Monkeys Don't Monkey With Math

June 1, 2006

When we count, we may mumble "one, two, three ..." to ourselves, but is language really essential to number perception? It's an intriguing question, because it pertains to how deeply ingrained numerical perception is in primate evolution. By testing how well monkeys compare numbers, Duke neuroscientists have found that the animals do semantically perceive numbers as we do, without language.

In their experiments, Elizabeth Brannon, assistant professor of psychological and brain sciences, and graduate student Jessica Cantlon sought to test whether macaque monkeys show a phenomenon known as "semantic congruity" when making numerical comparisons. Semantic congruity means that people are quicker at judging the smaller of two small animals--say an ant versus a rat--than judging which is the larger. Conversely, they're quicker at judging the larger of two large animals than judging which is the smaller. Semantic congruity holds for all kinds of comparisons, including numbers and distances.

"It would seem that this is entirely a linguistic effect, totally dependent on language," says Brannon. "We sought to understand whether monkeys showed this semantic effect, even though they don't have language."

In their experiments, Cantlon and Brannon presented monkeys with two arrays of randomized numbers of dots displayed on a computer touch screen. Unable to use language to instruct the monkeys to "choose larger" or "choose smaller," the researchers made the background blue if the monkeys were to choose the larger number and red if the smaller number. The monkeys were rewarded for correct answers with a sip of a sweet drink.

"Our results showed a very large semantic congruity effect," says Cantlon. "For example, when the number pair was small, such as two versus three, the monkeys were much faster at choosing the smaller compared to the larger of the pair. Clearly, even though their capability has nothing to do with language, it is nevertheless semantic in that the red and blue color cues carry meaning for the monkeys."

The researchers described their findings in an article in the Proceedings of the National Academies of Sciences.