Speciﬁcally, let the multimodal module be M , the action module be A , and the prediction module be P , this idea can be formulated as:

Original: [Equation]
Revised: [Equation]

Original: Hermann et al.
Revised: Hermann et al.

Original: ; Misra et al.
Revised: ; Misra et al.

Original: (2017); Chaplot et al.
Revised: (2017); Chaplot et al.

Original: all employ the above paradigm.
Revised: all employ the above paradigm.

Original: In their implementations, M is either vector concatenation or element-wise product.
Revised: In their implementations, M is either vector concatenation or element-wise product.

Original: For any particular word in the sentence, fusion with the image could happen anywhere starting from M all the way to the end, right before a label is output.
Revised: For any particular word in the sentence, fusion with the image could happen anywhere starting from M all the way to the end, right before a label is output.

Original: This is due to the fact that the RNN folds the string of words into a compact embedding which then goes through the subsequent blackbox computations.
Revised: This is due to the fact that the RNN folds the string of words into a compact embedding which then goes through the subsequent blackbox computations.

Original: Therefore, language grounding and other computational routines are entangled.

Original: Because of this, we

Original: L
Revised: L

Original: G r ound i ng “Move to the red apple.”
Revised: G r ound i ng “Move to the red apple.”

Original: “Say the color of avocado.”
Revised: “Say the color of avocado.”

Original: F V
Revised: F V

Original: i s i on
Revised: i s i on

Original: P
Revised: P

Original: P r e d i c ti on
Revised: P r e d i c ti on

Original: A A c ti on 𝑠 𝜙 𝑒 𝑐 𝑎 𝑣 ℎ ℎ 𝑥
Revised: A A c ti on 𝑠 𝜙 𝑒 𝑐 𝑎 𝑣 ℎ ℎ 𝑥

Original: *+,- 𝐌 / 𝐌
Revised: *+,- 𝐌 / 𝐌

Original: M u lti m od a l
Revised: M u lti m od a l

Original: NAV:
Revised: NAV:

Original: QA:  say that this paradigm has an implicit language grounding strategy.
Revised: QA: Therefore, language grounding and other computational routines are entangled. Because of this, we say that this paradigm has an implicit language grounding strategy.

Original: Such a strategy poses a great challenge for processing a ZS2 sentence because it is almost impossible to predict how a new word learned from language prediction would perform in the complex entanglement involved.
Revised: Such a strategy poses a great challenge for processing a ZS2 sentence because it is almost impossible to predict how a new word learned from language prediction would perform in the complex entanglement involved.

Original: Thus a careful inspection of the grounding process is needed.
Revised: Thus a careful inspection of the grounding process is needed.

Original: 2. A PPROACH
Revised: 2. A PPROACH

Original: The main idea behind our approach is to disentangle language grounding from other computations in the model.
Revised: The main idea behind our approach is to disentangle language grounding from other computations in the model.

Original: This disentanglement makes it possible for us to explicitly deﬁne language grounding around a core function that is also used by language prediction.
Revised: This disentanglement makes it possible for us to explicitly deﬁne language grounding around a core function that is also used by language prediction.

Original: Speciﬁcally, both grounding and prediction are cast as concept detection problems, where each word (embedding) is treated as a detector.
Revised: Speciﬁcally, both grounding and prediction are cast as concept detection problems, where each word (embedding) is treated as a detector.

Original: This opens up the possibility of transferring word meanings from the latter to the former.
Revised: This opens up the possibility of transferring word meanings from the latter to the former.

Original: The overall architecture of our model is shown in Figure 2.
Revised: The overall architecture of our model is shown in Figure 2.

Original: 2.2. E XPLICIT GROUNDING
Revised: 2.2. E XPLICIT GROUNDING

Original: We begin with our deﬁnition of “grounding.”
Revised: We begin with our deﬁnition of “grounding.”

Original: We deﬁne a sentence as generally a string of words of any length.
Revised: We deﬁne a sentence as generally a string of words of any length.

Original: A single word is a special case of a sentence.
Revised: A single word is a special case of a sentence.

Original: Given a sentence s and an image representation h “ CNN p e q , we say that s is grounded in h as x if
Revised: Given a sentence s and an image representation h “ CNN p e q , we say that s is grounded in h as x if

Original: I) h consists of M entities where an entity is a subset of visual features, and II) x
Revised: I) h consists of M entities where an entity is a subset of visual features, and II) x

Original: P t 0 , 1 u M with each entry x
Revised: P t 0 , 1 u M with each entry x

Original: r m s representing a binary selection of the m th entity of h .
Revised: r m s representing a binary selection of the m th entity of h .

Original: Thus x is a combinatorial selection over h .
Revised: Thus x is a combinatorial selection over h .

Original: Furthermore, x is explicit if
Revised: Furthermore, x is explicit if

Original: III) it is formed by the grounding results of (some) individual words of s (i
Revised: III) it is formed by the grounding results of (some) individual words of s (i

Original: e., compositionality).
Revised: e., compositionality).

Original: We say that a framework has an explicit grounding strategy if
Revised: We say that a framework has an explicit grounding strategy if

Original: IV) all language-vision fusions in the framework are explicit groundings.
Revised: IV) all language-vision fusions in the framework are explicit groundings.

Original: For our problem, we propose a new framework with an explicit grounding strategy:
Revised: For our problem, we propose a new framework with an explicit grounding strategy:

Original: [Equation]
Revised: [Equation]

Original: where the sole language-vision fusion x in the framework is an explicit grounding.
Revised: where the sole language-vision fusion x in the framework is an explicit grounding.

Original: Notice in the above how the grounding process, as a “bottleneck,” allows only x but not other linguistic information to ﬂow to the downstream of the network.
Revised: Notice in the above how the grounding process, as a “bottleneck,” allows only x but not other linguistic information to ﬂow to the downstream of the network.

Original: That is, M A , M P , A , and P all rely on grounded results but not on other sentence representations.
Revised: That is, M A , M P , A , and P all rely on grounded results but not on other sentence representations.

Original: By doing so, we expect x to summarize all the necessary linguistic information for performing the tasks.
Revised: By doing so, we expect x to summarize all the necessary linguistic information for performing the tasks.

Original: The beneﬁts of this framework are two-fold.
Revised: The beneﬁts of this framework are two-fold.

Original: First, the explicit grounding strategy provides a conceptual abstraction (Garnelo et al., 2016) that maps high-dimensional linguistic input to a lowerdimensional conceptual state space and abstracts away irrelevant input signals.
Revised: First, the explicit grounding strategy provides a conceptual abstraction (Garnelo et al., 2016) that maps high-dimensional linguistic input to a lowerdimensional conceptual state space and abstracts away irrelevant input signals.

Original: This improves the generalization for similar linguistic inputs.
Revised: This improves the generalization for similar linguistic inputs.

Original: Given e , all that matters for NAV and QA is x .
Revised: Given e , all that matters for NAV and QA is x .

Original: This guarantees that the agent will perform exactly in the same way on the same image e even given different sentences as long as their grounding results x are the same.
Revised: This guarantees that the agent will perform exactly in the same way on the same image e even given different sentences as long as their grounding results x are the same.

Original: It disentangles language grounding from subsequent computations such as obstacle detection, path planning, action making, and feature classiﬁcation, which all should be inherently language-independent routines.
Revised: It disentangles language grounding from subsequent computations such as obstacle detection, path planning, action making, and feature classiﬁcation, which all should be inherently language-independent routines.

Original: Second, because x is explicit, the roles played by the individual words of s in the grounding are interpretable.
Revised: Second, because x is explicit, the roles played by the individual words of s in the grounding are interpretable.

Original: This is in contrast to Eq.
Revised: This is in contrast to Eq.

Original: 1 where the roles of individual words are unclear.
Revised: 1 where the roles of individual words are unclear.

Original: The interpretability provides a possibility of establishing a link between language grounding and prediction, which we will perform in the remainder of this section.
Revised: The interpretability provides a possibility of establishing a link between language grounding and prediction, which we will perform in the remainder of this section.

Original: 2.2. I NSTANTIATION OF EXPLICIT GROUNDING
Revised: 2.2. I NSTANTIATION OF EXPLICIT GROUNDING

Original: Let h P R N ˆ
Revised: Let h P R N ˆ

Original: D be a spatially ﬂattened feature cube (originally in 3D, now the 2D spatial domain collapsed into 1D for notational simplicity), where D is the number of channels and N is the number of locations in the spatial domain.
Revised: D be a spatially ﬂattened feature cube (originally in 3D, now the 2D spatial domain collapsed into 1D for notational simplicity), where D is the number of channels and N is the number of locations in the spatial domain.

Original: We adopt three deﬁnitions for an entity:
Revised: We adopt three deﬁnitions for an entity:

Original: 1) a feature vector at a particular image location, 2) a particular feature map along the channel dimension, and 3) a scalar feature at the intersection of a feature vector and a feature map.
Revised: 1) a feature vector at a particular image location, 2) a particular feature map along the channel dimension, and 3) a scalar feature at the intersection of a feature vector and a feature map.

Original: Their grounding results are denoted as x loc p s, h q P t 0 , 1 u N , x feat p s, h q P t 0 , 1 u D , and x cube p s, h q P t 0 , 1 u N ˆ D , respectively.
Revised: Their grounding results are denoted as x loc p s, h q P t 0 , 1 u N , x feat p s, h q P t 0 , 1 u D , and x cube p s, h q P t 0 , 1 u N ˆ D , respectively.

