Justin Li


My Research

2010-05-21

I've wanted to write this post for a while, but have never found the motivation to do so. There's a workshop this coming week, however, and I am scheduled to present this stuff on Thursday (today... I did okay, I think). So, I decided this is the perfect time to given a non-technical (that is, non-mathematical) introduction to what I do... there truly is no greater motivator than procrastination.

For those of you who don't know, I'm a grad student in computer science at the University of Michigan, specializing in artificial intelligence. Within this field, which is wider and deeper than most people realize, I am looking at the problem of reinforcement learning (RL). The formulation of the problem is simple. An agent interacts with some environment by performing actions. The environment reacts, and may occasionally reward or punish the agent. The goal of the agent is to maximize the amount of reward (or equivalently, minimize the amount of punishment) it gets. Hence the name reinforcement learning: the reward and punishment terminology is borrowed from the theory of operant conditioning in psychology.

Does this problem sound easy? It's not. Let's use a computer game as an example... say, Pong. A Pong agent has two actions available to it: move up and move down. The environment includes the ball and the opposite agent. A simple reward function gives the agent 1 point for winning a game and -1 point for losing the game. Now, put yourself in the role of the agent. You might think, "Oh, just follow the ball by moving up and down, and don't let the ball get past me." Well, you know that, but how does the agent know that? Remember, the agent doesn't know it is in a Pong game - all it observes are a bunch of numbers. The agent doesn't know it will be rewarded for getting the ball past its opponent, nor that it will be punished for letting the ball past it. It doesn't know that the ball will "bounce" when it touches anything - or even what constitutes "touching", "opponent", and "past".

Without any knowledge, the agent will have to act randomly, at least at first. It might get lucky and defend its goal once or twice, but more likely it will let its opponent score and be punished. There comes the first hurdle of reinforcement learning: how does the agent know what it did wrong? The converse is also true if the agent scored - what did it do right? This is called the credit assignment problem, because the agent is trying to figure out what gets the credit (or blame) for the reward (or punishment) it received. Again, remember what while you intuitively know that the agent missed the ball, the agent doesn't have any model of cause and effect to realize this.

To understand the basic solution to reinforcement learning, I must say more about those numbers that the agent observes. For Pong, there might be four numbers: the agent's vertical position, its opponent's vertical position, and the vertical and horizontal position of the ball. Each of these numbers are a state variable, and the different values these numbers can take in conjunction is called the state space - as in "the state of the union", not "the state of Michigan" - because they can describe every situation in the environment. To make reinforcement learning somewhat easier, researchers tend to view tasks as a Markov decision process (MDP). The distinguishing property of an MDP is that the next state of the environment depends only on the current state. For the Pong example, the state space described by the four numbers listed above does not make the game an MDP, since the ball can be moving arbitrarily fast or slow. If instead six numbers are used - the represent the horizontal and vertical velocity of the ball - then the task would become an MDP.

To be concrete, every action the agent takes changes the state of the environment. At each new state, the agent may receive a reward or a punishment, and then it has to take another action, and so on.

Now that the agent has some idea of how to relate one set of numbers to another, it can start learning. In place of human level reasoning, the agent simply plans backwards. Intuitively, if you get punished for being in this state, you know you shouldn't have taken that last action from the previous state. For Pong, this partially corresponds to not moving up if the ball is below you and just about to pass you. This picture is not complete though, because if you are at the top of the screen and the ball is at the bottom, you would not have gotten to the ball in time to deflect it anyway. This means that the faulty action lies further back in time.

Here's what the agent does. The agent remembers the state in which it got a reward or a punishment. Knowing whether this state is good or bad, it knows that the previous action from the previous state is also good or bad. Now knowing about this previous state, and can know about the state two steps back, and three steps back, and so on. That is, the agent learns by propagating the rewards back through the states, so the next time it finds itself in the same situation, it will either avoid that action (because it was eventually punished for it) or do the same thing (because it was eventually rewarded for it).

This was the state of the art 25 years ago.

