Monkeys Recognize Number Values; Is Calculus — if not Shakespeare — Next?

November 13, 2007

A small study of rhesus monkeys seems to indicate that they recognize number values.

Andreas Nieder of the University of Tübingen, in Germany, and his colleagues came to this conclusion by measuring the electrical activity of individual neurons in the brains of the primates as they performed counting tasks involving the numbers one to four. The team focused specifically on the prefrontal cortex, a walnut-sized area involved in higher-level thinking.

The researchers trained the monkeys to pull a lever when an Arabic numeral matched the count of a preceding array of dots. The correct response earned the animals a treat. The researchers also reversed the experiment, revealing Arabic numerals several seconds before the dots appeared on a screen.

By recording the response rate of about 350 neurons located in each monkey's prefrontal cortex, the researchers discovered that 160 neurons had a strong preference for a specific value between one and four. For example, a given brain cell would send out a barrage of electrical signals whenever the animal saw three dots on the screen or the number three, but the same cell would not respond when the monkey was presented with other numerical values.

Nieder suggested that monkeys can count to far higher numbers. "I'm convinced," he told New Scientist, "that they could go to infinity."

Liz Brannon, of Duke University, agreed that the findings support the idea that non-human primates can understand the meaning of numerals. "Although monkeys don't have language, they can understand a symbol and what it refers to," she said.

However, it is impossible for every number to have a dedicated brain cell, she added. "There's a limit there," she said. "We just don't know what the limits are."

The findings appear in the article "Semantic Associations between Signs and Numerical Categories in the Prefrontal Cortex," published in PLoS Biology.

Source: PLoS Biology, Oct. 30, 2007; New Scientist, Oct. 30, 2007.