Grabbing subitizing with both hands: bimanual number processing.

Visual judgment of small numerosities (<4) is generally assumed to be done through subitizing, which is a faster process than counting. Subitizing has also been shown to occur in haptic judgment of the number of spheres in the hand. Furthermore, interactions have been shown to exist between visually perceived numbers and hand motor action. In this study, we compare enumeration of a set of spheres presented to one hand (unimanual) and enumeration of the same total number of spheres presented divided over the two hands (bimanual). Our results show that, like in vision, a combination of subitizing and counting is used to process numbers in active touch. This shows that numbers are processed in a modality-independent way. This suggests that there are not only interactions between perception of numbers and hand motor action, but rather that number representation is modality-independent.