The monkeys’ systematic errors .

To ensure that the monkeys hadn’t simply memorized every possible combination of symbols and associated a value with the combination—this wouldn’t be true addition—Livingstone’s team next taught the animals an entirely new set of symbols —Tetris-like blocks rather than letters and numbers. With the new symbols, the monkeys were again able to add—this time calculating the value of combinations they’d never seen before and confirming the ability to do basic addition, the team reports online today in the Proceedings of the National Academy of Sciences.

But when Livingstone and colleagues started analyzing the data in more detail—they had the results of hundreds of tests per day for months on end—they realized that the monkeys weren’t 100% accurate. They tended to underestimate a sum compared with a single symbol when the two were close in value—sometimes choosing, for example, a 13 over the sum of eight and six. The underestimation was systematic—when adding two numbers, the monkeys always paid attention to the larger of the two, and then added only a fraction of the smaller number to it.

The monkeys’ systematic errors argue against one theory of how the mammalian brain processes numbers. “What they’re doing is paying more attention to the big number than the little one,” Livingstone explains. But the altered values weren’t tied intrinsically to the symbols. If eight was the larger of two numbers in a sum, then its full value was considered, but if it was being added to a larger number, its value was diminished. One prevailing theory on number representation—dubbed logarithmic encoding—had proposed that the brain always underestimates the value of larger numbers in a systematic and unchangeable way. In such a case, the value of eight wouldn’t vary based on the situation as Livingstone observed.

“They’ve shown that it’s very unlikely that there’s some kind of logarithmic encoding of numbers,” says psychologist David Burr of the University of Florence in Italy, who was not involved in the new work. Further research on how humans and monkeys estimate the value of numbers, and how this plays a role in the brain’s ability to add two values, could shed light on dyscalculia—a human learning disability specific to mathematics. Children with dyscalculia often have trouble not only adding numbers, but quickly guessing how many objects are in a group. Together with the new results on rhesus macaques, this suggests that estimating values is key to the ability to add.

“Being able to estimate obviously has survival value;