Spaun sees a series of digits: 1 2 3; 5 6 7; 3 4 ?. Its neurons fire, and it calculates the next logical number in the sequence. It scrawls out a 5, in legible if messy writing. This is an unremarkable feat for a human, but Spaun is actually a simulated brain. It contains 2.5 million virtual neurons — many fewer than the 86 billion in the average human head, but enough to recognize lists of numbers, do simple arithmetic and solve reasoning problems.
Described for the first time in Science1, Spaun — the Semantic Pointer Architecture Unified Network — is the brainchild of Chris Eliasmith, a theoretical neuroscientist at the University of Waterloo in Canada, and his colleagues. It stands apart from other attempts to simulate a brain, such as the ambitious Blue Brain Project (see 'Brain in a box'), because it produces complex behaviours with fewer neurons. “Throwing a lot of neurons together and hoping something interesting emerges doesn’t seem like a plausible way of understanding something as sophisticated as the brain,” says Eliasmith. “Until now, the race was who could get a human-sized brain simulation running, regardless of what behaviours and functions such simulation exhibits,” says Eugene Izhikevich, chairman of the Brain Corporation in San Diego, California, who helped to develop some of the first large-scale neuronal models — including one with 100 billion neurons. “From now on, the race is more [about] who can get the most biological functions and animal-like behaviours. So far, Spaun is the winner.”