BRIAN MAY NOT BE HARD WIRED TO LINK NUMBERS AND SPACE.

Is our ability to map numbers onto a physical space – such as along a line – a cultural invention rather than an innate ability? Members of a remote tribe in Papua New Guinea understand the concept of numbers but do not map them along a line, which suggests that the 'number line' must be learned.

Researchers have long thought that the human brain is hard wired to associate numbers with physical space. The idea received a boost in 2002 when it was discovered that people with brain damage who were unable to fully perceive one side of their body had trouble interpreting the number line – they claimed, for example, that five lies between three and six (Nature, DOI:10.1038/417138a).

In 2008, Stanislas Dehaene of the National Institute of Health and Medical Research (INSERM) in Saclay, France, and colleagues found a subtle variation of the concept in the Mundurucu, an indigenous group in the Amazon with little or no formal education. The Mundurucu map numbers on to a line, but use a logarithmic scale rather than a typical linear scale – they allow plenty of room for small numbers but scrunch larger numbers together at the far end of the line. The finding suggested that the linear number line is a cultural invention, but the number line itself remained intact as an intuition shared by all humanity (Science, DOI: 10.1126/science.1156540).

This is now being called into question by Rafael Núñez of the University of California at San Diego and colleagues. Núñez's team trekked into the remote mountains of Papua New Guinea to study the Yupno, an indigenous group that live without electricity, roads or even domestic animals. "But they do have numbers," says Núñez. "They are number-savvy."

Number scrunchers

Núñez asked 20 Yupno adults (14 of whom were unschooled, while others had attended middle school) and 10 controls in California to place the numbers 1 to 10 along a 22-centimetre-long black line printed on a white card. The numbers were presented randomly, either as pre-recorded words in Yupno dialects, or as a sequence of tones, or as a set of dots printed on white paper.

The team found that both the controls and the schooled Yupno mapped the numbers on to the line, though the schooled Yupno did so less evenly, tending to place the numbers towards one of the two endpoints. But the unschooled Yupno completely ignored the extent of the number line. They lumped numbers 1 and 2 at one end of the line, and all other numbers at the other end.

According to Núñez, the finding reveals that the number line is a cultural metaphor for representing numbers, which has been used to build more sophisticated mathematics. Such metaphors recruit brain circuits and become ingrained.

"The number line doesn't seem to be something we have built into our brains," he says. "It's something that got concocted culturally at some point. Now we teach it and it's so natural that it's all over the place."

Cognitive linguist George Lakoff of the University of California, Berkeley, agrees the study knocks down the view that the number line is hardwired. He supports the notion that it is a metaphor, which the unschooled Yupno interpret in a unique way. "The brain mechanisms for [creating] metaphors are going to be the same for everybody, but the metaphors depend on your physical environment and your culture," Lakoff says.