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Summary by CodyWild 6 years ago
Reward functions are a funny part of modern reinforcement learning: enormously salient from the inside, if you’re coding or working with RL systems, yet not as clearly visible from the outside perspective, where we just see agents playing games in what seem to be human-like ways. Just seeing things from this angle, it can be easy to imagine that the mechanisms being used to learn are human-like as well. And, it’s true that some of the Atari games being examined are cases where there is in fact a clear, explicit reward in the form of points, that human players would also be trying to optimize. But in most cases, the world isn’t really in the habit of producing clear reward signals, and it definitely doesn’t typically do so on time scales that account for most of the learning humans do.
So, it’s generally hypothesized that in addition to updating on (sparse) environmental rewards, humans also operate according to certain pre-coded, possibly evolutionarily-engineered heuristics, of which one is curiosity. The intuition is: it sure seems like, especially early in life, humans learn by interacting with objects purely driven by curiosity, and we’d love to somehow harness that same drive to allow our learning systems to function in environments lacking dense, informative reward signals. One such environment is the video game Montezuma’s Revenge, which in addition to being amusingly difficult to search for, is a game with sparse, long-range rewards, on which typical reward-based agents have historically performed poorly, and on which this current paper focuses.
A strong existing tradition of curiosity objectives focuses on incentivizing agents to be able to better predict the next observation, given the current observation and their action within it. Intuitively, by training such a network on historical observations, and giving agents a bonus according to that prediction’s error on a given observation. The theory behind this is that if an agent isn’t able to predict the observation-transition dynamics at a given state, that probably means it hasn’t visited many nearby states, and so we want to incentivize it doing so to gain information. If this sounds familiar to the classic “explore vs exploit” trade-off, it’s very much a similar idea: in cases of clear reward, we should take the reward, but in cases of low or uncertain reward, there’s value to exploration.
One difficulty of systems like the one described above is that they reward the agent for being in environments where the next observation is difficult to predict from the current one. And while that could describe novel states about which the agent needs to gain information, it can also describe states that are inherently stochastic; the canonical example being random static on a TV screen. The agent has a lot of trouble predicting the next observation because it’s fundamentally non-deterministic to a greater degree than even the random-but-causal dynamics of most games. The proposed alternative of this paper is a little strange, but makes more sense in the context of responding to this stochasticity problem. The authors propose to create a random mapping, in the form of an initialized but untrained neural network, taking in observations and spitting out embedding vectors. Then, they incentivize their agent to go to places that have high prediction error on a network designed to predict these random embeddings. Since the output is just a function mapping, it’s deterministic with respect to observations. The idea here is that if you’ve seen observations similar to your current observation, you’ll be more able to predict the corresponding embedding, even if there’s no meaningful relationship that you’re learning.
https://i.imgur.com/Ds5gHDE.png
The authors found that this performed well on Montezuma’s Revenge and Private Eye, but only middlingly-well on other environments. I’m a bit torn on this paper overall. On one hand, it seems like a clever idea, and I’m in general interested in seeing more work on curiosity. It does clearly seem to be capturing something that corresponds to novelty-seeking, and the agent trained using it explores a higher number of rooms than alternative options. On the other, I’m a little skeptical of the fact that it only has consistent performance in two environments, and wish there had been more comparisons to simpler forms of observation similarity, since this really does just seem like a metric of “how similar of observation vectors to this have you seen before”. I find myself wondering if some sort of density modeling could even be effective here, especially if (as may be the case, I’m unsure) the input observations are metadata rather than pixels.
