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Summary by CodyWild 5 years ago
This was definitely one of the more conceptually nuanced and complicated papers I’ve read recently, and I’ve only got about 60% confidence that I fully grasp all of its intuitions. However, I’m going to try to collect together what I did understand.
There is a lot of research into generative models of text or image sequences, and some amount of research into building “models” in the reinforcement learning sense, where your model can predict future observations given current observations and your action. There’s an important underlying distinction here between model-based RL (where you learn a model of how the world evolves, and use that to optimize reward) and model-free RL (where you leave don’t bother explicitly learning a world model, and just directly try to optimize rewards) However, this paper identifies a few limitations of this research.
1) It’s largely focused on predicting observations, rather than predicting *state*. State is a bit of a fuzzy concept, and corresponds to, roughly, “the true underlying state of the game”. An example I like to use is a game where you walk in one door, and right next to it is a second door, which requires you to traverse the space and find rewards and a key before you can open. Now, imagine that the observation of your agent is it looking at the door. If the game doesn’t have any on-screen representation of the fact that you’ve found the key, it won’t be present in your observations, and you’ll observe the same thing at the point you have just entered and once you found the key. However, the state of the game at these two points will be quite different, in that in the latter case, your next states might be “opening the door” rather than “going to collect rewards”. Scenarios like this are referred to broadly as Partially Observable games or environments. This paper wants to build a model of how the game evolves into the future, but it wants to build a model of *state-to-state* evolution, rather than observation-to-observation evolution, since observations are typically both higher-dimensionality and also more noisy/less informative.
2) Past research has typically focused on predicting each next-step observation, rather than teaching models to be able to directly predict a state many steps in the future, without having to roll out the entire sequence of intermediate predictions. This is arguably quite valuable for making models that can predict the long term consequences of their decision
This paper approaches that with an approach inspired by the Temporal Difference framework used in much of RL, in which you update your past estimate of future rewards by forcing it to be consistent with the actual observed rewards you encounter in the future. Except, in this model, we sample two a state (z1) and then a state some distance into the future (z2), and try to make our backwards-looking prediction of the state at time 1, taking into account observations that happened in between, match what our prediction was with only the information at time one.
An important mechanistic nuance here is the idea of a “belief state”, something that captures all of your knowledge about game history up to a certain point. We can then directly sample a state Zt given the belief state Bt at that point. This isn’t actually possible with a model where we predict a state at time T given the state at time T-1, because the state at time Z-1 is itself a sample, and so in order to get a full distribution of Zt, you have to sample Zt over the distribution of Zt-1, and in order to get the distribution of Zt-1 you have to sample over the distribution of Zt-2, and so on and so on. Instead, we have a separate non-state variable, Bt that is created conditional on all our past observations (through a RNN).
https://i.imgur.com/N0Al42r.png
All said and done, the mechanics of this model look like:
1) Pick two points along the sequence trajectory
2) Calculate the belief state at each point, and, from that, construct a distribution over states at each timestep using p(z|b)
3) Have an additional model that predicts z1 given z2, b1, and b2 (that is, the future beliefs and states), and push the distribution over z1 from this model to be close to the distribution over z1 given only the information available at time t1
4) Have a model that predicts Z2 given Z1 and the time interval ahead that we’re jumping, and try to have this model be predictive/have high likelihood over the data
5) And, have a model that predicts an observation at time T2 given the state Z2, and train that so that we can convert our way back to an observation, given a state
They mostly test it on fairly simple environments, but it’s an interesting idea, and I’d be curious to see other people develop it in future.
(A strange aspect of this model is that, as far as I can tell, it’s non-interventionist, in that we’re not actually conditioning over agent action, or trying to learn a policy for an agent. This is purely trying to learn the long term transitions between states)

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