A popular metaphor for the representation of numbers in the brain is the 'mental number line', in which numbers are represented in a continuous, quantity-based analogical format. Here we show that patients with hemispatial neglect misplace the midpoint of a numerical interval when asked to bisect it (for example, stating that five is halfway between two and six), with an error pattern that closely resembles the bisection of physical lines. This new form of representational neglect constitutes strong evidence that the mental number line is more than simply a metaphor, and that its spatial nature renders it functionally isomorphic to physical lines.

Humans represent numbers along a mental number line (MNL), where smaller values are located on the left and larger on the right. The origin of the MNL and its connections with cultural experience are unclear: Pre-verbal infants and nonhuman species master a variety of numerical abilities, supporting the existence of evolutionary ancient precursor systems. In our experiments, 3-day-old domestic chicks, once familiarized with a target number (5), spontaneously associated a smaller number (2) with the left space and a larger number (8) with the right space. The same number (8), though, was associated with the left space when the target number was 20. Similarly to humans, chicks associate smaller numbers with the left space and larger numbers with the right space.

The authors suggest that VRMT may improve memory function in elderly adults by enhancing focused attention.

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