OPINION What does AI’s success playing complex board games tell brain scientists? OPINION Dale Purvesa,1 In research, it sometimes happens that advances in one beat the European and world champions at Go (2, 3), a field unexpectedly inform another in a fundamental territorial board game far more complicated than chess. way. A case in point may be what computer-generated The latest version in this effort, called AlphaZero (4), now gameplay suggests about how brains operate. Spear- beats the best players—human or machine—in chess headed by Demis Hassabis, David Silver, and their and shogi (Japanese chess) as well as Go. colleagues at the artificial intelligence (AI) company That machines can beat expert human players in Google DeepMind, a series of articles published over board games is not news. The achievement of this the past several years has reported ongoing improve- long-standing goal in computer science first attracted ment of a computational strategy (1–3) that now beats widespread attention in 1997 when IBM’s program all players in complex board games (4). Beginning in “Deep Blue” beat Garry Kasparov, then the world 2015 with a computer program that beat human oppo- champion chess player (5). Human players have since nents at all 49 games in the Atari 2600 suite (challenges been shown to be weak opponents in such games that include Pong, Space Invaders, and Pac-Man) (1), compared with a variety of machine programs. What the group progressed to a program called AlphaGo that is different about Google DeepMind’s AlphaZero is Fig. 1. The search space of the territorial board game Go is intractable using logical algorithms. Image credit: Shutterstock/Saran Poroong. aDuke Institute for Brain Sciences, Duke University, Durham, NC 27708 The author declares no conflict of interest. Published under the PNAS license. Any opinions, findings, conclusions, or recommendations expressed in this work are those of the author and have not been endorsed by the National Academy of Sciences. 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1909565116 PNAS | July 23, 2019 | vol. 116 | no. 30 | 14785–14787 Downloaded by guest on September 30, 2021 that the system operates by learning on a wholly empirical is beyond reckoning, it clearly dwarfs the complexity (trial and error) basis. of board games such as Go. Previous game-playing machines, including earlier versions in the AlphaGo series, used a combination of “Intelligent” Brains? brute force logic, tree search, successful moves made In thinking how these facts bear on understanding by experts in response to specific board positions, and brains, some caution is in order when using phrases other “hand-crafted” expedients. In contrast, AlphaZero such as “artificial intelligence (AI).” Although we use the uses only the empirical results of self-play. The starting word “intelligence”—artificial or otherwise—as if we point is simply a randomly initialized “deep” neural net- knew what it meant, the term simply refers to the ability work (an artificial neural network with multiple hidden to solve some kind of problem. The presumption is that layers) that gradually improves by playing against itself AI solves problems the way humans do, ignoring the millions of times using only the rules of the game and fact that the way we solve problems is largely a mystery. reinforcement learning. With respect to humans, “intelligence” usually re- What, then, is the key to this success, and what are fers to skills most of us lack, such as the ability to do the implications for understanding of how human higher mathematics. But this usage is misleading: is brains work? The lesson seems to be that neural the accomplished athlete solving complex problems on success hinges on connectivity generated by trial- the playing field any less “intelligent” than the math and-error learning over evolutionary and individual prodigy? This definitional deficiency has led to endless time rather than algorithms that compute behavioral arguments, not least the value of IQ tests and other answers. dubious measures of a vague concept whose determi- nation is subject to social bias and other influences. The Underlying Strategy We researchers accord a high status to the ability To understand the power of this approach in playing to think through logical problems and imagine that is board games, consider the search spaces involved. what brains must be doing. Psychologists showed The “game tree complexity” of tic-tac-toe—i.e., an long ago, however, that systematic thinking is awkward estimate of the number of possible positions that must for humans with consequences that can be embarrassing be evaluated to determine the worth of an initial position— (7). For instance, when high school students were briefly is about 2 × 104. This number of possibilities is easily presented with the sequences searched by an algorithm, as my children learned at a 1 × 2 × 3 × 4 × 5 × 6 × 7 × 8 theme park when they were beaten by a chicken pecking one of the tic-tac-toe squares in what amounted to a and cleverly designed Skinner box. 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 and asked to estimate the products, the median an- The take-home message may be that understanding swers given for the first series was 512 and for the neural circuitry in terms of steps specified by algorithms second 2,250. This result shows that when it comes to logic we are often flummoxed, in this case by failing to is not a path to success much beyond the challenge recognize that the sequences are multiplications of of tic-tac-toe. the same numbers and must, therefore, generate the same product (which is actually 40,320). Because ratio- nality and logic play little part in survival and reproduction, There are several different ways to measure the a generally feeble approach to problems such as this is complexity of more difficult board games, leading to what one might expect. These strategies are far too different estimates. But no matter the method used, as ponderous for generating answers to most biological the complexity of board games grows, the relevant problems, and problems whose solution demands rea- search spaces become unimaginably large. The game soning and logic are relatively rare outside the classroom. tree complexity is estimated to be about 1020 for checkers, about 10120 for chess, and an astonishing Implications 10320 for Go (6). For comparison, the estimated number AI in the guise of mimicking computations that brains of atoms in the universe is on the order of 1087.Unlike might be carrying out dates from a conference of math- tic-tac-toe, fully searching spaces of this magnitude is ematicians, computer scientists, information theo- not feasible. In the strategy used by Google DeepMind, rists, and others in 1956. The consensus was famously success follows not from a logical exploration of possible that this challenge should be relatively easy to solve. moves but from the instantiation of trial and error play More than 60 years on, however, the goal has not been in the network’s connectivity that beats a competing met. The central problem now as then is that no one machine playing in the same empirical way. knows what the operating principle of a brain actually is. The relevance of AlphaZero to neuroscience is the This may be where success in gameplay points the size of the search spaces organisms—and ultimately way. A wholly empirical approach to winning complex animals with brains—must contend with as they play board games revives a trial-and-error (neural network) “the game of life” by making “moves” that are rewarded approach that was introduced in the 1940s (8) but has by survival and reproductive success. Although the had a checkered history over the subsequent de- complexity of this biological “game tree” for humans cades (9). Solving problems by unsupervised learning 14786 | www.pnas.org/cgi/doi/10.1073/pnas.1909565116 Purves Downloaded by guest on September 30, 2021 using neural networks has often been overshadowed input board positions, the “synaptic weights” in the by the immense success of algorithmic computation network are updated based on the history of what (executing a series of specified steps), a strategy that worked well in millions of previous games. The Google has repeatedly proven its worth in applications as di- DeepMind group noted early on that the organization verse as manufacturing, genomics, and drug discovery. of networks thus trained bore little or no resemblance to But the success of reinforcement learning in tack- the organization of human brains (10). Indeed, the in- ling otherwise intractable games suggests that making scrutable connectivity of the trained AlphaZero net- behavioral responses based on what worked for a works may indicate why the detailed connectivity of species and its members over accumulated past ex- brains has failed to shed as much light on neural oper- perience is the most efficient way to win in life. Because ation as expected. the brain of any animal is the executor of behavioral The take-home message may be that understand- choices, it is hard to escape the implication that con- ing neural circuitry in terms of steps specified by al- tending with extremely large “search spaces” must be gorithms is not a path to success much beyond the predicated on neural (or machine) connectivity that re- flects accumulated learning by trial and error. This is, challenge of tic-tac-toe. If one accepts that playing after all, the strategy used in evolution. complex games empirically is a paradigm for playing If this empirical mode of operation is what brains the vastly more complex game of life, brain science such as ours are doing, then this game-related evidence could advance rapidly albeit in a framework different will have to be taken into account by brain scientists, from the rule-based computation that has often driven despite the daunting challenge of understanding nervous the field.