Original: In the rest of the paper, we remove s and h from x loc , x feat , and x cube for notational simplicity while always assuming a dependency on them.
Revised: In the rest of the paper, we remove s and h from x loc , x feat , and x cube for notational simplicity while always assuming a dependency on them.

Original: We assume that x cube is a low-rank matrix that can be decomposed into the two:
Revised: We assume that x cube is a low-rank matrix that can be decomposed into the two:

Original: [Equation]
Revised: [Equation]

Original: [Table]

Original: To make the model fully differentiable, in the following we relax the deﬁnition of grounding so that x loc P r 0 , 1 s N , x feat P r 0 , 1 s D , and x cube P r 0 , 1 s N ˆ D .
Revised: To make the model fully differentiable, in the following we relax the deﬁnition of grounding so that x loc P r 0 , 1 s N , x feat P r 0 , 1 s D , and x cube P r 0 , 1 s N ˆ D .

Original: The attention map x loc is responsible for image spatial attention.
Revised: The attention map x loc is responsible for image spatial attention.

Original: The channel mask x feat is responsible for selecting image feature maps, and is assumed to be independent of the speciﬁc h , namely, x feat p s, h q “ x feat p s q .
Revised: The channel mask x feat is responsible for selecting image feature maps, and is assumed to be independent of the speciﬁc h , namely, x feat p s, h q “ x feat p s q .

Original: Intuitively, h can be modulated by x feat before being sent to downstream processings.
Revised: Intuitively, h can be modulated by x feat before being sent to downstream processings.

Original: A recent paper by de Vries et al.
Revised: A recent paper by de Vries et al.

Original: (2017) proposes an even earlier modulation of the visual processing by directly conditioning some of the parameters of a CNN on the linguistic input.
Revised: (2017) proposes an even earlier modulation of the visual processing by directly conditioning some of the parameters of a CNN on the linguistic input.

Original: Finally, we emphasize that our explicit grounding, even though instantiated as a soft attention mechanism, is different from the existing visual attention models.
Revised: Finally, we emphasize that our explicit grounding, even though instantiated as a soft attention mechanism, is different from the existing visual attention models.

Original: Some attention models such as Xu et al.
Revised: Some attention models such as Xu et al.

Original: (2015); de Vries et al.
Revised: (2015); de Vries et al.

Original: (2017) violate deﬁnitions III and IV.
Revised: (2017) violate deﬁnitions III and IV.

Original: Some work (Andreas et al., 2016a;b; Lu et al., 2016) violates deﬁnition IV in a way that language is fused with vision by a multilayer perceptron (MLP) after image attention.
Revised: Some work (Andreas et al., 2016a;b; Lu et al., 2016) violates deﬁnition IV in a way that language is fused with vision by a multilayer perceptron (MLP) after image attention.

Original: Anderson et al.
Revised: Anderson et al.

Original: (2017) proposes a pipeline similar to ours but violates deﬁnition III in which the image spatial attention is computed from a compact question embedding output by an RNN.
Revised: (2017) proposes a pipeline similar to ours but violates deﬁnition III in which the image spatial attention is computed from a compact question embedding output by an RNN.

Original: 2.2. C ONCEPT DETECTION
Revised: 2.2. C ONCEPT DETECTION

Original: With language grounding disentangled, now we relate it to language prediction.
Revised: With language grounding disentangled, now we relate it to language prediction.

Original: This relation is a common concept detection function.
Revised: This relation is a common concept detection function.

Original: We assume that every word in a vocabulary, as a concept, is detectable against entities of type (1) as deﬁned in Section 2.2.1.
Revised: We assume that every word in a vocabulary, as a concept, is detectable against entities of type (1) as deﬁned in Section 2.2.1.

Original: For a meaningful detection of spatial-relation words that are irrelevant to image content, we incorporate parametric feature maps into h to learn spatial features.
Revised: For a meaningful detection of spatial-relation words that are irrelevant to image content, we incorporate parametric feature maps into h to learn spatial features.

Original: Assume a precomputed x feat , the concept detection operates by sliding over the spatial domain of the feature cube h , which can be written as a function φ :
Revised: Assume a precomputed x feat , the concept detection operates by sliding over the spatial domain of the feature cube h , which can be written as a function φ :

Original: [Equation]
Revised: [Equation]

Original: where χ P R N is a detection score map and u is a word embedding vector.
Revised: where χ P R N is a detection score map and u is a word embedding vector.

Original: This function scores the embedding u against each feature vector of h , modulated by x feat that selects which feature maps touse for the scoring.
Revised: This function scores the embedding u against each feature vector of h , modulated by x feat that selects which feature maps to

Original: Intuitively, each score on χ indicates the detection response of the feature vector in that location.

Original: A higher score represents a higher detection response.

Original: [Equation]
Revised: [Equation]

Original: “What is the color of the object in the northeast?” 
Revised: “What is the color of the object in the northeast?” use for the scoring.

Revised: Intuitively, each score on χ indicates the detection response of the feature vector in that location.

Revised: A higher score represents a higher detection response.

Original: While there are many potential forms for φ , we implement it as
Revised: While there are many potential forms for φ , we implement it as

Original: [Equation]
Revised: [Equation]

Original: where ˝ is the element-wise product.
Revised: where ˝ is the element-wise product.

Original: To do so, we have word embedding u P R D where D is equal to the number of channels of h .
Revised: To do so, we have word embedding u P R D where D is equal to the number of channels of h .

Original: 2.2. P REDICTION BY CONCEPT DETECTION
Revised: 2.2. P REDICTION BY CONCEPT DETECTION

Original: For prediction, we want to output a word given a question s and an image e .
Revised: For prediction, we want to output a word given a question s and an image e .

Original: Suppose that x loc and x feat are the grounding results of s .
Revised: Suppose that x loc and x feat are the grounding results of s .

Original: Based on the detection function φ , M P outputs a score vector m P R K over the entire lexicon, where each entry of the vector is:
Revised: Based on the detection function φ , M P outputs a score vector m P R K over the entire lexicon, where each entry of the vector is:

Original: [Equation]
Revised: [Equation]

Original: where u k is the k th entry of the word embedding table.
Revised: where u k is the k th entry of the word embedding table.

Original: The above suggests that m r k s is the result of weighting the scores on the map χ k by x loc .
Revised: The above suggests that m r k s is the result of weighting the scores on the map χ k by x loc .

Original: It represents the correctness of the k th lexical entry as the answer to the question s .
Revised: It represents the correctness of the k th lexical entry as the answer to the question s .

Original: To predict an answer
Revised: To predict an answer

Original: [Equation]
Revised: [Equation]

Original: Note that the role of x feat in the prediction is to select which feature maps are relevant to the question s .
Revised: Note that the role of x feat in the prediction is to select which feature maps are relevant to the question s .

Original: Otherwise it would be confusing for the agent about what to predict (e.g., whether to predict a color or an object name).
Revised: Otherwise it would be confusing for the agent about what to predict (e.g., whether to predict a color or an object name).

Original: By using x feat , we expect that different feature maps encode different image attributes (see an example in the caption of Figure 3).
Revised: By using x feat , we expect that different feature maps encode different image attributes (see an example in the caption of Figure 3).

Original: More analysis of x feat is performed in
Revised: More analysis of x feat is performed in

Original: Appendix A.
Revised: Appendix A.

Original: 2.2. G ROUNDING BY CONCEPT DETECTION
Revised: 2.2. G ROUNDING BY CONCEPT DETECTION

Original: To compute x cube , we compute x loc and x feat separately.
Revised: To compute x cube , we compute x loc and x feat separately.

Original: We want x loc to be built on the detection function φ .
Revised: We want x loc to be built on the detection function φ .

Original: One can expect to compute a series of score maps χ of individual words and merge them into x loc .
Revised: One can expect to compute a series of score maps χ of individual words and merge them into x loc .

Original: Suppose that s consists of L words t w l u with w l “ u k being some word k in the dictionary.
Revised: Suppose that s consists of L words t w l u with w l “ u k being some word k in the dictionary.

Original: Let τ p s q be a sequence of indices t l i u where 0 ď l i ă L .
Revised: Let τ p s q be a sequence of indices t l i u where 0 ď l i ă L .

Original: This sequence function τ decides which words of the sentence are selected and organized in what order.
Revised: This sequence function τ decides which words of the sentence are selected and organized in what order.

Original: We deﬁne x loc as
Revised: We deﬁne x loc as

Original: [Equation]
Revised: [Equation]

Original: ˘

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: where 1 P t 0 , 1 u D is a vector of ones, meaning that it selects all the feature maps for detecting w l i .
Revised: where 1 P t 0 , 1 u D is a vector of ones, meaning that it selects all the feature maps for detecting w l i .

Original: Υ is an aggregation function that combines the sequence of score maps
Revised: Υ is an aggregation function that combines the sequence of score maps

Original: χ l i of individual words.
Revised: χ l i of individual words.

Original: As such, φ makes it possible to transfer new words from Eq.
Revised: As such, φ makes it possible to transfer new words from Eq.

Original: 4 to Eq.
Revised: 4 to Eq.

Original: 5 during test time.
Revised: 5 during test time.

Original: If we were provided with an oracle that is able to output a parsing tree for any sentence, we could set τ and Υ according to the tree semantics.
Revised: If we were provided with an oracle that is able to output a parsing tree for any sentence, we could set τ and Υ according to the tree semantics.

Original: Neural module networks (NMNs) (Andreas et al., 2016a;b;
Revised: Neural module networks (NMNs) (Andreas et al., 2016a;b;

Original: Hu et al., 2017) rely on such a tree for language grounding.
Revised: Hu et al., 2017) rely on such a tree for language grounding.

