A head for numbers

The brain shows slightly different, but overlapping patterns when processing digits and dots of the same value

A 2 by any other name would not be the same to the brain.

NUMERIC BRAIN People process numbers in the parietal cortex (colored areas), but the brain reacts a bit differently when seeing a number of dots than when seeing a digit. A new study of brain images suggests that the parietal cortex processes quantities differently from symbols of those quantities. Eger, et al./Current Biology

Scientists can determine the number of dots a person has seen by looking at brain activity patterns using functional MRI. But when the brain views digits instead of dots, slightly different yet distinct activity patterns emerge. The findings suggest that people process the fundamental idea of a quantity differently from the way they process a symbol representing that quantity, French scientists report online September 24 in Current Biology.

“This paper takes a really important step toward using functional images to understand how numbers are represented in the brain,” says cognitive neuroscientist Daniel Ansari of the University of Western Ontario in Canada.

The brain identifies numbers in an area called the parietal cortex, near the upper back sides of the head. Scientists had speculated that the brain might use overlapping patches of neurons in this region to identify nonsymbolic quantities — such as the idea of one, two or three — and the symbolic numbers that people learn to associate with those values — such as 1, 2 or 3. But, Ansari says, no one knew for sure how the brain processed different representations of the same value. This experiment reveals that the brain codes number symbols and fundamental numerical quantities slightly differently, he adds.

Coauthor Evelyn Eger of the French research institute INSERM and her colleagues scanned the brains of 10 volunteers using fMRI while showing them collections of dots. Computer analyses revealed that viewing different quantities of dots led to distinct brain activity patterns. Then the team showed the volunteers either collections of dots or digits of the same value. Analyses revealed slightly different brain activation patterns for digits and dots of the same numerical value, Eger says.

In both cases, activity was primarily in the parietal cortex. And the analyses used to evaluate the patterns revealed there was some overlap. A volunteer’s brain activation signature from viewing numbers could predict how many dots a subject had viewed, but not the other way around.

The researchers also found that the brain patterns shifted gradually with increasing numbers of dots, suggesting that numbers that are close in value activate patches of neurons that are close to each other in the brain. Digits, on the other hand, did not create this pattern of change in brain activity as numbers increased.

Eger says the findings suggest that regions of the brain that process nonsymbolic numbers, the dots, are “broader” and “stronger” than those that develop later, as a person learns the symbols that represent numerical values. These symbolic numbers, the digits, may become mapped onto preexisting number-processing regions of the brain.

“Being able to resolve activities for individual numbers gives us more precise tools to track what happens in the brain when humans do calculations,” says Eger, who is also affiliated with the NeuroSpin imaging center near Paris. She says she hopes to use this knowledge to track how the brain’s representation of numbers changes as a person receives a mathematical education.

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