Grasping numbers in early development: a scoping review of manual action and magnitude perception in young infants

In adults, a reciprocal interaction exists between the processing of manual actions and different dimensions of magnitudes (number, size, time), as evidenced by cognitive and neurofunctional studies. Given that the ability to conceptualise numerical information in relation to spatial magnitude is functional at birth, we may hypothesise that the interrelation between the action perception system and the perception of magnitude develops throughout key stages of the early sensori-motor development. We present here a scoping review on how this interaction has been documented in early development. By systematically collecting articles reporting measures of both manual action (perception or production) and magnitude processing, we identified 21 studies conducted in human infants younger than three years of age. The majority of studies aimed at assessing infants' propensity to adapt their reaching or grasping actions to the size of objects (n = 15); others addressed the ability of infants to understand and predict other’s object-directed actions based on size information (n = 4). Finally, the most recent studies focused on infants’ ability to preferentially associate different hand apertures with congruent numerosities (n = 2), using habituation paradigms. The vast majority of these features were studied at the ages of 6 and 9 months. We present a state-of-the-art on the early development of the magnitude-action link, and suggest future research directions to help elucidate the ontogeny of the interaction between these two fundamental cognitive abilities.