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The neuronal code for number

Key Points

  • In primates, number neurons in a dedicated parieto-frontal network encode the number of elements in a stimulus.

  • Number neurons in the prefrontal cortex respond in an abstract manner, and their responses generalize across spatial, temporal and visuo-auditory item presentations.

  • Number neurons are present in numerically naive monkeys, suggesting that the brain is hard-wired to extract number.

  • Number processing provides mechanistic insight into how relevant information is selected and maintained in working memory.

  • Rule neurons and neuron populations in the frontal lobe guide decisions based on number information.

Abstract

Humans and non-human primates share an elemental quantification system that resides in a dedicated neural network in the parietal and frontal lobes. In this cortical network, 'number neurons' encode the number of elements in a set, its cardinality or numerosity, irrespective of stimulus appearance across sensory motor systems, and from both spatial and temporal presentation arrays. After numbers have been extracted from sensory input, they need to be processed to support goal-directed behaviour. Studying number neurons provides insights into how information is maintained in working memory and transformed in tasks that require rule-based decisions. Beyond an understanding of how cardinal numbers are encoded, number processing provides a window into the neuronal mechanisms of high-level brain functions.

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Figure 1: Representation of number in the primate brain.
Figure 2: Responses of neuron populations to number.
Figure 3: Microcircuit interactions of number neurons.
Figure 4: Cognitive inhibition of a prefrontal cortex cell during numerical distraction.
Figure 5: Numerical rules.

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Acknowledgements

The author thanks H. Ditz, T. Ott, L. Veit and S. Westendorff for valuable comments on the manuscript. H. Ditz helped with Figures 1b and 1c, and T. Ott provided Figure 5d.

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Glossary

Ordinal numbers

Numbers that relate to the empirical property of 'rank' in a sequence (for example, 'fifth place').

Nominal numbers

Strictly linguistic labels to identify objects (for example, 'Bus number 5').

Number estimation

(Also known as the analogue magnitude system and the analogue number system). A process of representing small and large set sizes that becomes systematically less precise with increasing numbers. Thus, number estimation obeys Weber's law.

Subitizing

(Also known as object file representation or object tracking system). The rapid tracking for up to approximately four items by assigning 'files' or 'pointers' to individual items.

Texture-like mechanism

A mechanism that allows the representation of very many and densely packed items and does not obey Weber's law.

Preadaptation

A trait that serves a different purpose from the one for which it evolved.

Labelled-line rate code

Relates to the discharge rates of neurons that belong to dedicated processing pathways and that convey information about specific stimulus parameters (it is a variation of a rate code).

Weber–Fechner law

Classic psychophysical law about the perception of magnitudes in relation to the physical intensity of a stimulus; it states that linear increments in sensation S are proportional to the logarithm of stimulus magnitude I (S = k * log(I)).

Rate code

Relates to the information encoded by the number of spikes during an interval.

Time code

Relates to the information encoded by temporal patterns of action potentials within an interval.

Global neuronal workspace

(GNW). A framework for the mechanism of consciousness. It consists of a network of distributed neurons with long-distance connectivity constituting a 'global workspace' that can potentially interconnect multiple specialized brain areas in a coordinated manner to give rise to a subjective feeling of conscious effort.

Homology

Refers to traits that share a common ancestry but may have different functions.

Homoplasy

Refers to traits with common functions but that are associated with different underlying structures and origins, and have evolved by convergent evolution.

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Nieder, A. The neuronal code for number. Nat Rev Neurosci 17, 366–382 (2016). https://doi.org/10.1038/nrn.2016.40

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