Curiosity in minds and machine learners

In the “classic” machine learning paradigm of supervised learning, there’s no role for curiosity. The goal of a supervised learning algorithm is simply to match a set of labels for a provided dataset as closely as possible. The task is clearly defined, and even if the algorithm was capable of wondering about the meaning of the data pouring into it, this wouldn’t help with the learning task at all.

For humans, on the other hand, curiosity seems to be an integral part of how we learn. Even before starting school, children are hard at work figuring out the world around them. They crave novelty and surprise, and delight in finding solutions to new challenges. Their choice of things to investigate or focus on is internally motivated: they target objects and skills that present just the right amount of challenge (too little is boring, too much is frustrating). Although adults might provide toys and encouragement, children seem to find exploring and gaining knowledge intrinsically rewarding. (This isn’t just limited to humans, either – for example, mice will endure an electric shock to explore a new environment.1) “Curiosity” is difficult to define precisely, but surely choosing to learn about something just for the sake of it gets at the heart of what it means to be curious.

It shouldn’t come as a surprise that humans and machine learning algorithms learn in very different ways. However, unlike supervised learning, there seems to be a role for curiosity in reinforcement learning (RL). In RL, an algorithm (also called an agent) must choose actions in an attempt to reach some particular goal state, where it receives a reward that allows it to learn which actions are beneficial. For simple goals, an RL agent may be able to stumble upon a solution simply by choosing random actions until it receives a reward for the first time. However, this strategy doesn’t work so well if the task demands a precise sequence of actions. For example, when playing a video game, many hazards may need to be avoided before any points can be scored. 

In these cases, the RL agent has no idea what to do to reach the goal, and it might try random actions for years without success. However, the failures provide important information about what kinds of states don’t lead to a reward. This suggests a new strategy: systematically trying as many new things as possible, while keeping track of what has been tried so far. Various algorithms have been proposed to do this, and when implemented, the RL agent purposefully seeks out new situations, looking very much as if it is curious about its environment. One of these new situations will eventually be the goal, and the RL agent will successfully learn to complete the task.

The simplest way to make a “curious” RL agent is to have it try to predict what is going to happen next, and give it an “exploration bonus” when this prediction fails. Intuitively, the agent can make good predictions about situations it has encountered before, but not about novel ones. Therefore, it receives the exploration bonus for learning how to achieve new kinds of states. How well does this behavior reflect human-like curiosity? Well, the exploration bonus causes the agent to seek out novelty, just like a curious human. However, this version of curiosity seems strangely passive: anything that’s unpredictable is equally rewarding to the RL agent, and it has no concept of some kinds of novelty being more interesting or relevant than others. This turns out to be a very practical concern, because the RL agent will seek out any source of unpredictability in its environment, such as random TV static.

Several methods proposed in the last several years, such as Random Network Distillation (RND) and episodic curiosity, have solved the “TV static problem.” In fact, these kinds of approaches to exploration are so successful that they enabled RL agents to achieve superhuman performance on difficult Atari video games like Montezuma’s Revenge. However, they achieve this through algorithmic tricks that rule out randomness in the environment when calculating exploration bonuses. An RND agent still has no basis for deciding that some kinds of novelty are more important than others.2

This is important because, unlike in video games, the real world is incredibly complex. Predicting the consequence of every action, especially in the presence of active humans and a dynamic environment, is intractable. Humans deal with this in part by having relatively narrow interests and things that matter to us, either because they are extrinsically rewarding (e.g. income, food), or because we simply get curious about specific things. Exploration and play, especially in children, are almost certainly tied to overall learning, development, and gaining mastery in interacting with the world. But children learn actively; they choose their own problems to solve, mysteries to explore, and questions to try to find answers to. They introduce their own structure into a world that resists simple rules and explanations.

A related idea in machine learning is that of automatic curricula, where algorithms attempt to successively choose learning tasks that are neither too easy nor too difficult in an attempt to smooth the learning process. Similarly, in active learning3 algorithms seek out more information when they’re uncertain. But the truth is that we have very little idea why children (and humans in general) get curious about some things rather than others, even though this is probably crucial to how we learn to make sense of our highly complicated environments. Human curiosity has already proved a fruitful source of inspiration for machine learning; future insights from cognitive scientists about how and why we get curious are likely to propel further developments and even better algorithms.

A note of caution

As AI agents become more competent and have more influence over our lives, ensuring that their behavior is truly beneficial becomes increasingly important. Even when AI agents only optimize for a single human-provided objective, it has been well established that there could be serious unintended consequences (for example, since the AI has to be operational to complete its task, it’s incentivized to resist attempts to turn it off for almost any objective). From this view, the idea of AI agents actively exploring and choosing their own goals sounds especially risky.4 If this turns out to be helpful or even necessary for robust learning in complex environments, how could we ensure that the things an AI decides to attempt aren’t harmful? Learning more about how young children play and explore may help, but children have societal scaffolds (like being physically weaker than adults) that may not apply to robots and AI algorithms. Ensuring safety while supporting learning is an important topic for research.

  1. This is stated in this paper, but I wasn’t able to get access to the cited source to verify.
  2. As an example, imagine entering a huge library, where the contents of each book follows a different kind of pattern. An RND agent would happily gain large exploration bonuses for each new book it opened until it figured out the pattern, and then move on to the next one, in effect staying trapped like that forever. A human, selecting a few books at random, would quickly realize that their contents have no relevance to their life or things they care about, and leave the library. If they got curious about something, it would most likely be about the library’s history and purpose, rather than about the contents of each individual book.
  3. For example, see this paper: Permalink (not open access), pdf.
  4. As an illustration, consider an analogy that’s commonly drawn between humans, ants, and AIs. Humans don’t care so much about accidentally stepping on ants, because our minds are so much more complex than theirs that we struggle to take the possibility of them being conscious seriously – so we really don’t want to create an AI that feels about humans the way we feel about ants. Now, consider that instead of indifference, this same AI was really curious about humans – but in the same sort of way that cats are curious about how the mice they catch will behave if let go for a moment. From our perspective, the “wrong” kind of curiosity in a sufficiently advanced AI could look outright malicious.

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