You may not want a monkey to balance your chequebook, but you still have to give them credit – new research supports the idea that not only can monkeys understand written numbers, but that individual brain cells may become dedicated to specific numbers.

The small study of two rhesus monkeys reveals that cells in their brains respond selectively to specific number values – regardless of whether the amount is represented by dots on a screen or an Arabic numeral.

For example, a given brain cell in the monkey will respond to the number three, but not the number one. The results suggest that individual cells in human brains might also have a fine-tuned preference for specific numerical values.

While monkeys might not yet have mastered calculus, recent studies have shown that they can learn understand some basic aspects of arithmetic and, in a rare case, multiplication.


Andreas Nieder at the University of Tübingen in Germany and colleagues trained two rhesus monkeys to count by showing them various numbers of dots on a screen followed by Arabic numerals.

Excited cells

The monkeys had to pull a lever to indicate when the numeral matched the preceding count of dots. An accurate response earned the animals a cup of apple juice, which they consider a treat. The researchers also reversed the task, showing the Arabic numerals several seconds before the dots.

Nieder and his colleagues used special probes to measure the electrical activity of individual neurons in the brains of the two primates performing this counting task for the numbers one through to four. The team focused specifically on a walnut-sized area involved in higher-level thinking, known as the prefrontal cortex.

The researchers painstakingly recorded the response rate of about 350 neurons located in each monkey’s prefrontal cortex. Of the roughly 700 neurons they examined, 160 appeared to have 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 points out that there is redundancy in that multiple cells appear fine-tuned to the same numerical values.

‘Count to infinity’

Liz Brannon at Duke University in Durham, North Carolina, US says that the new findings support the notion that non-human primates really can understand the meaning of numerals.

“Although monkeys don’t have language they can understand a symbol and what it refers to,” she explains.

Nieder, meanwhile, believes that the monkeys can count to far higher numbers. “I’m convinced that they could go to infinity,” he says.

Brannon notes, however, that it is impossible for every number to have a dedicated cell in the brain: “There’s a limit there – we just don’t know what the limits are.” For example, she says she strongly doubts there is a cell that responds to the number 2017 only.

The results are not the first to suggest there may be specific brain cells tied to individual concepts. In 2005 researchers discovered that individual neurons become activated by images of specific celebrities such as Jennifer Aniston and Halle Berry.

Journal reference: PLoS Biology (DOI: 10.1371/journal.pbio.0050294)

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