Single brain cells count
Monkey neurons can recognize numbers.
06 September 2002
JOHN WHITFIELD
 |
| Individual
cells and whole monkeys
have the same numerical
abilities. |
| ©
Getty Images |
|
|
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.
Call weighting
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." |
