Monkey neurons can recognize numbers.
When a monkey looks at two dots, apples or other monkeys, single nerve cells recognize the groups' 'twoness', researchers have found1. The discovery shows that the brain's ability to deal with abstract concepts can be traced right down to individual cells.
Some neurons respond to one item, others to two, or three, and so on, say Earl Miller and his colleagues at the Massachusetts Institute of Technology, Cambridge.
The discovery "decodes the brain's code for number", says neuroscientist Stanislas Dehaene of Frédéric Joliot Hospital in Orsay, France. The neurons respond to numbers in the same way that whole monkeys - or people - do.
Monkeys' numerical abilities could be an evolutionary shadow of our own, says Dehaene. "Even behaviour which we think of as sophisticated and based in culture ultimately has biological roots."
Spot the difference
Miller's team showed groups of dots to macaques, and recorded the output from individual neurons in the monkeys' prefrontal cortex. This area receives inputs from the visual system.
The neurons ignore the dots' size, shape and arrangement and hone in on their number. Each cell's response peaks at its preferred number and tails off on either side.
This could explain why animals can spot large differences more easily than small - there's less overlap between the responses of different specialist cells.
For cells tickled by bigger numbers, the peak of the curve is flatter. It's as if these neurons hedge their bets about the number of items they see.
This matches another feature of number perception. It takes animals and humans longer to distinguish between groups of 15 and 16 than it does between one and two, for example.
The US team tested the monkeys using groups of up to five dots. For bigger numbers, says Dehaene, cells could respond to approximate quantities rather than individual numbers - between 10 and 12, for example, or 30 to 35.
Many animals can recognize and distinguish different quantities - pigeons can tell between different-sized groups of objects up to about 50 items.
Neurons in the hearing system can also count, another study revealed this week2.
“Even sophisticated behaviour has biological roots Stanislas Dehaene , Frédéric Joliot Hospital, Orsay, France”
The Pacific tree frog (Hyla regilla) makes an aggressive call to other males and an advertisement call to attract females. The only difference between the two calls is their speed.
"The slow call is like a ribbit, the fast one more like a drawn-out croak," says biologist Gary Rose of the University of Utah. Different frog nerve cells distinguish rapid and slow pulses.
Female frogs' male-detector neurons fire only after they hear five or more rapid pulses, Rose and his colleagues find. If the pulses are too close or too far apart, the counter resets to zero - as if the nerve cells measure the spaces between pulses, rather than the sounds themselves.
We may perceive some elements of speech and music in a similar way, says Rose: "Humans are much better at detecting changes in rhythm when there are a larger number of notes."
Nieder, A., Freedman, D. J. & Miller, E. K Representation of the quantity of visual items in the primate prefrontal cortex. Science, 297, 1708 - 1711, (2002).
Edwards, C. J., Alder, T. B. & Rose, G. J. Auditory midbrain neurons that count. Nature Neuroscience, Advance online publication, (2002).
Frédéric Joliot Hospital, Orsay, France