Evidence adds up that monkeys can count
Study: Brain neurons seem to provide knack for numbers
September 5, 2002 Posted: 3:14 PM EDT (1914 GMT)
By Marsha Walton
CNN Sci-Tech
(CNN) -- Animal rescue workers tell stories of mama dogs and cats in the wild "counting noses" of their babies to make sure none of the little ones has wandered off. Other mammals and birds often make lifesaving decisions based on a sort of numerical ability: To fight or to flee based on how many predators they encounter; or to pick an area that has more nuts or bananas, rather than fewer.
In recent studies of rhesus monkeys, scientists at the Massachusetts Institute of Technology (MIT) determined that specific neurons in the monkey's brain are "tuned to" certain numbers, and play a major role in the animal's ability to determine quantities.
It is a bit different from counting, which involves the unique human capability of language, said Andreas Nieder, who reports his findings in this week's edition of the journal Science.
"Our evidence is imprecise, but the monkeys showed a language-independent ability to discriminate numbers," Nieder said.
Connecting the dots
Nieder and his colleagues spent about nine months training and testing two rhesus monkeys. The monkeys looked at a computer screen, which displayed from one to five dots. After a one second interval, a second screen appeared. If it had the same number of dots as the first, the monkeys were taught to release a lever. Their reward for recognizing matching numbers of dots was a sip of apple juice.
Scientists found that rhesus monkeys could tell whether two successive displays on the computer contained the same number of dots.
Researchers monitored the monkeys through microelectrodes in the prefrontal cortex of their brains. This region has a variety of functions that involve some type of abstract thinking. In a human, for example, learning how to respond to traffic lights would involve the prefrontal cortex; pulling a hand away from a hot stove would not.
Different neurons in the monkeys showed more activity when encountering different numbers of dots. For example, one neuron would "like" the number three, and show heavy brain activity; another would get excited by the number five.
"There may be other places in the brain where numerical information is encoded," said Nieder. But, he said, the information gained from the prefrontal cortex indicates it plays a key part in the process.
The size, shape, and placement of the dots on the screen varied during the tests. Four dots might be in the shape of a square on one screen, then in a straight line on the next. This, according to Nieder, showed that the monkeys were not just responding to a common shape, but to a specific number of items being displayed.
Precursor to humans' ability to count?
This numeric ability in monkeys may be a biological precursor of what has developed in humans as precise language and sometimes culture-based counting systems
"Human babies also have a rudimentary ability to estimate small quantities," said Nieder. Both human infants and monkeys can accurately discriminate quantities up to five.
In reviewing the MIT findings, other scientists say training alone could not explain the monkeys' numerical abilities -- the wide variations of the tests indicate some of the knowledge has to be innate.
"We are clearly not the only species with a knack for numbers," said Stanislas Dehaene in a perspectives article, also in Science.
Nieder said the research may someday help understand how neurons encode information, and give clues to how humans develop mathematical skills.
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