Behavioral and near-infrared spectroscopy study of the effects of distance and choice in a number comparison task.

Extensive behavioral and neurophysiological numerical comparison studies have shown that response times are longer and parietal activities are stronger when the numerical distance between two digits is smaller (the distance effect). However, only a few behavioral studies have considered the effect of the choice of larger or smaller numerals in numerical comparisons. Using near-infrared spectroscopy (NIRS), we investigated the neural basis of choosing larger/smaller numerals in number comparison tasks in which subjects were required to choose a larger or smaller digit. Our results showed that choosing a smaller digit induced significantly longer response times (the choice effect) and stronger parietal activities. We also obtained significantly longer response times as the distance effect in accordance with previous works. However, NIRS data did not show any significant difference corresponding to distance effect. Our results and previous studies suggest that the parietal cortex is involved not only in measuring numerical quantities, but also in choosing a numerically larger/smaller quantity among the categories of choice. Potentials and limitations of NIRS were discussed.