Monkey see, monkey add. And in the same test of high-speed arithmetic, it turns out, people see and people add using what looks like the monkey method for doing rough sums without counting.
“What we’re doing is accessing a primitive system for nonverbal arithmetic,” says Elizabeth Brannon of Duke University in Durham, N.C. She and Duke colleague Jessica Cantlon tested both rhesus macaques and college students for their ability to do split-second addition. Similarities in the two groups’ performance support the idea of an evolutionarily ancient capacity for dealing with numerical quantities, the researchers say in the December PLoS Biology.
“The paper marks an important milestone in the development of our understanding of the roots of mathematical cognition,” comments Charles R. Gallistel of Rutgers University in Piscataway, N.J.
Experiments with rats, pigeons, and other creatures have already shown that non-human animals have some ability to do approximate computations, says Gallistel. Experiments have even shown pigeons doing a form of subtraction.
Brannon says she wanted to devise a test that both adult humans and monkeys could take. Earlier tests that worked for monkeys didn’t make sense for people. In one such protocol, for example, monkeys watched as researchers at Harvard University put a group of lemons, and then a second group, behind a screen. When the researchers lifted the screen, they might reveal the correct “sum” of the two groups of lemons—or they might have sneaked in some extras. Monkeys stared longer when a revealed pile had the wrong number of lemons, suggesting that the animals have some computational ability. But measuring gaze length isn’t an ideal way to assess math skills in human adults.
So Brannon and Cantlon set up computer tests. One set of dots flashed on the screen for a half-second. After a delay, another set appeared. Finally a screen showed two boxed sets of dots, one representing the correct sum of the previous sets and the other showing an incorrect sum. Test subjects had to tap the correct box. The computer changed the size of dots in the possible answers, so that relying on area alone wouldn’t yield a correct score.
Two female macaques got lessons on how to do sums, with correct answers earning sips of Kool-Aid. The lessons used only even numbers as totals, but the tests included odd sums, up to totals of 17. Fourteen college students also went through the screens, receiving $10 instead of Kool-Aid.
Brannon says she told the college students to tap an answer quickly without counting. The students answered each problem in about a second, which is indeed too fast to count both dot sets.
The students reached 94 percent accuracy; macaques averaged 76 percent. Both primates and students lost accuracy when the two sets of possible answers differed by only a few dots.
“These data are very good for telling us where our sophisticated human minds came from,” says Cantlon.