Can developmental disorders provide evidence for two systems of number computation in humans?

Typically developing 6-month-olds can discriminate between small and large numerosities. Discrimination between small numbers is only possible in young infants when variables continuous with number (e.g. area or circumference) are also present in the display. However, they succeed on large number discrimination even when such variables are controlled for. This points to the possible existence of two different number systems in infancy. Williams syndrome (WS), is a genetic disorder, for which numerical deficits have been identified in older children and adults. The current study examined whether such deficits originate in infancy. We tested small and large number discrimination in infants and toddlers with WS. While they succeeded in discriminating between 2 and 3 elements when numerosity was confounded with total area, the same infants failed to discriminate between 8 and 16 elements, when continuous variables were controlled for. These findings suggest that a system for tracking certain features of small numbers of objects may be functional in WS, while large number discrimination reveals deficits already in infancy. We conclude that individual differences in infancy in large number discrimination is probably a better predictor of subsequent development of numerical cognition than small number discrimination.