Number–Space Interactions in the Human Parietal Cortex: Enlightening the SNARC Effect with Functional Near-Infrared Spectroscopy

Abstract

Interactions between numbers and space have become a major issue in cognitive neuroscience, because they suggest that numerical representations might be deeply rooted in cortical networks that also subserve spatial cognition. The spatial-numerical association of response codes (SNARC) is the most robust and widely replicated demonstration of the link between numbers and space: in magnitude comparison or parity judgments, participants' reaction times to small numbers are faster with left than right effectors, whereas the converse is found for large numbers. However, despite the massive body of research on number-space interactions, the nature of the SNARC effect remains controversial and no study to date has identified its hemodynamic correlates. Using functional near-infrared spectroscopy, we found a hemodynamic signature of the SNARC effect in the bilateral intraparietal sulcus, a core region for numerical magnitude representation, and left angular gyrus (ANG), a region implicated in verbal number processing. Activation of intraparietal sulcus was also modulated by numerical distance. Our findings point to number semantics as cognitive locus of number-space interactions, thereby revealing the intrinsic spatial nature of numerical magnitude representation. Moreover, the involvement of left ANG is consistent with the mediating role of verbal/cultural factors in shaping interactions between numbers and space.

Different Brains Process Numbers Differently: Structural Bases of Individual Differences in Spatial and Nonspatial Number Representations

“…This study is one of the first to demonstrate a direct brain correlate of the well-known association between numbers and spatial responses reflected by the SNARC effect. The results of a recent functional near-infrared spectroscopy study show a functional activation of bilateral intraparietal sulcus and left angular gyrus when participants are engaged in a spatial number response interference task (Cutini, Scarpa, Scatturin, Dellʼacqua, & Zorzi, 2012). This study now extends these findings and shows that the existing individual preferences in the association between numbers and space (e.g., Fischer, 2006) relate to structural variance in right precuneus.…”

Counteracting Implicit Conflicts by Electrical Inhibition of the Prefrontal Cortex

“…Regarding its cognitive foundations, it has been suggested that space-number associations are driven by flexible ordinality representations of the current number set in serial order working memory (WM; van Dijck, Abrahamse, Acar, Ketels, & Fias, 2014; van Dijck & Fias, 2011), possibly emphasized by cultural learning (Patro, Nuerk, Cress, & Haman, 2014;Shaki, Fischer, & Petrusic, 2009). The established neurophysiological view on the representation of number magnitude and its association with space highlights specifically the contribution of parietal areas (Cutini, Scarpa, Scatturin, Dell'Acqua, & Zorzi, 2014;Krause, Lindemann, Toni, & Bekkering, 2014;Cohen Kadosh & Walsh, 2009;Cohen Kadosh, Cohen Kadosh, Kaas, Henik, & Goebel, 2007;Dehaene, Piazza, Pinel, & Cohen, 2003;Fias, Lammertyn, Reynvoet, Dupont, & Orban, 2003). Only recently, however, studies indicated that prefrontal contributions to advanced numerical cognition might be essential (Klein et al, 2014;Arsalidou & Taylor, 2011), especially in terms of frontoparietal circuits (Nieder, 2016;Rusconi, Dervinis, Verbruggen, & Chambers, 2013;Göbel, Johansen-Berg, Behrens, & Rushworth, 2004).…”

Larger, smaller, odd or even? Task-specific effects of optokinetic stimulation on the mental number space

“…Numerous studies have shown effects of number processing on spatially encoded responses; for example, when participants are asked to classify numbers, they are typically faster in responding to smaller numbers with left-sided responses, whereas they are faster in responding to larger numbers with right-sided responses (the Spatial-Numerical Association of Response Codes (SNARC) effect; Dehaene, Bossini, & Giraux, 1993; see Wood, Willmes, Nuerk, & Fischer, 2008, for a review). This relation between numbers and space is independent of the effector used (e.g., hands: Dehaene et al, 1993; fingers of the same hand: feet: Schwarz & Müller, 2006;saccades: Fischer, Warlop, Hill, & Fias, 2004;Schwarz & Keus, 2004) and it has a key neural correlate in the human posterior parietal cortex (Cutini, Scarpa, Scatturin, Dell'Acqua, & Zorzi, 2014). The classic explanation of the SNARC effect is based on the notion of an analogue, left-to-right oriented mental number line (MNL; Restle, 1970), with relatively small numbers on the left and relatively large numbers on the right (Dehaene et al, 1993).…”