When it is not in our power to determine what is true, we ought to act in accordance with what is most probable. - Descartes
While neural networks are responsible for recent breakthroughs in problems like computer vision, machine translation and time series prediction – they can also combine with reinforcement learning algorithms to create something astounding like AlphaGo.
Reinforcement learning refers to goal-oriented algorithms, which learn how to attain a complex objective (goal) or maximize along a particular dimension over many steps; for example, maximize the points won in a game over many moves. They can start from a blank slate, and under the right conditions they achieve superhuman performance. Like a child incentivized by spankings and candy, these algorithms are penalized when they make the wrong decisions and rewarded when they make the right ones – this is reinforcement.
Reinforcement algorithms that incorporate deep learning can beat world champions at the game of Go as well as human experts playing numerous Atari video games. While that may sound trivial, it’s a vast improvement over their previous accomplishments, and the state of the art is progressing rapidly.
Reinforcement learning solves the difficult problem of correlating immediate actions with the delayed returns they produce. Like humans, reinforcement learning algorithms sometimes have to wait a while to see the fruit of their decisions. They operate in a delayed return environment, where it can be difficult to understand which action leads to which outcome over many time steps.
Reinforcement learning algorithms can be expected to perform better and better in more ambiguous, real-life environments while choosing from an arbitrary number of possible actions, rather than from the limited options of a video game. That is, with time we expect them to be valuable to achieve goals in the real world.
Reinforcement learning can be understand using the concepts of agents, environments, states, actions and rewards, all of which we’ll explain below. Capital letters tend to denote sets of things, and lower-case letters denote a specific instance of that thing; e.g.
A is all possible actions, while
a is a specific action contained in the set.
Ais the set of all possible moves the agent can make. An action is almost self-explanatory, but it should be noted that agents choose among a list of possible actions. In video games, the list might include running right or left, jumping high or low, crouching or standing still. In the stock markets, the list might include buying, selling or holding any one of an array of securities and their derivatives. When handling aerial drones, alternatives would include many different velocities and accelerations in 3D space.
0.8³ x 10. A discount factor of 1 would make future rewards worth just as much as immediate rewards.
Vπ(s)is defined as the expected long-term return of the current state under policy
π. We discount rewards, or lower their estimated value, the further into the future they occur. See discount factor.
Qπ(s, a)refers to the long-term return of the current state
s, taking action a under policy
π. Q maps state-action pairs to rewards. Note the difference between Q and policy.
So environments are functions that transform an action taken in the current state into the next state and a reward; agents are functions that transform the new state and reward into the next action. We can know the agent’s function, but we cannot know the function of the environment. It is a black box where we only see the inputs and outputs. Reinforcement learning represents an agent’s attempt to approximate the environment’s function, such that we can send actions into the black-box environment that maximize the rewards it spits out.
In the feedback loop above, the subscripts denote the time steps
t+1, each of which refer to different states: the state at moment
t, and the state at moment
t+1. Unlike other forms of machine learning – such as supervised and unsupervised learning – reinforcement learning can only be thought about sequentially in terms of state-action pairs that occur one after the other.
Reinforcement learning judges actions by the results they produce. It is goal oriented, and its aim is to learn sequences of actions that will lead an agent to achieve its goal, or maximize its objective function. Here are some examples:
Here’s an example of an objective function for reinforcement learning; i.e. the way it defines its goal.
We are summing reward function r over t, which stands for time steps. So this objective function calculates all the reward we could obtain by running through, say, a game. Here, x is the state at a given time step, and a is the action taken in that state. r is the reward function for x and a. (We’ll ignore γ for now.)
Reinforcement learning differs from both supervised and unsupervised learning by how it interprets inputs. We can illustrate their difference by describing what they learn about a “thing.”
One way to imagine an autonomous reinforcement learning agent would be as a blind person attempting to navigate the world with only their ears and a white cane. Agents have small windows that allow them to perceive their environment, and those windows may not even be the most appropriate way for them to perceive what’s around them.
(In fact, deciding which types of input and feedback your agent should pay attention to is a hard problem to solve. This is known as domain selection. Algorithms that are learning how to play video games can mostly ignore this problem, since the environment is man-made and strictly limited. Thus, video games provide the sterile environment of the lab, where ideas about reinforcement learning can be tested. Domain selection requires human decisions, usually based on knowledge or theories about the problem to be solved; e.g. selecting the domain of input for an algorithm in a self-driving car might include choosing to include radar sensors in addition to cameras and GPS data.)
