The role of numerical and non-numerical cues in non-symbolic number processing: evidence from the line bisection task

Abstract In line bisection tasks, adults and children bisect towards the numerically larger of two non-symbolic numerosities (de Hevia & Spelke, 2009). However, it is not clear whether this effect is driven by number itself or rather by visual cues such as subtended area (Gebuis & Gever, 2011). Furthermore, this effect has only been demonstrated with flanking displays of two and nine items. Here, we report three studies that examined whether this "spatial bias" effect occurs across a range of absolute and ratio numerosity differences; in particular, we examined whether the bias would occur when both flankers were outside the subitizing range. Additionally, we manipulated the subtended area of the stimulus and the aggregate surface area to assess the influence of visual cues. We found that the spatial bias effect occurred for a range of flanking numerosities, and for ratios of 3:5 and 5:6 when subtended area was not controlled (Experiment 1). However, when subtended area and aggregate surface area were held constant, the biasing effect was reversed such that participants bisected towards the flanker with fewer items (Experiment 2). Moreover, when flankers were identical, participants bisected towards the flanker with larger subtended area or larger aggregate surface area (Experiments 2 and 3). On the basis of these studies, we conclude that the spatial bias effect for non-symbolic numerosities is primarily driven by visual cues.