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Title: Probing Emergent Semantics in Predictive Agents via Question Answering

Paper content:
Recent work has demonstrated how predictive modeling can endow agents with rich knowledge of their surroundings, improving their ability to act in complex environments.
We propose question-answering as a general paradigm to decode and understand the representations that such agents develop, applying our method to two recent approaches to predictive modeling – action-conditional CPC (Guo et al., 2018) and SimCore (Gregor et al., 2019).
After training agents with these predictive objectives in a visually-rich, 3D environment with an assortment of objects, colors, shapes, and spatial configurations, we probe their internal state representations with a host of synthetic (English) questions, without backpropagating gradients from the question-answering decoder into the agent.
The performance of different agents when probed in this way reveals that they learn to encode detailed, and seemingly compositional, information about objects, properties and spatial relations from their physical environment.
Our approach is intuitive, i.e. humans can easily interpret the responses of the model as opposed to inspecting continuous vectors, and model-agnostic, i.e. applicable to any modeling approach.
By revealing the implicit knowledge of objects, quantities, properties and relations acquired by agents as they learn, question-conditional agent probing can stimulate the design and development of stronger predictive learning objectives.
Some of the biggest successes in artificial intelligence have relied on learning representations from large labeled datasets (Krizhevsky et al., 2012; Sutskever et al., 2014) or dense reward signals (Mnih et al., 2015; .
However, an intelligent agent that is capable of functioning in a complex, open-ended environment should be capable of learning general representations that are task-agnostic, and should not require exhaustive labeled data collection or careful reward design.
One of the main challenges in developing such agents is the need for general approaches to evaluate and analyze agents' internal states.
In this work, we propose question-answering as an evaluation paradigm for analyzing how much objective knowledge about the external environment is encoded in an agent's internal representation.
Our motivation to do so is twofold.
First, question-answering provides an intuitive investigative tool for humans -one can simply ask an agent what it knows about its environment and get an answer back, without having to inspect internal activations.
Second, the space of questions is fairly open-ended -we can pose arbitrarily complex questions to an agent, enabling a comprehensive analysis of its internal states.
Question-answering has previously been studied in textual (Rajpurkar et al., 2016; 2018) , visual (Malinowski & Fritz, 2014; Antol et al., 2015; Das et al., 2017) and embodied (Gordon et al., 2018; Das et al., 2018a) settings.
Crucially, however, these systems are trained end-to-end for the goal of answering questions.
Here, we utilize questionanswering simply to probe an agent's internal representation, without backpropagating gradients from the question-answering decoder into the agent.
That is, we view question-answering as a general purpose decoder of environment knowledge designed to assist the development of agents.
We are particularly interested in agents that can learn general task-agnostic representations of the external world.
One promising way to achieve this is via self-supervised predictive modeling.
Inspired by learning in humans (Elman, 1990; Quiroga et al., 2005; Nortmann et al., 2013; Rao & Ballard, 1999; Clark, 2016; Hohwy, 2013; Seth, 2015) , predictive modeling, i.e. predicting future sensory observations, has emerged as a powerful method to learn general-purpose neural network Figure 1 : We train predictive agents to explore a visually-rich 3D environment with an assortment of objects of different shapes, colors and sizes.
As the agent navigates (trajectory shown in white on the top-down map), it contains an auxiliary network that learns to simulate representations of future observations (labeled 'Simulation Network') say k steps into the future self-supervised by a loss on the agent's future prediction against the ground-truth egocentric observation at t`k.
Simultaneously, another decoder network is trained to extract answers to a variety of questions about the environment, conditioned on the agent's internal memory but without affecting it (notice 'stop gradient' -gradients from the QA decoder are not backpropagated into the agent).
We use this question-answering paradigm to decode and understand the internal representations that such agents develop.
Note that the top-down map is only shown for illustration purposes and not available to the agent.
representations (Elias, 1955; Atal & Schroeder, 1970; Schmidhuber, 1991; Schaul & Ring, 2013; Schaul et al., 2015; Silver et al., 2017; Wayne et al., 2018; Guo et al., 2018; Gregor et al., 2019; Recanatesi et al., 2019) .
