A symmetry-based mechanism for perceptual grouping in preverbal infants.

Newborn chicks can solve complex numerical discriminations through a purely perceptual mechanism based on the possibility of creating all equal-sized subsets. The present work investigates whether this is a prerogative of precocial birds or whether an analogous mechanism can be found in humans. We familiarized 8-month-olds with sets of even numerosities (i.e., decomposable in equal-sized subsets), and then we tested them for their spontaneous preference between two novel sets, one that cannot be divided into equal-sized subsets (i.e., a prime numerosity) and one decomposable in equal-sized subsets (i.e., a non-prime odd numerosity). Infants were tested with either the 7vs.9 comparison (i.e., the prime being smaller) or the 9vs.11 comparison (i.e., the prime being larger). Infants oriented their gaze more often toward the prime set of elements, irrespective of whether it was smaller or larger in the comparison, similar to the preference observed in baby chicks. Overall, our results suggest an early-emerging perceptual mechanism that can support complex numerical discriminations, that might be shared between distantly related and ecologically different species.