Partially overlapping regions in the parietal cortex are involved in both numerical and visuo-spatial tasks, ,, and cortical areas associated to saccadic movements are recruited during arithmetical performance, suggesting that numerical processing drives participants' shifts of attention along a representational space. Neuroimaging and neuropsychological studies also support the existence of this phenomenon. For instance, an Arabic digit prime in a detection task boosts attention orientation towards the left or right spatial hemifield depending on its magnitude. Numerous experimental paradigms have provided further evidence for a mapping whereby numbers are associated to different spatial positions, suggesting that the number-space mapping is not a mere association but it can impact our visuo-spatial processing. Therefore, this spatially oriented mapping is not fixed but easily adapts depending on the task demands, being currently considered a strategy to mentally organize ordinal information. This phenomenon depends on the relative, rather than absolute, magnitude since the same number can be associated to opposite lateralized responses depending on the numerical interval in which it is embedded (e.g., ‘5’ is associated to the right in the ‘1–5’ interval, and to the left in the ‘5–9’ interval ). In its mature form, the association of numerical order and spatial orientation gives rise to a Spatial Numerical Association of Response Codes (or SNARC) effect, which refers to the advantage for a spatial congruency between numbers and responses : Western adults respond faster to small numbers with the left hand, and to large numbers with the right hand (see also ). Within this view, numbers correspond to different spatial extensions and positions along an oriented horizontal axis. Nowadays, a growing amount of research on adults' numerical abilities is suggestive of the idea that numbers are represented as inherently variable distributions of activation over a spatially oriented number line. The idea that numbers are mentally represented in a stable spatial layout was first informally documented more than a century ago with the description of a series of ‘number forms’ or spatial configurations containing the series of numbers. One prominent spatial coding is that of numbers, which appears to be highly relevant for human innovation, as witnessed by the ubiquitous use of rulers, graphs, and other measurement tools. For instance, most cultures picture the future as if it were ahead and the past behind, in reference to one's own body. The coding of ordinal information in spatial terms is a widespread phenomenon. These findings are suggestive of an early predisposition in humans to link numerical order with a left-to-right spatial orientation, which precedes the acquisition of symbolic abilities, mathematics education, and the acquisition of reading and writing skills. The different pattern of results was congruent with the presence of a malleable, context-dependent baseline preference for increasing, left-to-right oriented, numerosities (Experiment 3). This pattern did not hold when infants were presented with the same ordinal numerical information displayed from right to left (Experiment 2). Infants habituated to left-to-right oriented increasing or decreasing numerical sequences showed an overall higher looking time to new left-to-right oriented increasing numerical sequences at test (Experiment 1). Here we show that preverbal infants aged 7 months, who lack symbolic knowledge and mathematics education, show a preference for increasing magnitude displayed in a left-to-right spatial orientation. The commonly accepted view is that this number-space association is a product of human invention, with accounts proposing that culture, symbolic knowledge, and mathematics education are at the roots of this phenomenon. While associations between number and space, in the form of a spatially oriented numerical representation, have been extensively reported in human adults, the origins of this phenomenon are still poorly understood.
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