Original: They generate a network of modules where each module corresponds to a tree node.
Revised: They generate a network of modules where each module corresponds to a tree node.

Original: However, labeled trees are needed for training.
Revised: However, labeled trees are needed for training.

Original: Below we propose to learn τ and Υ based on word attention (Bahdanau et al., 2015) to bypass the need for labeled structured data.
Revised: Below we propose to learn τ and Υ based on word attention (Bahdanau et al., 2015) to bypass the need for labeled structured data.

Original: We start by feeding a sentence s “ t w l u of length L to a bidirectional RNN (Schuster & Paliwal, 1997).
Revised: We start by feeding a sentence s “ t w l u of length L to a bidirectional RNN (Schuster & Paliwal, 1997).

Original: It outputs a compact sentence embedding s emb and a sequence of L word context vectors w l .
Revised: It outputs a compact sentence embedding s emb and a sequence of L word context vectors w l .

Original: Each w l summarizes the sentential pattern around that word.
Revised: Each w l summarizes the sentential pattern around that word.

Original: We then employ a meta controller called interpreter in an iterative manner.
Revised: We then employ a meta controller called interpreter in an iterative manner.

Original: For the i th interpretation step, the interpreter computes the word attention as:
Revised: For the i th interpretation step, the interpreter computes the word attention as:

Original: [Equation]
Revised: [Equation]

Original: where S cos is cosine similarity and GRU is the gated recurrent unit (Cho et al., 2014).
Revised: where S cos is cosine similarity and GRU is the gated recurrent unit (Cho et al., 2014).

Original: Here we use τ ˚ to represent an approximation of τ via soft word attention.
Revised: Here we use τ ˚ to represent an approximation of τ via soft word attention.

Original: We set p 0 to the compact sentence embedding s emb .
Revised: We set p 0 to the compact sentence embedding s emb .

Original: After this, the attended word s i is fed to the detection function φ .
Revised: After this, the attended word s i is fed to the detection function φ .

Original: The interpreter aggregates the score map of s i by:
Revised: The interpreter aggregates the score map of s i by:

Original: [Equation]
Revised: [Equation]

Original: where ˚ denotes a 2D convolution, σ is sigmoid, and ρ i is a scalar.
Revised: where ˚ denotes a 2D convolution, σ is sigmoid, and ρ i is a scalar.

Original: W and b are parameters to be learned.
Revised: W and b are parameters to be learned.

Original: Finally, the interpreter outputs x I loc as x loc , where I is the predeﬁned maximum step.
Revised: Finally, the interpreter outputs x I loc as x loc , where I is the predeﬁned maximum step.

Original: Note that in the above we formulate the map transform as a 2D convolution.
Revised: Note that in the above we formulate the map transform as a 2D convolution.

Original: This operation enables the agent to reason about spatial relations.
Revised: This operation enables the agent to reason about spatial relations.

Original: Recall that each attention map x loc is egocentric.
Revised: Recall that each attention map x loc is egocentric.

Original: When the agent needs to attend to a region speciﬁed by a spatial relation referring to an object, it can translate the object attention with the attention map of the spatial-relation word which serves as a 2D convolutional offset ﬁlter (Figure 4).
Revised: When the agent needs to attend to a region speciﬁed by a spatial relation referring to an object, it can translate the object attention with the attention map of the spatial-relation word which serves as a 2D convolutional offset ﬁlter (Figure 4).

Original: For this reason, we set y 0 as a one-hot map where the map center is one, to represent the identity translation.
Revised: For this reason, we set y 0 as a one-hot map where the map center is one, to represent the identity translation.

Original: A similar mechanism of spatial reasoning via convolution was explored by Kitaev & Klein (2017) for a voxel-grid 3D representation.
Revised: A similar mechanism of spatial reasoning via convolution was explored by Kitaev & Klein (2017) for a voxel-grid 3D representation.

Original: By assumption, the channel mask x feat is meant to be determined solely from s ; namely, which features to use should only depend on the sentence itself, not on the value of the feature cube h .
Revised: By assumption, the channel mask x feat is meant to be determined solely from s ; namely, which features to use should only depend on the sentence itself, not on the value of the feature cube h .

Original: Thus it is computed as
Revised: Thus it is computed as

Original: [Equation]
Revised: [Equation]

Original: where the RNN returns an average state of processing s , followed by an MLP with the sigmoid activation.
Revised: where the RNN returns an average state of processing s , followed by an MLP with the sigmoid activation.

Original:  R ELATED WORK
Revised:  R ELATED WORK

Original: Our XWORLD is similar to the 2D block world MaseBase (Sukhbaatar et al., 2016).
Revised: Our XWORLD is similar to the 2D block world MaseBase (Sukhbaatar et al., 2016).

Original: However, we emphasize the problem of grounded language acquisition and generalization, while they do not.
Revised: However, we emphasize the problem of grounded language acquisition and generalization, while they do not.

Original: There have been several 3D simulated worlds such as ViZDoom (Kempka et al., 2016), DeepMind
Revised: There have been several 3D simulated worlds such as ViZDoom (Kempka et al., 2016), DeepMind

Original: Lab (Beattie et al., 2016), and Malmo (Johnson et al., 2016).
Revised: Lab (Beattie et al., 2016), and Malmo (Johnson et al., 2016).

Original: Still, these other settings intended for visual perception and control, with less or no language.
Revised: Still, these other settings intended for visual perception and control, with less or no language.

Original: Our problem setting draws inspirations from the AI roadmap delineated by Mikolov et al.
Revised: Our problem setting draws inspirations from the AI roadmap delineated by Mikolov et al.

Original: Like theirs, we have a teacher in the environment that assigns tasks and rewards to the agent, potentially with a curriculum.
Revised: Like theirs, we have a teacher in the environment that assigns tasks and rewards to the agent, potentially with a curriculum.

Original: Unlike their proposal of using only the linguistic channel, we have multiple perceptual modalities, the fusion of which is believed to be the basis of meaning (Kiela et al., 2016).
Revised: Unlike their proposal of using only the linguistic channel, we have multiple perceptual modalities, the fusion of which is believed to be the basis of meaning (Kiela et al., 2016).

Original: Contemporary to our work, several end-to-end systems also address language grounding problems in a simulated world with deep RL.
Revised: Contemporary to our work, several end-to-end systems also address language grounding problems in a simulated world with deep RL.

Original: Misra et al.
Revised: Misra et al.

Original: (2017) maps instructions and visual observations to actions of manipulating blocks on a 2D plane.
Revised: (2017) maps instructions and visual observations to actions of manipulating blocks on a 2D plane.

Original: Hermann et al.
Revised: Hermann et al.

Original: (2017); Chaplot et al.
Revised: (2017); Chaplot et al.

Original: (2018) learn to navigate in 3D under instructions, and both evaluate ZS1 generalization.
Revised: (2018) learn to navigate in 3D under instructions, and both evaluate ZS1 generalization.

Original: Despite falling short of the vision challenges found in these other worlds, we have a much larger space of zero-shot sentences and additionally require ZS2 generalization, which is in fact a transfer learning (Pan & Yang, 2010) problem.
Revised: Despite falling short of the vision challenges found in these other worlds, we have a much larger space of zero-shot sentences and additionally require ZS2 generalization, which is in fact a transfer learning (Pan & Yang, 2010) problem.

Original: Other recent work (Andreas et al., 2017; Oh et al., 2017) on zero-shot multitask learning treats language tokens as (parsed) labels for identifying skills.
Revised: Other recent work (Andreas et al., 2017; Oh et al., 2017) on zero-shot multitask learning treats language tokens as (parsed) labels for identifying skills.

Original: In these papers, the zero-shot settings are not intended for language understanding.
Revised: In these papers, the zero-shot settings are not intended for language understanding.

Revised: The problem of grounding language in perception can perhaps be traced back to the early work of Siskind (1994; 1999), although no statistical learning was adopted at that time.

Original: Our  learning problem is related to some recent work on grounding language in images and videos (Yu & Siskind, 2013; Gao et al., 2016; Rohrbach et al., 2016).
Revised: Our language learning problem is related to some recent work on learning to ground language in images and videos (Yu & Siskind, 2013; Gao et al., 2016; Rohrbach et al., 2016).

Original: The NAV task is relevant to robotics navigation under language commands (Chen & Mooney, 2011; Tellex et al., 2011; Barrett et al., 2015).
Revised: The navigation task is relevant to robotics navigation under language commands (Chen & Mooney, 2011; Tellex et al., 2011; Barrett et al., 2017).

Original: The QA task is relevant to image question answering (VQA) (Antol et al., 2015; 
Revised: The question-answering task is relevant to image question answering (VQA) (Antol et al., 2015; Gao et al., 2015;

Original: Gao et al., 2015; Ren et al., 2015; Lu et al., 2016; Yang et al., 2016; Anderson et al., 2017; de Vries et al., 2017).
Revised: Ren et al., 2015; Lu et al., 2016; Yang et al., 2016; Anderson et al., 2017; de Vries et al., 2017).

Original: The interactive setting of learning to accomplish tasks is similar to that of learning to play video games from pixels (Mnih et al., 2015).
Revised: The interactive setting of learning to accomplish tasks is similar to that of learning to play video games from pixels (Mnih et al., 2015).

Original:  E XPERIMENTS
Revised:  E XPERIMENTS

Original: We design a variety of experiments to evaluate the agent’s language acquisition and generalization ability.
Revised: We design a variety of experiments to evaluate the agent’s language acquisition and generalization ability.

