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Abstract:
Abstract: When subjects compare two numbers, response times
and error rates decrease systematically as a function of numerical
distance. This result is called the semantic distance effect. We
used single-event functional magnetic resonance imaging (fMRI) to
test the hypothesis that the distance effect originates in the
parietal lobes, which contain a representation of numerical
quantity in the form of a spatially extended number line. Nine
subjects compared 2-digit numbers between 30 and 99 to a fixed
reference of 65. Three levels of numerical distance were
contrasted. The numbers could be presented in two notations, arabic
numerals or spelled-out words. Behaviorally, reaction times showed
additive effects of notation and distance, confirming that the
distance effect originates from a notation-independent
representation of quantity. fMRI revealed a left posterior
parietal/precuneus activation whose activation decreased
monotonically with increasing numerical distance, and was
unaffected by number notation. A more extended bilateral
parieto-frontal network was non-monotonically affected by numerical
distance, and with apparent differences as a function of notation.
Overall, the results suggest that the posterior parietal region,
which has been previously implicated in various numerical and
non-numerical spatial tasks such as mental rotation and navigation,
may be one of the cerebral substrates of the semantic distance
effect in number comparison.
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