A research team led by neurobiologist Margaret Livingstone trained three rhesus macaques to identify symbols representing the numbers zero to 25. When given the choice between two symbols, monkeys reliably chose the larger to get a correspondingly larger number of droplets of water, apple juice, or orange soda as a reward. To test whether the monkeys could add these values, the researchers began giving them a choice between a sum and a single symbol rather than two single symbols. Within 4 months, the monkeys had learned how the task worked and were able to effectively add two symbols and compare the sum to a third, single symbol.
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. This challenges the idea that mammalian brains perceive numbers logarithmically and may help researchers better understand how human beings process numbers.
“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; you want to be able to glance up and see how many lions are about to attack you,” Burr says. “The remaining goal is developing a model to explain how that happens in the brain.”