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Monkeys Learn Their Numbers

Diester I,Nieder A (2007) Semantic Associations between Signs and Numerical Categories in the Prefrontal Cortex. PLoS Biol 5(11): e294 doi:10.1371/journal.pbio.0050294

From the Abstract:

• The utilization of symbols such as words and numbers as mental tools endows humans with unrivalled cognitive flexibility.
  
• In the number domain, a fundamental first step for the acquisition of numerical symbols is the semantic association of signs with cardinalities.
  
• We explored the primitives of such a semantic mapping process by recording single-cell activity in the monkey prefrontal and parietal cortices, brain structures critically involved in numerical cognition.
  
• Monkeys were trained to associate visual shapes with varying numbers of items in a matching task.
  
• After this long-term learning process, we found that the responses of many prefrontal neurons to the visual shapes reflected the associated numerical value in a behaviorally relevant way.
  
• In contrast, such association neurons were rarely found in the parietal lobe.
  
• These findings suggest a cardinal [probably no pun intended] role of the prefrontal cortex in establishing semantic associations between signs and abstract categories, a cognitive precursor that may ultimately give rise to symbolic thinking in linguistic humans.
  

Author Summary:

We use symbols, such as numbers, as mental tools for abstract and precise representations. Humans share with animals a language-independent system for representing numerical quantity, but number symbols are learned during childhood. A first step in the acquisition of number symbols constitutes an association of signs with specific numerical values of sets. To investigate the single-neuron mechanisms of semantic association, we simulated such a mapping process in rhesus monkeys by training them to associate the visual shapes of Arabic numerals with the numerosity of multiple-dot displays. We found that many individual neurons in the prefrontal cortex, but only a few in the posterior parietal cortex, responded in a tuned fashion to the same numerical values of dot sets and associated shapes. We called these neurons association neurons since they establish an associational link between shapes and numerical categories. The distribution of these association neurons across prefrontal and parietal areas resembles activation patterns in children and suggests a precursor of our symbol system in monkeys.

Copyright: © 2007 Diester and Nieder. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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