Visual and proprioceptive feedback differently modulate the spatial representation of number and time in children

There has been compelling evidence favouring the idea that human adults similarly represent number and time along a horizontal Mental Number Line (MNL) and Mental Time Line (MTL), respectively. Yet, analogies drawn between the MNL and MTL have been challenged by recent studies suggesting that adults' representations of number and time arise from different spatial frames of reference: whereas the MNL relies on both hand-centred and object-centred coordinates, the MTL appears to be exclusively anchored on object-centred coordinates. To directly test this possibility, here we explored the extent to which visual and proprioceptive feedback affect children’s performance in a Number Comparison task (Experiment 1) and a Time Comparison task (Experiment 2), in which participants had to associate a lateralised key to numerical and temporal words, respectively. Five- and six-year-old children performed the task with either their hands uncrossed or crossed over the body midline (i.e., manipulation of proprioceptive feedback) and with either visual control over their hands allowed or precluded under blindfolds (i.e., manipulation of visual feedback). Results showed that children were facilitated in associating smaller/larger numbers with the left/right side of the external space, but only when hands were uncrossed and visual feedback was available. On the contrary, blindfolding and crossing their hands over the midline did not affect spatial-time mapping, as 6-year-old children showed facilitation in associating words referring to the past/future with the left/right side of the external space, irrespective of visual and proprioceptive feedback. This same effect was also present in 5-year-olds despite their difficulty in performing the Time Comparison task. Together, these findings show, for the first time, that - just like adults - young children (1) map temporal events onto space in a rightward direction as they do for numbers, and (2) anchor their spatial representation of time and numbers to different spatial frames of reference.