Original: Our model is ﬁrst compared with several methods to demonstrate the challenges in XWORLD .
Revised: Our model is ﬁrst compared with several methods to demonstrate the challenges in XWORLD .

Original: We then evaluate the agent’s language generalization ability in two different zero-shot conditions.
Revised: We then evaluate the agent’s language generalization ability in two different zero-shot conditions.

Original: Finally, we conclude with preliminary thoughts on how to scale our model to a 3D world. 
Revised: Finally, we conclude with preliminary thoughts on how to scale our model to a 3D world. [Figure] (~ [Figure]

Revised: [Figure]

Revised: [Figure]

Original:  4. G ENERAL SETUP
Revised: [Figure] (~ [Figure] 4. G ENERAL SETUP

Original: For all the experiments, both the sentences and the environments change from session to session, and from training to testing.
Revised: For all the experiments, both the sentences and the environments change from session to session, and from training to testing.

Original: The sentences are drawn conforming to the teacher’s grammar.
Revised: The sentences are drawn conforming to the teacher’s grammar.

Original: There are three types of language data: NAV command, QA question, and QA answer, which are illustrated[Figure] [Figure] in Figure 5.
Revised: There are three types of language data: NAV command, QA question, and QA answer, which are illustrated in Figure 5.

Original: In total, there are „ 570k NAV commands, „ 1m QA questions, and 136 QA answers (all the content words plus “nothing” and minus “between”).
Revised: In total, there are „ 570k NAV commands, „ 1m QA questions, and 136 QA answers (all the content words plus “nothing” and minus “between”).

Original: The environment conﬁgurations are randomly generated from octillions of possibilities of a 7 ˆ 7 map, conforming to some high-level speciﬁcations such as the numbers of objects and wall blocks.
Revised: The environment conﬁgurations are randomly generated from octillions of possibilities of a 7 ˆ 7 map, conforming to some high-level speciﬁcations such as the numbers of objects and wall blocks.

Original: For NAV, our model is evaluated on four types of navigation commands:
Revised: For NAV, our model is evaluated on four types of navigation commands:

Original: nav obj :
Revised: nav obj :

Original: Navigate to an object.
Revised: Navigate to an object.

Original: nav col obj :
Revised: nav col obj :

Original: Navigate to an object with a speciﬁc color.
Revised: Navigate to an object with a speciﬁc color.

Original: nav nr obj :
Revised: nav nr obj :

Original: Navigate to a location near an object.
Revised: Navigate to a location near an object.

Original: nav bw obj
Revised: nav bw obj

Original: : Navigate to a location between two objects.
Revised: : Navigate to a location between two objects.

Original: For QA, our model is evaluated on twelve types of questions ( rec * in Table 2).
Revised: For QA, our model is evaluated on twelve types of questions ( rec * in Table 2).

Original: We refer the reader to Appendix B for a detailed description of the experiment settings.
Revised: We refer the reader to Appendix B for a detailed description of the experiment settings.

Original: 4. C OMPARISON METHODS
Revised: 4. C OMPARISON METHODS

Original: Four comparison methods and one ablation method are described below:
Revised: Four comparison methods and one ablation method are described below:

Original: ContextualAttention [CA]
Revised: ContextualAttention [CA]

Original: A variant of our model that replaces the interpreter with acontextual attention model.
Revised: A variant of our model that replaces the interpreter with a

Revised: contextual attention

Revised: model

Original: This attention model uses a gated RNN to convert a sentence to a ﬁlter which isthen convolved with the feature cube h to obtain the attention map x loc .
Revised: This attention model uses a gated RNN to convert a sentence to a ﬁlter which is

Revised: then

Original: The ﬁlter covers the3 ˆneighborhood of each feature vector in the spatial domain.
Revised: convolved with the feature cube h to obtain the attention map x loc .

Revised: The

Revised: ﬁlter

Revised: covers the

Revised: ˆ

Revised: neighborhood of each feature

Revised: vector

Revised: in the spatial domain.

Original: The rest of the model is unchanged.
Revised: The rest of the model is unchanged.

Original: StackedAttentionNet [SAN] An adaptation of a model devised by Yang et al.
Revised: StackedAttentionNet [SAN An adaptation of a model devised by Yang et al

Original: (2016) which was originally proposed for VQA.
Revised: ) which was originally proposed for VQA.

Original: We replace our interpreter with their stacked attention model to compute the attention map x loc .
Revised: We replace our interpreter with their stacked attention model to compute the attention map x loc .

Original: Instead of employing a pretrained CNN as they did, we train a CNN from scratch to accommodate to XWORLD .
Revised: Instead of employing a pretrained CNN as they did, we train a CNN from scratch to accommodate to XWORLD .

Original: The CNN is conﬁgured as the one employed by our model.
Revised: The CNN is conﬁgured as the one employed by our model.

Original: The rest of our model is unchanged.
Revised: The rest of our model is unchanged.

Original: VIS-LSTM
Revised: VIS-LSTM

Original: VL] An adaptation of a model devised by Ren et al.
Revised: VL] An adaptation of a model devised by Ren et al.

Original: (2015) which was originally proposed for VQA.
Revised: (2015) which was originally proposed for VQA.

Original: We ﬂatten h and project it to the word embedding space R D .
Revised: We ﬂatten h and project it to the word embedding space R D .

Original: Then it is appended to the input sentence s as the ﬁrst word.
Revised: Then it is appended to the input sentence s as the ﬁrst word.

Original: The augmented sentence goes through an LSTM whose last state is used for both NAV and QA (Figure 17, Appendix D).
Revised: The augmented sentence goes through an LSTM whose last state is used for both NAV and QA (Figure 17, Appendix D).

Original: ConcatEmbed [CE] An adaptation of a model proposed by Mao et al. (2015) which was originally proposed for image captioning.
Revised: ConcatEmbed [CE] An adaptation of a model proposed by Mao et al. (2015) which was originally proposed for image captioning.

Original: It instantiates L as a vanilla LSTM which outputs a sentence embedding.
Revised: It instantiates L as a vanilla LSTM which outputs a sentence embedding.

Original: Then h is projected and concatenated with the embedding.
Revised: Then h is projected and concatenated with the embedding.

Original: The concatenated vector is used for both NAV and QA (Figure 18 Appendix D).
Revised: The concatenated vector is used for both NAV and QA (Figure 18 Appendix D).

Original: This concatenation mechanism is also employed by Hermann et al.
Revised: This concatenation mechanism is also employed by Hermann et al.

Original: ; Misra et al.
Revised: ; Misra et al.

Original: NavAlone [NAVA] An ablation of our model that does not have the QA pipeline ( M P and P ) and is trained only on the NAV tasks.
Revised: NavAlone [NAVA] An ablation of our model that does not have the QA pipeline ( M P and P ) and is trained only on the NAV tasks.

Original: The rest of the model is the same.
Revised: The rest of the model is the same.

Original: In the following experiments, we train all six approaches (four comparison methods, one ablation, and our model) with a small learning rate of 1 ˆ 10 ´ 5 and a batch size of 16, for a maximum of 200k minibatches.

Original: Additional training details are described in Appendix C. After training, we test each approach on 50k sessions.

Original: For NAV, we compute the average success rate of navigation where a success is deﬁned as reaching the correct location before the time out of a session.

Original: For QA, we compute the average accuracy in answering the questions.

Original: Number of training examples ( × 10 6 ) −−−−−
Revised: Number of training examples ( × 10 6 ) −−−−−

Original: [Figure]
Revised: [Figure]

Original: Ours
Revised: Ours

Original: CA
Revised: CA

Original: SAN
Revised: SAN

Original: CE
Revised: CE

Original: VL
Revised: VL

Original: NAVA
Revised: NAVA

Original: [Figure]
Revised: [Figure]

Revised: In the following experiments, we train all six approaches (four comparison methods, one ablation, and our model) with a small learning rate of 1 ˆ 10 ´ 5 and a batch size of 16, for a maximum of 200k minibatches.

Revised: Additional training details are described in Appendix C. After training, we test each approach on 50k sessions.

Revised: For NAV, we compute the average success rate of navigation where a success is deﬁned as reaching the correct location before the time out of a session.

Revised: For QA, we compute the average accuracy in answering the questions.

Original: 4. B ASIC EVALUATION
Revised: 4. B ASIC EVALUATION

Original: In this experiment, the training and testing sentences (including NAV commands and QA questions) are sampled from the same distribution over the entire sentence space.
Revised: In this experiment, the training and testing sentences (including NAV commands and QA questions) are sampled from the same distribution over the entire sentence space.

Original: We call it the normal language setting.
Revised: We call it the normal language setting.

Original: The training reward curves and testing results are shown in Figure 6.
Revised: The training reward curves and testing results are shown in Figure 6.

Original: VL and CE have quite low rewards without convergences.
Revised: VL and CE have quite low rewards without convergences.

Original: These two approaches do not use the spatial attention x loc , and thus always attend to whole images with no focus.
Revised: These two approaches do not use the spatial attention x loc , and thus always attend to whole images with no focus.

Original: The region of a target location is tiny compared to the whole egocentric image (a ratio of 1 : p
Revised: The region of a target location is tiny compared to the whole egocentric image (a ratio of 1 : p

Original: 7 ˆ 2 ´ 1 q 2 “ 1 : 169 ).
Revised: 7 ˆ 2 ´ 1 q 2 “ 1 : 169 ).

