## A number is a number is a number

Numbers in the world always come in a concrete form - but they have a meaning independent of this form: the digit "2" and the word "two" mean the same number. How is this reflected in processing in the brain?

In elementary school we learn the essence of mathematics: three apples, three dots and the symbol "3" have one thing in common. However, the meaning of a number is independent of its symbolic representation. Researchers are trying different ways to find out whether there is a neural representation for this abstract meaning of a number. First clues to number processing revealed lesions in certain brain regions that led to deficits in mathematical skills. Techniques such as functional magnetic resonance imaging (fMRI) are now available, which, by measuring blood flow, allow conclusions to be drawn about the activity of neurons in a volume of the brain.

Today, scientists agree that the intraparietal sulcus (IPS), the cleft between the cerebral hemispheres in the parietal lobe, plays an important role in the representation and processing of numerical information. The influence of the format in which a number is perceived, for example as a word ("three"), digit ("3") or dot pattern ("…"), has not yet been clarified. One reason for this is the poor resolution of fMRI measurements (in the millimeter range) compared to the size of neurons (in the micrometer range).

Two new studies circumvent this shortcoming elegantly by using the adaptation effect: When a certain stimulus is repeated, the neuronal activation gradually decreases. When a new stimulus is presented that differs from the old in certain aspects, the activity of the very regions processing these changed aspects increases again. The effect can be interpreted physiologically as the decaying firing rate of repeatedly activated, as it were, exhausted neurons. If the stimulus changes, other neurons, which are still fresh, become active and fire more strongly again.

A research group led by Manuela Piazza at INSERM in Orsay presented test subjects with various numbers as dot patterns and Arabic numerals [1]. With an abrupt change in the numerical value, they were able to observe the adaptation effect in the IPS. Activation depended on how far the new value deviated from the old. In the right hemisphere, the effect was also independent of whether the number format changed.

Roi Cohen Kadosh at Israel's Ben Gurion University and his colleagues used numbers in the form of words (rather than dots) and Arabic numerals in a similar experiment [2]. They came to a different conclusion: the adaptation effect was independent of number format in the left hemisphere.

To clarify the contradiction between the two results, experimental differences must not be overlooked: Cohen Kadosh used only culturally appropriated symbols ("3", "three") and small values (2-9), while Piazza used culturally appropriated " combined "natural" number representations ("3", "…") and also used much larger numerical values (17-49).

Roughly simplified, both results and older studies allow us to conclude what neuropsychology had already suspected: the left hemisphere specializes in culturally appropriated number symbols, it does not differentiate between "3" and "three". The right hemisphere, on the other hand, is responsible for non-symbolic estimates, for which a "17" and (approximately) seventeen points are equivalent.

There is a format-independent number representation in the parietal cortex. The abstract mathematical term number is not just a pipe dream, the brain handles abstract numerical values directly.