Neurons in primate cortex associate numerical meaning with visual signs

Humans share with animals a language-independent system for representing numerical quantity, but number symbols have to be acquired throughout childhood.

A first step in the acquisition of number symbols entails the association of a sign, such as “4,” with a specific quantity. However, it is not known how the brain accomplishes this feat. To investigate the neural mechanisms underlying this process of semantic association, Ilka Diester and Andreas Nieder from the University of Tübingen recorded from neurons in cortical brain regions of monkeys as they learned to associate shapes with Arabic numerals.

Published this week online in the open-access journal PLoS Biology, the scientists show how they first simulated the process of symbolic mapping by training rhesus monkeys to associate the visual shapes of Arabic numerals with the numerosity of multiple-dot displays. They found that many individual neurons in the prefrontal cortex, but only few in the posterior parietal cortex, responded in a tuned fashion to the same numerical values of dot sets and associated shapes. The distribution of these “association neurons” across prefrontal and parietal areas resembles activation patterns in children and suggests how symbolic numerical abilities might have evolved in the human brain, building on existing precursor systems that are used by other animals for numerical judgments.