Original: It is possible that this difﬁculty drives the models towards overﬁtting QA without learning useful representations for NAV.
Revised: It is possible that this difﬁculty drives the models towards overﬁtting QA without learning useful representations for NAV.

Original: Both CA and SAN obtain rewards and success rates slightly worse than ours.
Revised: Both CA and SAN obtain rewards and success rates slightly worse than ours.

Original: This suggests that in a normal language setting, existing attention models are able to handle previously seen sentences.
Revised: This suggests that in a normal language setting, existing attention models are able to handle previously seen sentences.

Original: However, their language generalization abilities, especially on the ZS2 sentences, are usually very weak, as we will demonstrate in the next section.
Revised: However, their language generalization abilities, especially on the ZS2 sentences, are usually very weak, as we will demonstrate in the next section.

Original: The ablation NAVA has a very large variance in its performance.
Revised: The ablation NAVA has a very large variance in its performance.

Original: Depending on the random seed, its reward can reach that of SAN ( ´ 0 . 8 ), or it can be as low as that of CE ( ´ 3 . 0 ).
Revised: Depending on the random seed, its reward can reach that of SAN ( ´ 0 . 8 ), or it can be as low as that of CE ( ´ 3 . 0 ).

Original: Comparing it to our full method, we conclude that even though the QA pipeline operates on a completely different set of sentences, it learns useful local sentential knowledge that results in an effective training of the NAV pipeline.
Revised: Comparing it to our full method, we conclude that even though the QA pipeline operates on a completely different set of sentences, it learns useful local sentential knowledge that results in an effective training of the NAV pipeline.

Original: Note that all the four comparison methods obtained high QA accuracies ( ą 70%, see Figure 6c), despite their distinct NAV results.
Revised: Note that all the four comparison methods obtained high QA accuracies ( ą 70%, see Figure 6c), despite their distinct NAV results.

Original: This suggests that QA, as a supervised learning task, is easier than NAV as an RL task in our scenario.
Revised: This suggests that QA, as a supervised learning task, is easier than NAV as an RL task in our scenario.

Original: One reason is that the groundtruth label in QA is a much stronger training signal than the reward in NAV.
Revised: One reason is that the groundtruth label in QA is a much stronger training signal than the reward in NAV.

Original: Another reason might be that NAV additionally requires learning the control module, which is absent in QA.
Revised: Another reason might be that NAV additionally requires learning the control module, which is absent in QA.

Original: 4. L ANGUAGE GENERALIZATION
Revised: 4. L ANGUAGE GENERALIZATION

Original: Our more important question is whether the agent has the ability of interpreting zero-shot sentences.
Revised: Our more important question is whether the agent has the ability of interpreting zero-shot sentences.

Original: For comparison, we use CA and SAN from the previous section, as they achieved good performance in the normal language setting.
Revised: For comparison, we use CA and SAN from the previous section, as they achieved good performance in the normal language setting.

Original: For a zero-shot setting, we set up two language conditions:
Revised: For a zero-shot setting, we set up two language conditions:

Original: NewWordCombination [ZS1] Some word pairs are excluded from both the NAV commands and the QA questions.
Revised: NewWordCombination [ZS1] Some word pairs are excluded from both the NAV commands and the QA questions.

Original: We consider three types of unordered combinations of the content words: ( object , spatial relation ), ( object , color ), and ( object , object ).
Revised: We consider three types of unordered combinations of the content words: ( object , spatial relation ), ( object , color ), and ( object , object ).

Original: We randomly hold out X % of the word pairs during the training.
Revised: We randomly hold out X % of the word pairs during the training.

Original: NewWord [ZS2] Some single words are excluded from both the NAV commands and the QA questions, but can appear in the QA answers.
Revised: NewWord [ZS2] Some single words are excluded from both the NAV commands and the QA questions, but can appear in the QA answers.

Original: We randomly hold out X % of the object words during the training.
Revised: We randomly hold out X % of the object words during the training.

Original: We vary the value of X (12.5, 20.0, 50.0, 66.7, or 90.0) in both conditions to test how sensitive the generalization is to the amount of the held-out data.
Revised: We vary the value of X (12.5, 20.0, 50.0, 66. or 90.0) in both conditions to test how sensitive the generalization is to the amount of the held-out data.

Original: For evaluation, we report the test results only for the zero-shot sentences that contain the held-out word pairs or words.
Revised: For evaluation, we report the test results only for the zero-shot sentences that contain the held-out word pairs or words.

Original: The results are shown in
Revised: The results are shown in

Original: Figure 7.
Revised: Figure 7.

Original: We draw three conclusions from the results.
Revised: We draw three conclusions from the results.

Original: First, the ZS1 sentences are much easier to interpret than the ZS2 sentences.
Revised: First, the ZS1 sentences are much easier to interpret than the ZS2 sentences.

Original: Neural networks seem to inherently have this capability to some extent.
Revised: Neural networks seem to inherently have this capability to some extent.

Original: This is consistent with what has been observed in some previous work (Hermann et al., 2017; Chaplot et al., 2018) that addresses the generalization on new word combinations.
Revised: This is consistent with what has been observed in some previous work (Hermann et al., 2017; Chaplot et al., 2018) that addresses the generalization on new word combinations.

Original: Second, the ZS2 sentences are difﬁcult for CA and SAN .
Revised: Second, the ZS2 sentences are difﬁcult for CA and SAN .

Original: Even with a held-out portion as small as X % “ 12 . 5% , their navigation success rates and QA accuracies drop up to 80% and 35%, respectively, compared to those in the normal language setting.
Revised: Even with a held-out portion as small as X % “ 12 . 5% , their navigation success rates and QA accuracies drop up to 80% and 35%, respectively, compared to those in the normal language setting.

Original: In contrast, our model tends to maintain the same results until X “ 90 . 0 .
Revised: In contrast, our model tends to maintain the same results until X “ 90 . 0 .

Original: Impressively, it achieves an average success rate of 60% and an average accuracy of 83% even when the number of new object words is 9 times that of seen object words in the NAV commands and
Revised: Impressively, it achieves an average success rate of 60% and an average accuracy of 83% even when the number of new object words is 9 times that of seen object words in the NAV commands and

Original: QA questions, respectively!
Revised: QA questions, respectively!

Original: Third, in ZS2 , if we compare the slopes of the success-rate curves with those of the accuracy curves (as shown in Figures 7e and 7f), we notice that the agent generalizes better on QA than on NAV.
Revised: Third, in ZS2 , if we compare the slopes of the success-rate curves with those of the accuracy curves (as shown in Figures 7e and 7f), we notice that the agent generalizes better on QA than on NAV.

Original: This further veriﬁes our ﬁnding in the previous section that QA is in general an easier task than NAV in XWORLD .
Revised: This further veriﬁes our ﬁnding in the previous section that QA is in general an easier task than NAV in XWORLD .

Original: This demonstrates the necessity of evaluating NAV in addition to QA, as NAV requires additional language grounding to control.
Revised: This demonstrates the necessity of evaluating NAV in addition to QA, as NAV requires additional language grounding to control.

Original: Interestingly, we notice that nav bw obj is an outlier command type for which CA is much less sensitive to the increase of X in ZS2 .
Revised: Interestingly, we notice that nav bw obj is an outlier command type for which CA is much less sensitive to the increase of X in ZS2 .

Original: A deep analysis reveals that for nav bw obj , CA learns to cheat by looking for the image region that contains the special pattern of object pairs in the image without having to recognize the objects.
Revised: A deep analysis reveals that for nav bw obj , CA learns to cheat by looking for the image region that contains the special pattern of object pairs in the image without having to recognize the objects.

Original: This suggests that neural networks tend to exploit data in an unexpected way to achieve tasks if no constraints are imposed (Kottur et al., 2017).
Revised: This suggests that neural networks tend to exploit data in an unexpected way to achieve tasks if no constraints are imposed (Kottur et al., 2017).

Original: To sum up, our model exhibits a strong generalization ability on both ZS1 and ZS2 sentences, the latter of which pose a great challenge for traditional language grounding models.
Revised: To sum up, our model exhibits a strong generalization ability on both ZS1 and ZS2 sentences, the latter of which pose a great challenge for traditional language grounding models.

Original: Although we use a particular 2D world for evaluation in this work, the promising results imply the potential for scaling to an even larger vocabulary and grammar with a much larger language space. 
Revised: Although we use a particular 2D world for evaluation in this work, the promising results imply the potential for scaling to an even larger vocabulary and grammar with a much larger language space. [Figure] [Table] [Figure]

Revised: [Table]

Original: 4. H OW DOES IT ADAPT TO 3D?
Revised: [Figure] [Table] [Figure] [Table]

Original:  We discuss the possibility of adapting our model to an agent with similar language abilities in a 3D world (e.g., Beattie et al.
Revised: [Figure] [Figure] 4. H OW DOES IT ADAPT TO 3D? We discuss the possibility of adapting our model to an agent with similar language abilities in a 3D world (e.g., Beattie et al.

Original: ; Johnson et al.
Revised: ; Johnson et al.

Original: This is our goal for the future, but here we would like to share some preliminary thoughts.
Revised: This is our goal for the future [Figure] but here we would like to share some preliminary thoughts.

Original: Generally speaking, a 3D world will pose a greater challenge for vision-related computations.
Revised: Generally speaking, a 3D world will pose a greater challenge for vision-related computations.

Original: The key element of our model is the attention cube x cube that is intended for an explicit language grounding, including the channel mask x feat and [Figure] [Table] [Figure]
Revised: The key element of our model is the attention cube x cube that is intended for an explicit language grounding, including the channel mask x feat and the attention map x loc .