The goal of reinforcement learning is to pick the best known action for any given state, which means the actions have to be ranked, and assigned values relative to one another. Since those actions are state-dependent, what we are really gauging is the value of state-action pairs; i.e. an action taken from a certain state, something you did somewhere. Here are a few examples to demonstrate that the value and meaning of an action is contingent upon the state in which it is taken:
If the action is marrying someone, then marrying a 35-year-old when you’re 18 probably means something different than marrying a 35-year-old when you’re 90, and those two outcomes probably have different motivations and lead to different outcomes.
If the action is yelling “Fire!”, then performing the action a crowded theater should mean something different from performing the action next to a squad of men with rifles. We can’t predict an action’s outcome without knowing the context.
We map state-action pairs to the values we expect them to produce with the Q function, described above. The Q function takes as its input an agent’s state and action, and maps them to probable rewards.
Reinforcement learning is the process of running the agent through sequences of state-action pairs, observing the rewards that result, and adapting the predictions of the Q function to those rewards until it accurately predicts the best path for the agent to take. That prediction is known as a policy.
Reinforcement learning is an attempt to model a complex probability distribution of rewards in relation to a very large number of state-action pairs. This is one reason reinforcement learning is paired with, say, a Markov decision process, a method to sample from a complex distribution to infer its properties. It closely resembles the problem that inspired Stan Ulam to invent the Monte Carlo method; namely, trying to infer the chances that a given hand of solitaire will turn out successful.
Any statistical approach is essentially a confession of ignorance. The immense complexity of some phenomena (biological, political, sociological, or related to board games) make it impossible to reason from first principles. The only way to study them is through statistics, measuring superficial events and attempting to establish correlations between them, even when we do not understand the mechanism by which they relate. Reinforcement learning, like deep neural networks, is one such strategy, relying on sampling to extract information from data.
After a little time spent employing something like a Markov decision process to approximate the probability distribution of reward over state-action pairs, a reinforcement learning algorithm may tend to repeat actions that lead to reward and cease to test alternatives. There is a tension between the exploitation of known rewards, and continued exploration to discover new actions that also lead to victory. Just as oil companies have the dual function of pumping crude out of known oil fields while drilling for new reserves, so too, reinforcement learning algorithms can be made to both exploit and explore to varying degrees, in order to ensure that they don’t pass over rewarding actions at the expense of known winners.
Reinforcement learning is iterative. In its most interesting applications, it doesn’t begin by knowing which rewards state-action pairs will produce. It learns those relations by running through states again and again, like athletes or musicians iterate through states in an attempt to improve their performance.
You could say that an algorithm is a method to more quickly aggregate the lessons of time. Reinforcement learning algorithms have a different relationship to time than humans do. An algorithm can run through the same states over and over again while experimenting with different actions, until it can infer which actions are best from which states. Effectively, algorithms enjoy their very own Groundhog Day, where they start out as dumb jerks and slowly get wise.
Since humans never experience Groundhog Day outside the movie, reinforcement learning algorithms have the potential to learn more, and better, than humans. Indeed, the true advantage of these algorithms over humans stems not so much from their inherent nature, but from their ability to live in parallel on many chips at once, to train night and day without fatigue, and therefore to learn more. An algorithm trained on the game of Go, such as AlphaGo, will have played many more games of Go than any human could hope to complete in 100 lifetimes.2
Where do neural networks fit in? Neural networks are the agent that learns to map state-action pairs to rewards. Like all neural networks, they use coefficients to approximate the function relating inputs to outputs, and their learning consists to finding the right coefficients, or weights, by iteratively adjusting those weights along gradients that promise less error.
In reinforcement learning, convolutional networks can be used to recognize an agent’s state; e.g. the screen that Mario is on, or the terrain before a drone. That is, they perform their typical task of image recognition.
But convolutional networks derive different interpretations from images in reinforcement learning than in supervised learning. In supervised learning, the network applies a label to an image; that is, it matches names to pixels.
In fact, it will rank the labels that best fit the image in terms of their probabilities. Shown an image of a donkey, it might decide the picture is 80% likely to be a donkey, 50% likely to be a horse, and 30% likely to be a dog.
