A left-to-right bias in spatial numerical associations with dots and symbols

Number and space are intertwined in human and non-human cognition. A substantial body of research has shown that numerical magnitudes are mentally represented along a spatial continuum, akin to a "mental number line". Some suggested that its directionality is determined by culture and context. Nevertheless, evidence from preverbal infants and non-human animals indicates a consistent left-to-right directional mapping of numerosities, suggesting a biologically predisposed Spatial-Numerical Association (SNA) that may precede cultural factors. A recent study has shown that an implicit association between "left" and "small" emerges not only in literate adults, but also in unschooled indigenous populations and preschool Western children. This finding suggests that SNAs may originate from universal innate mechanisms rather than being solely a by-product of cultural learning. However, while the study reported a strong association between "left" and "decreasing" numerosity, there was only a very weak association between "right" and "increasing" numerosity. This asymmetry was not predicted and needs further investigations to be understood. Here we further investigated the number/space association in implicit tasks in educated Western adults by using more variable and better controlled stimuli compared to the ones used in the previous study, and also manipulating stimulus format, using both dot patterns and symbolic numbers. Fifty-one adult participants performed a numerical comparison task within a Go-No-Go paradigm on subsequent pairs of visual stimuli (with ratios spanning from 0.75 to 0.94) that could appear on the left or on the right of a fixation point and completed two different tasks: "press when more" and "press when less". Results revealed distinct response patterns depending on the symbolic/non-symbolic nature of the stimuli. When non-symbolic stimuli were used, a consistent association between small numerosities and the left side and large numerosities and the right side was observed. When symbolic stimuli were used, only an association between large numerosities and the right side was observed. These findings support the hypothesis that SNAs may reflect a biological predisposition associated with brain asymmetry, and that task demands may interact with the underlying hemispheric specializations.