Original: [Table]

Original: [Figure]

Original: [Table]

Original: [Figure]

Original: [Table]

Original: [Figure]

Original: [Figure]

Original: the attention map x loc .

Original: The channel mask only depends on the sentence, and thus is expected to work regardless of the world’s dimensionality.
Revised: The channel mask only depends on the sentence, and thus is expected to work regardless of the world’s dimensionality.

Original: The interpreter depends on a sequence of score maps χ which for now are computed as multiplying a word embedding with the feature cube (Eq. 3).
Revised: The interpreter depends on a sequence of score maps χ which for now are computed as multiplying a word embedding with the feature cube (Eq. 3).

Original: A more sophisticated deﬁnition of φ will be needed to detect objects in a 3D environment.
Revised: A more sophisticated deﬁnition of φ will be needed to detect objects in a 3D environment.

Original: Additionally, the interpreter models the spatial transform of attention as a 2D convolution (Eq. 7).
Revised: Additionally, the interpreter models the spatial transform of attention as a 2D convolution (Eq. 7).

Original: This assumption will be not valid for reasoning 3D spatial relations on 2D images, and a better transform method that accounts for perspective distortion is required.
Revised: This assumption will be not valid for reasoning 3D spatial relations on 2D images, and a better transform method that accounts for perspective distortion is required.

Original: Lastly, the surrounding environment is only partially observable to a 3D agent.
Revised: Lastly, the surrounding environment is only partially observable to a 3D agent.

Original: A working memory, such as an LSTM added to the action module A , will be important for navigation in this case.
Revised: A working memory, such as an LSTM added to the action module A , will be important for navigation in this case.

Original: Despite these changes to be made, we believe that our general explicit grounding strategy and the common detection function shared by language grounding and prediction have shed some light on the adaptation.
Revised: Despite these changes to be made, we believe that our general explicit grounding strategy and the common detection function shared by language grounding and prediction have shed some light on the adaptation.

Original:  C ONCLUSION
Revised:  C ONCLUSION

Original: We have presented an end-to-end model of a virtual agent for acquiring language from a 2D world in an interactive manner, through the visual and linguistic perception channels.
Revised: We have presented an end-to-end model of a virtual agent for acquiring language from a 2D world in an interactive manner, through the visual and linguistic perception channels.

Original: After learning, the agent is able to both interpolate and extrapolate to interpret zero-shot sentences that contain new word combinations or even new words.
Revised: After learning, the agent is able to both interpolate and extrapolate to interpret zero-shot sentences that contain new word combinations or even new words.

Original: This generalization ability is supported by an explicit grounding strategy that disentangles the language grounding from the subsequent languageindependent computations.
Revised: This generalization ability is supported by an explicit grounding strategy that disentangles the language grounding from the subsequent languageindependent computations.

Original: It also depends on sharing a detection function between the language grounding and prediction as the core computation.
Revised: It also depends on sharing a detection function between the language grounding and prediction as the core computation.

Original: This function enables the word meanings to transfer from the prediction to the grounding during the test time.
Revised: This function enables the word meanings to transfer from the prediction to the grounding during the test time.

Original: Promising language acquisition and generalization results have been obtained in the 2D XWORLD .
Revised: Promising language acquisition and generalization results have been obtained in the 2D XWORLD .

Original: We hope that this work can shed some light on acquiring and generalizing language in a similar way in a 3D world.
Revised: We hope that this work can shed some light on acquiring and generalizing language in a similar way in a 3D world.

Original: A CKNOWLEDGMENTS
Revised: A CKNOWLEDGMENTS

Original: We thank the anonymous reviewers for providing valuable comments and suggestions.
Revised: We thank the anonymous reviewers for providing valuable comments and suggestions.

Original: We thank the other team members, Yuanpeng Li, Liang Zhao, Yi Yang, Zihang Dai, Qing Sun, Jianyu Wang, and
Revised: We thank the other team members, Yuanpeng Li, Liang Zhao, Yi Yang, Zihang Dai, Qing Sun, Jianyu Wang and

Original: Xiaochen Lian, for helpful discussions.
Revised: Xiaochen Lian, for helpful discussions.

Original: We thank Jack Feerick and Richard Mark for proofreading.
Revised: We thank Jack Feerick and Richard Mark for proofreading.

Original: Finally, we specially thank Dhruv Batra for his feedback on an early version of this paper.
Revised: Finally, we specially thank Dhruv Batra for his feedback on an early version of this paper

Original: A PPENDICES A V ISUALIZATION AND ANALYSIS
Revised: A PPENDICES A V ISUALIZATION AND ANALYSIS

Original: In this section, we visualize and analyze some intermediate results of our trained model.
Revised: In this section, we visualize and analyze some intermediate results of our trained model.

Original: [Figure]
Revised: [Figure]

Original: We inspect the channel mask x feat which allows the model to select certain feature maps from a feature cube h and predict an answer to the question s .
Revised: We inspect the channel mask x feat which allows the model to select certain feature maps from a feature cube h and predict an answer to the question s .

Original: We randomly sample 10k QA questions and compute x feat for each of them using the grounding module L .
Revised: We randomly sample 10k QA questions and compute x feat for each of them using the grounding module L .

Original: We divide the 10k questions into 134 groups, where each group corresponds to a different answer.
Revised: We divide the 10k questions into 134 groups, where each group corresponds to a different answer.

Original: Then we compute an Euclidean distance matrix D
Revised: Then we compute an Euclidean distance matrix D

Original: where entry
Revised: where entry

Original: D r i, j s is the average distance between the x feat of a question from the i th group and that from the j th group (Figure 8).
Revised: D r i, j s is the average distance between the x feat of a question from the i th group and that from the j th group (Figure 8).

Original: It is clear that
Revised: It is clear that

Original: Where is green apple located ?
Revised: Where is green apple located ?

Original: What is between lion and hedgehog ?
Revised: What is between lion and hedgehog ?

Original: The grid in east to cherry ?
Revised: The grid in east to cherry ?

Original: The location of the fish is ?
Revised: The location of the fish is ?

Original: What is in blue ?
Revised: What is in blue ?

Original: low high
Revised: low high

Original: [Figure]
Revised: [Figure]

Original: there are three topics (object, color, and spatial relation) in the matrix.
Revised: there are three topics (object, color, and spatial relation) in the matrix.

Original: The distances computed within a topic are much smaller than those computed across topics.
Revised: The distances computed within a topic are much smaller than those computed across topics.

Original: This demonstrates that with the channel mask, the model is able to look at different subsets of features for questions of different topics, while using the same subset of features for questions of the same topic.
Revised: This demonstrates that with the channel mask, the model is able to look at different subsets of features for questions of different topics, while using the same subset of features for questions of the same topic.

Original: It also implies that the feature cube h is learned to organize feature maps according to image attributes.
Revised: It also implies that the feature cube h is learned to organize feature maps according to image attributes.

Original: Word context & attention.
Revised: Word context & attention.

Original: To intuitively demonstrate how the interpreter works, we visualize the word context vectors w l in Eq.
Revised: To intuitively demonstrate how the interpreter works, we visualize the word context vectors w l in Eq.

Original: 6 for a total of 20k word locations l (10k from QA questions and the other 10k from NAV commands).
Revised: 6 for a total of 20k word locations l (10k from QA questions and the other 10k from NAV commands).

Original: Each word context vector is projected down to a space ofdimensions using PCA (Jolliffe, 1986), after which we use t-SNE (van der Maaten & Hinton, 2008;
Revised: Each word context vector is projected down to a space ofdimensions using PCA (Jolliffe, 1986), after which we use t-SNE (van der Maaten & Hinton, 2008;

Original: Ulyanov, 2016) to further reduce the dimensionality to 2.
Revised: Ulyanov, 2016) to further reduce the dimensionality to 2.

Original: The t-SNE method uses a perplexity ofand a learning rate of 200, and runs for 1k iterations.
Revised: The t-SNE method uses a perplexity ofand a learning rate of 200, and runs for 1k iterations.

Original: The visualization of the 20k points is shown in
Revised: The visualization of the 20k points is shown in

Original: Figure 9a.
Revised: Figure 9a.

Original: Recall that in Eq.
Revised: Recall that in Eq.

Original: 6 the word attention is computed by comparing the interpreter state p i ´with the word context vectors w l .
Revised: 6 the word attention is computed by comparing the interpreter state p i ´with the word context vectors w l .

Original: In order to select the content words to generate meaningful score maps via φ , the interpreter is expected to differentiate them from the remaining grammatical words based on the contexts.
Revised: In order to select the content words to generate meaningful score maps via φ , the interpreter is expected to differentiate them from the remaining grammatical words based on the contexts.

Original: This expectation is veriﬁed by the above visualization in which the context vectors of the content words (in blue, green, and red) and those of the grammatical words (in black) are mostly separated.
Revised: This expectation is veriﬁed by the above visualization in which the context vectors of the content words (in blue, green, and red) and those of the grammatical words (in black) are mostly separated.

Original: Figure 9b shows some example questions whose word attentions are computed from the word context vectors.
Revised: Figure 9b shows some example questions whose word attentions are computed from the word context vectors.

Original: It can be seen that the content words are successfully selected by the interpreter.
Revised: It can be seen that the content words are successfully selected by the interpreter.

Original: Attention map x loc .
Revised: Attention map x loc .

Original: Finally, we visualize the computation of the attention map x loc .
Revised: Finally, we visualize the computation of the attention map x loc .

