Two steps to space for numbers

Citation: Fischer MH and Shaki S (2015) Two steps to space for numbers. The study of spatial-numerical associations (SNAs) is an active research project that was triggered by a landmark publication reporting several simple reaction time experiments: Adults classified visually presented numbers according to their parity by using left and right response keys (Dehaene et al., 1993). The main finding was that small numbers, such as 1 or 2, were classified faster on the left side and larger numbers, such as 8 or 9, were classified faster on the right side. This specific instance of a SNA has been replicated and extended in numerous studies (recent review by Fischer and Shaki, 2014). The original interpretation of the effect assumed a " spill-over " from reading habits into the number domain but subsequent work has pushed back the time line to preschoolers, infants, and even neonates (for recent review, see Patro et al., 2014). Our own work (e.g., Shaki et al., 2009; Fischer and Shaki, 2015) confirmed that reading habits contribute to the direction and strength of SNAs but has also indicated that they are not the only and not even the strongest determinant (e.g., Fischer et al., 2010). In the following paragraphs we propose a processing principle for SNAs and describe two successive steps by which the mapping of numbers onto space might occur. Our proposed processing principle is that spatial mapping is an integral part of semantic number processing. This is evident from the ubiquity of SNAs: They have been reported with various stimulus formats, in many different tasks, and while studying a wide range of responses (for recent review, see Fischer and Shaki, 2014). SNAs modulate the cortical region underlying semantic number processing (i.e., bilateral hIPS; Cutini et al., 2012). Moreover, the association between numbers and space is bi-directional: numerical magnitude can serve as a spatial cue and vice versa (Stoianov et al., 2008; Shaki and Fischer, 2014a). Most studies of SNAs have used centrally presented numbers in combination with spatial responses, which may have encouraged participants to use spatial number mapping strategies (Fischer, 2006). However, today it is clear that the very appearance of numerical stimuli is enough for SNAs to appear, even when removing, in healthy adults (cf. Zorzi et al., 2002), spatial features from both stimuli and responses (Fischer and Shaki, 2015; Ranzini et al., 2015). Evidence for such a purely conceptual link between numbers …