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Title: Modular Continual Learning in a Unified Visual Environment

Paper content:
A core aspect of human intelligence is the ability to learn new tasks quickly and switch between them flexibly.
Here, we describe a modular continual reinforcement learning paradigm inspired by these abilities.
We first introduce a visual interaction environment that allows many types of tasks to be unified in a single framework.
We then describe a reward map prediction scheme that learns new tasks robustly in the very large state and action spaces required by such an environment.
We investigate how properties of module architecture influence efficiency of task learning, showing that a module motif incorporating specific design principles (e.g. early bottlenecks, low-order polynomial nonlinearities, and symmetry) significantly outperforms more standard neural network motifs, needing fewer training examples and fewer neurons to achieve high levels of performance.
Finally, we present a meta-controller architecture for task switching based on a dynamic neural voting scheme, which allows new modules to use information learned from previously-seen tasks to substantially improve their own learning efficiency.
In the course of everyday functioning, people are constantly faced with real-world environments in which they are required to shift unpredictably between multiple, sometimes unfamiliar, tasks BID2 .
They are nonetheless able to flexibly adapt existing decision schemas or build new ones in response to these challenges BID1 .
How humans support such flexible learning and task switching is largely unknown, both neuroscientifically and algorithmically BID28 BID5 .We
investigate solving this problem with a neural module approach in which simple, task-specialized decision modules are dynamically allocated on top of a largely-fixed underlying sensory system BID0 BID14 . The
sensory system computes a general-purpose visual representation from which the decision modules read. While
this sensory backbone can be large, complex, and learned comparatively slowly with significant amounts of training data, the task modules that deploy information from the base representation must, in contrast, be lightweight, quick to be learned, and easy to switch between. In the
case of visually-driven tasks, results from neuroscience and computer vision suggest the role of the fixed general purpose visual representation may be played by the ventral visual stream, modeled as a deep convolutional neural network (Yamins & DiCarlo, 2016; BID23 . However
, the algorithmic basis for how to efficiently learn and dynamically deploy visual decision modules remains far from obvious. The TouchStream
environment is a touchscreen-like GUI for continual learning agents, in which a spectrum of visual reasoning tasks can be posed in a large but unified action space. On each timestep
, the environment (cyan box) emits a visual image (xt) and a reward (rt). The agent recieves
xt and rt as input and emits an action at. The action represents
a "touch" at some location on a two-dimensional screen e.g. at ∈ {0, . . . , H − 1} × {0, . . . , W − 1}, where H and W are the screen height and width. The environment's policy
is a program computing xt and rt as a function of the agent's action history. The agent's goal is to learn
how to choose optimal actions to maximize the amount of reward it recieves over time. The agent consists of several
component neural networks including a fixed visual backbone (yellow inset), a set of learned neural modules (grey inset), and a meta-controller (red inset) which mediates the deployment of these learned modules for task solving. The modules use the ReMaP algorithm
§ 2 to learn how to estimate reward as a function of action (heatmap), conditional on the agent's recent history. Using a sampling policy on this reward
map, the agent chooses an optimal action to maximize its aggregate reward.In standard supervised learning, it is often assumed that the output space of a problem is prespecified in a manner that just happens to fit the task at hand -e.g. for a classification task, a discrete output with a fixed number of classes might be determined ahead of time, while for a continuous estimation problem, a one-dimensional real-valued target might be chosen instead. This is a very convenient simplification
in supervised learning or single-task reinforcement learning contexts, but if one is interested in the learning and deployment of decision structures in a rich environment defining tasks with many different natural output types, this simplification becomes cumbersome.To go beyond this limitation, we build a unified environment in which many different tasks are naturally embodied. Specifically, we model an agent interacting
with a two-dimensional touchscreenlike GUI that we call the TouchStream, in which all tasks (discrete categorization tasks, continuous estimation problems, and many other combinations and variants thereof) can be encoded using a single common and intuitive -albeit large -output space. This choice frees us from having to hand-design
or programmatically choose between different output domain spaces, but forces us to confront the core challenge of how a naive agent can quickly and emergently learn the implicit "interfaces" required to solve different tasks.We then introduce Reward Map Prediction (ReMaP) networks, an algorithm for continual reinforcement learning that is able to discover implicit task-specific interfaces in large action spaces like those of the TouchStream environment. We address two major algorithmic challenges associated
with learning ReMaP modules. First, what module architectural motifs allow for efficient
task interface learning? We compare several candidate architectures and show that those
incorporating certain intuitive design principles (e.g. early visual bottlenecks, low-order polynomial nonlinearities and symmetry-inducing concatenations) significantly outperform more standard neural network motifs, needing fewer training examples and fewer neurons to achieve high levels of performance. Second, what system architectures are effective for switching
between tasks? We present a meta-controller architecture based on a dynamic
neural voting scheme, allowing new modules to use information learned from previously-seen tasks to substantially improve their own learning efficiency.In § 1 we formalize the TouchStream environment. In § 2, we introduce the ReMaP algorithm. In § 3, we describe
and evaluate comparative performance of multiple
ReMaP module architectures on a variety of TouchStream tasks. In § 4, we describe the Dynamic Neural Voting meta-controller, and evaluate
its ability to efficiently transfer knowledge between ReMaP modules on task switches.
In this work, we introduce the TouchStream environment, a continual reinforcement learning framework that unifies a wide variety of spatial decision-making tasks within a single context.
We describe a general algorithm (ReMaP) for learning light-weight neural modules that discover implicit task interfaces within this large-action/state-space environment.
We show that a particular module architecture (EMS) is able to remain compact while retaining high task performance, and thus is especially suitable for flexible task learning and switching.
We also describe a simple but general dynamic task-switching architecture that shows substantial ability to transfer knowledge when modules for new tasks are learned.A crucial future direction will be to expand insights from the current work into a more complete continual-learning agent.
We will need to show that our approach scales to handle dozens or hundreds of task switches in sequence.
We will also need to address issues of how the agent determines when to build a new module and how to consolidate modules when appropriate (e.g. when a series of tasks previously understood as separate can be solved by a single smaller structure).
It will also be critical to extend our approach to handle visual tasks with longer horizons, such as navigation or game play with extended strategic planning, which will likely require the use of recurrent memory stores as part of the feature encoder.From an application point of view, we are particularly interested in using techniques like those described here to produce agents that can autonomously discover and operate the interfaces present in many important real-world two-dimensional problem domains, such as on smartphones or the internet BID12 .
We also expect many of the same spatially-informed techniques that enable our ReMaP/EMS modules to perform well in the 2-D TouchStream environment will also transfer naturally to a three-dimensional context, where autonomous robotics applications BID7 Friedemann Zenke, Ben Poole, and Surya Ganguli.
Improved multitask learning through synaptic intelligence.
arXiv preprint arXiv:1703.04200, 2017.

assistant: text: We propose a neural module approach to continual learning using a unified visual environment with a large action space.