In reinforcement learning, given an image that represents a state, a convolutional net can rank the actions possible to perform in that state; for example, it might predict that running right will return 5 points, jumping 7, and running left none.
The above image illustrates what a policy agent does, mapping a state to the best action.
A policy maps a state to an action.
If you recall, this is distinct from Q, which maps state action pairs to rewards.
To be more specific, Q maps state-action pairs to the highest combination of immediate reward with all future rewards that might be harvested by later actions in the trajectory. Here is the equation for Q, from Wikipedia:
Having assigned values to the expected rewards, the Q function simply selects the state-action pair with the highest so-called Q value.
At the beginning of reinforcement learning, the neural network coefficients may be initialized stochastically, or randomly. Using feedback from the environment, the neural net can use the difference between its expected reward and the ground-truth reward to adjust its weights and improve its interpretation of state-action pairs.
This feedback loop is analogous to the backpropagation of error in supervised learning. However, supervised learning begins with knowledge of the ground-truth labels the neural network is trying to predict. Its goal is to create a model that maps different images to their respective names.
Reinforcement learning relies on the environment to send it a scalar number in response to each new action. The rewards returned by the environment can be varied, delayed or affected by unknown variables, introducing noise to the feedback loop.
This leads us to a more complete expression of the Q function, which takes into account not only the immediate rewards produced by an action, but also the delayed rewards that may be returned several time steps deeper in the sequence.
Like human beings, the Q function is recursive. Just as calling the wetware method
human() contains within it another method
human(), of which we are all the fruit, calling the Q function on a given state-action pair requires us to call a nested Q function to predict the value of the next state, which in turn depends on the Q function of the state after that, and so forth.
1) It might be helpful to imagine a reinforcement learning algorithm in action, to paint it visually. Let’s say the algorithm is learning to play the video game Super Mario. It’s trying to get Mario through the game and acquire the most points. To do that, we can spin up lots of different Marios in parallel and run them through the space of all possible game states. It’s as though you have 1,000 Marios all tunnelling through a mountain, and as they dig (e.g. as they decide again and again which action to take to affect the game environment), their experience-tunnels branch like the intricate and fractal twigs of a tree. The Marios’ experience-tunnels are corridors of light cutting through the mountain. And as in life itself, one successful action may make it more likely that successful action is possible in a larger decision flow, propelling the winning Marios onward. You might also imagine, if each Mario is an agent, that in front of him is a heat map tracking the rewards he can associate with state-action pairs. (Imagine each state-action pair as have its own screen overlayed with heat from yellow to red. The many screens are assembled in a grid, like you might see in front of a Wall St. trader with many monitors. One action screen might be “jump harder from this state”, another might be “run faster in this state” and so on and so forth.) Since some state-action pairs lead to significantly more reward than others, and different kinds of actions such as jumping, squatting or running can be taken, the probability distribution of reward over actions is not a bell curve but instead complex, which is why Markov and Monte Carlo techniques are used to explore it, much as Stan Ulam explored winning Solitaire hands. That is, while it is difficult to describe the reward distribution in a formula, it can be sampled. Because the algorithm starts ignorant and many of the paths through the game-state space are unexplored, the heat maps will reflect their lack of experience; i.e. there could be blanks in the heatmap of the rewards they imagine, or they might just start with some default assumptions about rewards that will be adjusted with experience. The Marios are essentially reward-seeking missiles guided by those heatmaps, and the more times they run through the game, the more accurate their heatmap of potential future reward becomes. The heatmaps are basically probability distributions of reward over the state-action pairs possible from the Mario’s current state.
2) The correct analogy may actually be that a learning algorithm is like a species. Each simulation the algorithm runs as it learns could be considered an individual of the species. Just as knowledge from the algorithm’s runs through the game is collected in the algorithm’s model of the world, the individual humans of any group will report back via language, allowing the collective’s model of the world, embodied in its texts, records and oral traditions, to become more intelligent (At least in the ideal case. The subversion and noise introduced into our collective models is a topic for another post, and probably for another website entirely.). This puts a finer point on why the contest between algorithms and individual humans, even when the humans are world champions, is unfair. We are pitting a civilization that has accumulated the wisdom of 10,000 lives against a single sack of flesh.
Reinforcement Learning Methods