In humans, the representation of numbers and space are deeply intertwined. We create mental representations of numbers on a line from left to right, with the smallest numbers on the left and the largest numbers on the right. Italian researchers have shown that hours after birth, newborns already have a rudimentary sense of numbers. Their study raises the following question: is the mental spatial representation of numbers innate?

In the preamble to their study, Rosa Rugani and her collegues (Universities of Padua and Trento, Italy) present recent research that calls into question the belief that humans represent numbers on a mental number line (MNL) based on their reading/writing habits (for Western cultures). Indeed, the role of language may not be so central in the determination of the left-right direction in numerical-spatial association.

Moreover, the SNARC effect (Spatial Numerical Association of Response Code, a systematic association between numbers and space), demonstrated for the first time by Stanislas Dehaene and colleagues, strengthens the association in humans of small numbers to the left and larger numbers to the right. Indeed, in numerical equality judgements (comparing two sets of numbers and determining whether or not they are equal), the researchers discovered that the subjects always responded faster when presented with smaller numbers if they were clicking with their left hand, but were faster with their right hand for the larger numbers.

In order to determine if the MNL is innate or determined by language and culture, R. Rugani’s team studied the direction of numbers in newborns between 12 and 117 hours after birth (average age = 55 hours). In one of their experiments, they habituated 24 infants to the number 12 by presenting them with a stimuli consisting of 12 black squares positioned spatially in different ways. The stimuli were displayed very quickly, appearing on the screen for only 500 milliseconds. After this habituation phase, a sequence of two different trials was administered. Half of the newborns saw a small number (4) in the first trial and a big number (36) in the second. The protocol was reversed for the other infants. In each trial, the same stimulus was presented simultaneously on the right and left sides of the computer screen. A camera located above the screen recorded the babies' eye movements as they sat on the experimenter's knees. What did the results reveal?

When the babies were shown 4 black squares on the left and 4 squares on the right, they tended to gaze longer at the left-hand stimuli, whereas when they were shown 36 black squares to the left and 36 black squares to the right, they fixated longer on the left stimulus. Thus, once habituated to the number (12 in this instance), the babies spontaneously associated smaller numbers with the left side of the screen and larger numbers with the right side of the screen. In another experiment, the researchers showed that the MNL was relative. When the infants saw the same set of 12 black squares on both sides of the screen, the direction of their gaze depended on the preceding stimuli. If the baby had seen 36 black squares on both sides of the previous screen, his/her gaze focused on the 12 black squares on the left side of the screen. If the baby had seen 4 black squares on both sides of the previous screen, his/her gaze focused on the 12 black squares on the right side of the screen.

According to the study's authors: “These results constitute strong evidence that in our species Spatial Numerical Association originates from pre-linguistic and biological precursors in the brain.”