These representations can be learned while exploring in and interacting with an environment in a task-agnostic manner, and later exploited for goal-directed behavior.
We evaluate predictive vs. non-predictive agents (both trained for exploration) on our questionanswering testbed to investigate how much objective knowledge about environment semantics can be captured solely by egocentric prediction.
By semantics, here we specifically refer to information about objects -quantity, colors, shapes, spatial relations.
The set of questions is intended to be holistic, i.e. they require a representation of relevant aspects of the whole environment and in general cannot be answered from a single observation, nor a few consecutive observations of the episodeand test a variety of local and global scene understanding, visual reasoning, and recall skills.
Concretely, we make the following contributions:
• In a visually rich 3D room environment developed in the Unity game engine 1 , we define and develop a set of questions designed to probe a range of semantic, relational and spatial knowledge -from identifying shapes and colors ('What shape is the red object?') to counting ('How many blue objects are there?') to spatial relations ('What is the color of the chair near the table?'), exhaustive search ('Is there a cushion?'), and comparisons ('Are there the same number of tables as chairs?').
• We train RL agents augmented with predictive loss functions -1) action-conditional CPC (Guo et al., 2018) and 2) SimCore (Gregor et al., 2019 ) -for an exploration task and analyze the internal representations they develop by decoding answers to our suite of questions.
Crucially, the QA decoder is trained independent of the predictive agent and we find that QA performance is indicative of the agent's ability to capture global environment structure and semantics solely through egocentric prediction.
We compare these predictive agents to strong non-predictive LSTM baselines as well as to an agent that is explicitly optimized for the question-answering task.
• We establish generality of the semantic knowledge by testing zero-shot generalization of a trained QA decoder to compositionally novel questions (unseen combinations of seen attributes), suggesting a degree of compositionality in the internal representations captured by predictive agents.
We introduced question-answering as a paradigm to evaluate and analyze representations learned by artificial agents.
In particular, we tested how much knowledge about the external environment can be decoded from predictive vs. non-predictive RL agents.
We started by developing a range of question-answering tasks in a visually-rich 3D environment serving as a diagnostic test of an agent's scene understanding, visual reasoning, and memory skills.
Next, we trained agents to optimize an exploration objective with and without auxiliary self-supervised predictive losses, and evaluated the representations they form as they explore an environment via this question-answering testbed.
We found that predictive agents (in particular SimCore (Gregor et al., 2019) ) are able to reliably capture detailed environment semantics in their internal states, which can be easily decoded as answers to questions, while non-predictive agents do not, even if they optimize the exploration objective well.
Interestingly, not all predictive agents are equally good at forming these representations.
We compared a model explicitly learning the probability distribution of future frames in pixel space via a generative model (SimCore (Gregor et al., 2019) ) with a model based on discriminating frames through contrastive estimation (CPC|A (Guo et al., 2018) ).
We found that while both learned to navigate well, only the former developed representations that could be used for answering questions about the environment.
Gregor et al. (2019) previously showed that the choice of predictive model has a significant impact on the ability to decode an agent's position, orientation and top-down map reconstructions of the environment.
Here we extend this idea to more high-level and complex aspects of the environment and show the value of our question-answering approach in comparing existing agents and its potential utility as a tool for developing better ones.
Finally, the fact that we can even decode answers to questions (i.e. symbolic information) from an agent's internal representations learned solely from egocentric future predictions without exposing the agent to any questions is encouraging.
It indicates that the agent is learning to form and maintain invariant object identities and properties (modulo limitations in decoder capacity) in its internal state without explicit supervision.
It is almost 30 years since Elman (1990) showed how syntactic structures and semantic organization can emerge in the units of a neural network as a consequence of the simple objective of predicting the next word in a sequence.
This work corroborates Elman's belief in the power of prediction by demonstrating the diversity of knowledge that can emerge when a situated neural-network agent is endowed with powerful predictive objectives applied to raw pixel observations.
We think we have just scratched the surface of this problem, and hope our work inspires future research in evaluating predictive agents using natural linguistic interactions.

assistant: text: Proposes QA as a tool to investigate what agents learn about in the world, arguing this as an intuitive method for humans which allows for arbitrary complexity.