Original: In each of the following six examples, the intermediate attention maps x i loc and word attentions o il (in Eq. 7) of the three interpretation steps are shown from top to bottom.
Revised: In each of the following six examples, the intermediate attention maps x i loc and word attentions o il (in Eq. 7) of the three interpretation steps are shown from top to bottom.

Original: Each step shows the current attention map x i loc overlaid on the environment image e .
Revised: Each step shows the current attention map x i loc overlaid on the environment image e .

Original: The last attention map x 3 loc is the ﬁnal output of the interpreter at the current time.
Revised: The last attention map x 3 loc is the ﬁnal output of the interpreter at the current time.

Original: Note that not all the results of the three steps are needed to generate the ﬁnal output.
Revised: Note that not all the results of the three steps are needed to generate the ﬁnal output.

Original: Some results might be discarded according to the value of the update gate ρ i .
Revised: Some results might be discarded according to the value of the update gate ρ i .

Original: As a result, sometimes the interpreter may produce “bogus” intermediate attention maps which do not contribute to x loc .
Revised: As a result, sometimes the interpreter may produce “bogus” intermediate attention maps which do not contribute to x loc .

Original: Each example also visualizes the environment terrain map x terr (deﬁned in Appendix C) that perfectly detects all the objects and blocks, and thus provides a good guide for the agent to navigate successfully without hitting walls or confounding targets.
Revised: Each example also visualizes the environment terrain map x terr (deﬁned in Appendix C) that perfectly detects all the objects and blocks, and thus provides a good guide for the agent to navigate successfully without hitting walls or confounding targets.

Original: For a better visualization, the egocentric images are converted back to the normal view.
Revised: For a better visualization, the egocentric images are converted back to the normal view.

Original: “Please move to east of cabbage .”
Revised: “Please move to east of cabbage .”

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: “Navigate to west of garlic please .”
Revised: “Navigate to west of garlic please .”

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: high

Original: “Navigate to red beet .”
Revised: “Navigate to red beet .”

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: “Tell the name of the object which is west to snake .”
Revised: “Tell the name of the object which is west to snake .”

Original: [Figure]
Revised: [Figure]

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: “What is north near vacuum?”
Revised: “What is north near vacuum

Revised: [Figure]

Original: [Equation]
Revised: [Equation]

Original: [Figure]
Revised: [Figure]

Original: B XWORLD SETUP
Revised: B XWORLD SETUP

Original: XWORLD is conﬁgured with 7 ˆ 7 grid maps.
Revised: XWORLD is conﬁgured with 7 ˆ 7 grid maps.

Original: On each map, the open space for the agent has a size ranging from 3 ˆ 3 to 7 ˆ 7 .
Revised: On each map, the open space for the agent has a size ranging from 3 ˆ 3 to 7 ˆ 7 .

Original: Smaller open spaces are set up for curriculum learning (Appendix F), but not for testing.
Revised: Smaller open spaces are set up for curriculum learning (Appendix F), but not for testing.

Original: To keep the size of the environment image ﬁxed, we pad the map with wall blocks if the open space has a size less than 7 .
Revised: To keep the size of the environment image ﬁxed, we pad the map with wall blocks if the open space has a size less than 7 .

Original: The agent has four navigation actions in total: left , right , up , and down .
Revised: The agent has four navigation actions in total: left , right , up , and down .

Original: In each session,
Revised: In each session,

Original: I The maximum number of time steps is four times the map size.
Revised: I The maximum number of time steps is four times the map size.

Original: That is, the agent only has 7 ˆ 4 “ 28 steps to reach a target.
Revised: That is, the agent only has 7 ˆ 4 “ 28 steps to reach a target.

Original: II The number of objects on the map ranges from 1 to 5.
Revised: II The number of objects on the map ranges from 1 to 5.

Original: III
Revised: III

Original: The number of wall blocks on the map ranges from 0 to 15.
Revised: The number of wall blocks on the map ranges from 0 to 15.

Original: IV
Revised: IV

Original: The positive reward when the agent reaches the correct location is 1 . 0 .
Revised: The positive reward when the agent reaches the correct location is 1 . 0 .

Original: V The negative rewards for hitting walls and for stepping on non-target objects are ´ 0 .
Revised: V The negative rewards for hitting walls and for stepping on non-target objects are ´ 0 .

Original: 2 and ´ 1 . 0 , respectively.
Revised: 2 and ´ 1 . 0 , respectively.

Original: VI The time penalty of each step is ´ 0 .
Revised: VI The time penalty of each step is ´ 0 .

Original: [Table]
Revised: [Table]

Original: The teacher has a vocabulary size of 185.
Revised: The teacher has a vocabulary size of 185.

Original: There are 9 spatial relations, 8 colors, 119 distinct object classes, and 50 grammatical words.
Revised: There are 9 spatial relations, 8 colors, 119 distinct object classes, and 50 grammatical words.

Original: Every object class contains 3 different instances.
Revised: Every object class contains 3 different instances.

Original: All object instances are shown in Figure 16.
Revised: All object instances are shown in Figure 16.

Original: Every time the environment is reset, a number of object classes are randomly sampled and an object instance is randomly sampled for each class.
Revised: Every time the environment is reset, a number of object classes are randomly sampled and an object instance is randomly sampled for each class.

Original: There are in total 16 types of sentences that the teacher can speak, including 4 types of NAV commands andtypes of QA questions.
Revised: There are in total 16 types of sentences that the teacher can speak, including 4 types of NAV commands andtypes of QA questions.

Original: Each sentence type has several non-recursive templates, and corresponds to a concrete type of tasks the agent must learn to accomplish.
Revised: Each sentence type has several non-recursive templates, and corresponds to a concrete type of tasks the agent must learn to accomplish.

Original: In total there are 1,639,015 distinct sentences with 567,579 for NAV and 1,071,436 for QA.
Revised: In total there are 1,639,015 distinct sentences with 567,579 for NAV and 1,071,436 for QA.

Original: The sentence length varies between 2 and 13.
Revised: The sentence length varies between 2 and 13.

Original: The object, spatial-relation, and color words of the teacher’s language are listed in Table 1.
Revised: The object, spatial-relation, and color words of the teacher’s language are listed in Table 1.

Original: These are the content words that can be grounded in XWORLD .
Revised: These are the content words that can be grounded in XWORLD .

Original: All the others are grammatical words.
Revised: All the others are grammatical words.

Original: Note that the differentiation between the content and the grammatical words is automatically learned by the agent according to the tasks.
Revised: Note that the differentiation between the content and the grammatical words is automatically learned by the agent according to the tasks.

Original: Every word is represented by an entry in the word embedding table.
Revised: Every word is represented by an entry in the word embedding table.

Original: The sentence types that the teacher can speak are listed in Table 2.
Revised: The sentence types that the teacher can speak are listed in Table 2.

Original: Each type has a triggering condition about when the teacher says that type of sentences.
Revised: Each type has a triggering condition about when the teacher says that type of sentences.

Original: Besides the shown conditions, an extra condition for NAV commands is that the target must be reachable from the current agent location.
Revised: Besides the shown conditions, an extra condition for NAV commands is that the target must be reachable from the current agent location.

Original: An extra condition for color-related questions is that the object color must be one of the eight deﬁned colors.
Revised: An extra condition for color-related questions is that the object color must be one of the eight deﬁned colors.

Original: If at any time step there are multiple types triggered, we randomly sample one for NAV and another for QA.
Revised: If at any time step there are multiple types triggered, we randomly sample one for NAV and another for QA.

Original: After a sentence type is sampled, we generate a sentence according to the corresponding sentence templates.
Revised: After a sentence type is sampled, we generate a sentence according to the corresponding sentence templates.

Original: C I MPLEMENTATION DETAILS
Revised: C I MPLEMENTATION DETAILS

Original: The environment image e is a 156 ˆ 156 egocentric RGB image.
Revised: The environment image e is a 156 ˆ 156 egocentric RGB image.

Original: The CNN in F has four convolutional layers: p 3 , 3 , 64 q , p 2 , 2 , 64 q , p 2 , 2 , 256 q , p 1 , 1 , 256 q , where p a, b, c q represents a layer conﬁguration of c kernels of size a applied at stride width b .
Revised: The CNN in F has four convolutional layers: p 3 , 3 , 64 q , p 2 , 2 , 64 q , p 2 , 2 , 256 q , p 1 , 1 , 256 q , where p a, b, c q represents a layer conﬁguration of c kernels of size a applied at stride width b .

Original: All the four layers are ReLU activated.
Revised: All the four layers are ReLU activated.

Original: To enable the agent to reason about spatial-relation words (e.g., “north”), we append an additional parametric cube to the convolutional output to obtain h .
Revised: To enable the agent to reason about spatial-relation words (e.g., “north”), we append an additional parametric cube to the convolutional output to obtain h .

Original: This parametric cube has the same number of channels with the CNN output, and it is initialized with a zero mean and a zero standard deviation.
Revised: This parametric cube has the same number of channels with the CNN output, and it is initialized with a zero mean and a zero standard deviation.

Original: The word embedding table is initialized with a zero mean and a unit standard deviation.
Revised: The word embedding table is initialized with a zero mean and a unit standard deviation.

Original: All the gated
Revised: All the gated

Original: RNNs (including the bidirectional RNN) in L have 128 units.
Revised: RNNs (including the bidirectional RNN) in L have 128 units.