Before I talk about more recent developments in the field, I want to raise a few problems in the solution above. The shallowest, but also most thought provocative, is this: how does the agent know it's doing its best? Imagine the task is to go from the bedroom to the bathroom to pick up an object. Through pure chance, the agent does this by going through the living room, the kitchen, and the broom closet, despite there being a door directly between the two rooms. Further imagine that the agent is rewarded based on how quickly it gets to its destination (I'm sure you can think of a reasonable, real life scenario for this). How would the agent know that the path it found is the shortest one? This problem is known as the exploration-exploitation problem. It is thus named because the agent needs to explore to know more about the environment, but this often means not exploiting the best action for the agent. In practice, researchers simply make the agent act randomly some small percentage of the time, so it will do its best for the most part but be constantly exploring. For the jargon-philiac, this is called an epsilon-greedy exploration strategy, where epsilon is the small percentage mentioned above.

There are other variations to this general framework which researchers are working on, such as partial observability (what if the agent doesn't know the value of some state variables?) and stochastic actions (what if actions only succeed some of the time?). I will skip the details on these topics and instead focus on what I'm looking into.

Pong, while an illustrative example, is not the most complicated environment for an agent to learn in. For one, there are only a limited number of situations for the agent to be in: in the original pixelated arcade game, there might only be a thousand or so different states. A modern computer running the algorithm I described above would find the optimal strategy (or policy) in less than a minute. Consider instead the rooms example I just gave, where the agent must move from one room to another. There can be an arbitrary number of rooms, and each room itself can be arbitrarily large. Even if the starting and ending positions are unchanged every time the agent must complete the task, it might still take the agent a long time simply due to how much exploring it has to do.

Yet, unlike Pong, this more complicated problem also presents more information for the agent to use. For example, completely exploring one room is useless when the goal is in another room. If you were to give directions to a human, you might tell them to go out of the room, walk down the hallway, and take the last right. Alternately, you might tell them that the thing they're looking for is not in the living room, but in the kitchen. The other person, on hearing these instructions, could then ignore everything in their current room.

What these two instructions have in common is abstraction. The first instruction abstracts over actions (it doesn't say "take 10 steps forward, then 2 steps left,..."), while the second instruction abstracts over states (it's saying that everything outside the kitchen is one room which doesn't matter). Despite this distinction, the two types of abstractions are related: the first instruction is implicitly saying that the current room and the hallway are not worth exploring, and the second instruction can be thought of as the action "go to the kitchen".

My research is related to this idea, although it's a little more specific. Expanding the example beyond rooms, if I'm giving instructions for someone to get to the Eiffel Tower in Paris, I would tell them to get to the airport, take a plane, etc. Each of these "actions" can be further broken down: order a cab, get out of the house, get in the cab,... And even further: walk to the phone, call the cab company,... This is called an action hierarchy, as the first actions "contain" the second ones, which "contain" the third, and so on, until at the lowest level the actions are simply "move this muscle". How do humans break down such a complex task, and can a computer do the same thing?

So far, the state of the art (which is about 5 years old) is "sort of". Given just the MDP assumption, there are proposals for what subgoals should be. The most general ones are different ways of identifying bottlenecks - that is, states which an agent must go through to reach a goal. Think of the door to a room, and in order to get anywhere else you must first go through that door. Other ideas include things like looking at what states you have commonly visited in your experience, or perhaps looking at what you're rewarded handsomely for.

Even knowing what appropriate subgoals are, the agent is not done. Imagine that both the door to your room and the door to the apartment are given as subgoals. How will you know that the door to your room is the first thing to go for, before trying to read the door to your apartment? Depending on the viewpoint, this could either be a problem of restricting the proposal of actions, or it could be one of inducing a preference on different possible actions. This appears to be a slightly easier problem to solve, and I was surprised to find that there is almost no prior work in this area. I intend to look into this question further, and hopefully by the end of the summer I will have some intuition as to what might work and what won't.

Post script: to people not in computer science or perhaps in but not in AI, the problem of action hierarchies might sound insignificant. Despite my previous misgivings about being limited to too specific a field, I find this problem genuinely interesting. Although it may not change the world (yet), I do think its solution will contribute to an understanding of humans and intelligence.

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