Original: All the layers in L , unless otherwise stated, use tanh as the activation function. 
Revised: All the layers in L , unless otherwise stated, use tanh as the activation function. [Table] objects

Original: [Table]
Revised: [Table]

Original: rec bw obj2col
Revised: rec bw obj2col

Original: [Table]
Revised: [Table]

Original: For NAV, x loc is used as the target to reach on the image plane.
Revised: For NAV, x loc is used as the target to reach on the image plane.

Original: However, knowing this alone does not sufﬁce.
Revised: However, knowing this alone does not sufﬁce.

Original: The agent must also be aware of walls and possibly confounding targets (other objects) in the environment.
Revised: The agent must also be aware of walls and possibly confounding targets (other objects) in the environment.

Original: Toward this end, M A further computes an environment terrain map x terr “ σ p hf q where f P R D is a parameter vector to be learned and σ is sigmoid.
Revised: Toward this end, M A further computes an environment terrain map x terr “ σ p hf q where f P R D is a parameter vector to be learned and σ is sigmoid.

Original: We expect that x terr detects any blocks informative for navigation.
Revised: We expect that x terr detects any blocks informative for navigation.

Original: Note that x terr is unrelated to the speciﬁc command; it solely depends on the current environment.
Revised: Note that x terr is unrelated to the speciﬁc command; it solely depends on the current environment.

Original: After stacking x loc and x terr together, M A feeds them to another
Revised: After stacking x loc and x terr together, M A feeds them to another

Original: CNN followed by an MLP.
Revised: CNN followed by an MLP.

Original: The CNN has two convolutional layers p 3 , 1 , 64 q and p 3 , 1 , 4 q , both with paddings of 1 .
Revised: The CNN has two convolutional layers p 3 , 1 , 64 q and p 3 , 1 , 4 q , both with paddings of 1 .

Original: It is followed by a three-layer MLP where each layer has 512 units and is ReLU activated.
Revised: It is followed by a three-layer MLP where each layer has 512 units and is ReLU activated.

Original: The action module A contains a two-layer MLP of which the ﬁrst layer has 512 ReLU activated units and the second layer is softmax whose output dimension is equal to the number of actions.
Revised: The action module A contains a two-layer MLP of which the ﬁrst layer has 512 ReLU activated units and the second layer is softmax whose output dimension is equal to the number of actions.

Original: We use adagrad (Duchi et al., 2011) with a learning rate of 10 ´ 5 for stochastic gradient descent (SGD).
Revised: We use adagrad (Duchi et al., 2011) with a learning rate of 10 ´ 5 for stochastic gradient descent (SGD).

Original: The reward discount factor is set to 0 .
Revised: The reward discount factor is set to 0 .

Original: All the parameters have a default weight decay of 10 ´ 4 ˆ 16 .
Revised: All the parameters have a default weight decay of 10 ´ 4 ˆ 16 .

Original: For each layer, its parameters have zero mean and a standard deviation of 1 { ?
Revised: For each layer, its parameters have zero mean and a standard deviation of 1 { ?

Original: K , where K is the number of parameters of that layer.
Revised: K , where K is the number of parameters of that layer.

Original: We set the maximum interpretation step I “ 3 .
Revised: We set the maximum interpretation step I “ 3 .

Original: The whole model is trained end to end.
Revised: The whole model is trained end to end.

Original: D B ASELINE DETAILS
Revised: D B ASELINE DETAILS

Original: Some additional implementation details of the baselines in Section 4.3 are described below.
Revised: Some additional implementation details of the baselines in Section 4.3 are described below.

Original: [CA] Its RNN has 512 units.
Revised: [CA] Its RNN has 512 units.

Original: [Figure]
Revised: [Figure]

Original: [Figure]
Revised: [Figure]

Original: [VL] Its CNN has four convolutional layers p 3 , 2 , 64 q , p 3 , 2 , 64 q , p 3 , 2 , 128 q , and p 3 , 1 , 128 q .
Revised: [VL] Its CNN has four convolutional layers p 3 , 2 , 64 q , p 3 , 2 , 64 q , p 3 , 2 , 128 q , and p 3 , 1 , 128 q .

Original: This is followed by a fully-connected layer of size 512 , which projects the feature cube to the word embedding space.
Revised: This is followed by a fully-connected layer of size 512 , which projects the feature cube to the word embedding space.

Original: The RNN has 512 units.
Revised: The RNN has 512 units.

Original: For either QA or NAV, the RNN’s last state goes through a three-layer MLP of which each layer has 512 units (Figure 17).
Revised: For either QA or NAV, the RNN’s last state goes through a three-layer MLP of which each layer has 512 units (Figure 17).

Original: [CE] It has the same layer-size conﬁguration with VL (Figure 18).
Revised: [CE] It has the same layer-size conﬁguration with VL (Figure 18).

Original: [SAN] Its RNN has 256 units.
Revised: [SAN] Its RNN has 256 units.

Original: Following the original approach (Yang et al., 2016), we use two attention layers.
Revised: Following the original approach (Yang et al., 2016), we use two attention layers.

Original: All the layers of the above baselines are ReLU activated.
Revised: All the layers of the above baselines are ReLU activated.

Original: E E XPLORATION AND E XPERIENCE R EPLAY
Revised: E E XPLORATION AND E XPERIENCE R EPLAY

Original: The agent has one million exploration steps in total, and the exploration rate λ decreases linearly from 1 to 0 .
Revised: The agent has one million exploration steps in total, and the exploration rate λ decreases linearly from 1 to 0 .

Original: At each time step, the agent takes an action a P t left , right , up , down u with a probability of
Revised: At each time step, the agent takes an action a P t left , right , up , down u with a probability of

Original: [Equation]
Revised: [Equation]

Original: where π θ is the current policy, and s and e denote the current command and environment image, respectively.
Revised: where π θ is the current policy, and s and e denote the current command and environment image, respectively.

Original: To stabilize the learning, we also employ experience replay (ER) (Mnih et al., 2015).
Revised: To stabilize the learning, we also employ experience replay (ER) (Mnih et al., 2015).

Original: The environment inputs, rewards, and the actions taken by the agent in the most recent 10k time steps are stored in a replay buffer.
Revised: The environment inputs, rewards, and the actions taken by the agent in the most recent 10k time steps are stored in a replay buffer.

Original: During training, each time a minibatch t a i , s i , e i , r
Revised: During training, each time a minibatch t a i , s i , e i , r

Original: i u
Revised: i u

Original: Ni “ 1 is sampled from the buffer, using the rank-based sampler (Schaul et al., 2016) which has proven to increase the training efﬁciency by prioritizing rare experiences.
Revised: Ni “ 1 is sampled from the buffer, using the rank-based sampler (Schaul et al., 2016) which has proven to increase the training efﬁciency by prioritizing rare experiences.

Original: Then we compute the gradient as:
Revised: Then we compute the gradient as:

Original: [Equation]
Revised: [Equation]

Original: where i is the sample index in the batch, s 1 i and e 1 i are the command and image in the next time step, v is the value function, θ are the current parameters, θ ´ are the target parameters that have an update delay, and γ is the discount factor.
Revised: where i is the sample index in the batch, s 1 i and e 1 i are the command and image in the next time step, v is the value function, θ are the current parameters, θ ´ are the target parameters that have an update delay, and γ is the discount factor.

Original: This gradient maximizes the expected reward while minimizing the temporal-difference (TD) error.
Revised: This gradient maximizes the expected reward while minimizing the temporal-difference (TD) error.

Original: Note that because of ER, our AC method is off-policy.
Revised: Note that because of ER, our AC method is off-policy.

Original: To avoid introducing biases into the gradient, importance ratios are needed.
Revised: To avoid introducing biases into the gradient, importance ratios are needed.

Original: However, we ignored them in the above gradient for implementation simplicity.
Revised: However, we ignored them in the above gradient for implementation simplicity.

Original: We found that the current implementation worked well in practice for our problem.
Revised: We found that the current implementation worked well in practice for our problem.

Original: F C URRICULUM LEARNING
Revised: F C URRICULUM LEARNING

Original: We exploit curriculum learning (Bengio et al., 2009) to gradually increase the environment complexity to help the agent learn.
Revised: We exploit curriculum learning (Bengio et al., 2009) to gradually increase the environment complexity to help the agent learn.

Original: The following quantities are increased in proportional to min p 1 , G 1 { G q , where G 1 is the number of sessions trained so far and G is the total number of curriculum sessions:
Revised: The following quantities are increased in proportional to min p 1 , G 1 { G q , where G 1 is the number of sessions trained so far and G is the total number of curriculum sessions:

Original: I The size of the open space on the environment map.
Revised: I The size of the open space on the environment map.

Original: II The number of objects in the environment.
Revised: II The number of objects in the environment.

Original: III
Revised: III

Original: The number of wall blocks.
Revised: The number of wall blocks.

Original: IV
Revised: IV

Original: The number of object classes that can be sampled from.
Revised: The number of object classes that can be sampled from.

Original: V The lengths of the NAV command and the QA question.
Revised: V The lengths of the NAV command and the QA question.

Original: We found that this curriculum is important for an efﬁcient learning.
Revised: We found that this curriculum is important for an efﬁcient learning.

Revised: Speciﬁcally, the gradual changes of quantities IV and V are supported by the ﬁndings of Siskind (1996) that children learn new words in a linguistic corpus much faster after partial exposure to the corpus.

Original: In the experiments, we set
Revised: In the experiments, we set

Original: G “ 25k during training while do not have any curriculum during test (i.e., testing with the maximum difﬁculty).
Revised: G “ 25k during training while do not have any curriculum during test (i.e., testing with the maximum difﬁculty).