The role of continuous quantities in non-symbolic number processing: theoretical implications and methodological challenges

In recent years, the interaction between numerosity and continuous quantities has grown from a methodological nuisance to a theoretically relevant issue to understand the mechanisms of numerosity perception and representation, as well as to clarify the connection between basic numerical abilities and formal mathematical skills. This thesis explores the interplay between numerosity and continuous magnitude processing by investigating the role of spatial and temporal features in numerosity judgments. After a first overview of the relevant literature, we describe two studies where we introduce an innovative stimulus space inspired by a recently developed method to assess the influence of temporal information during sequential numerosity judgments. Based on this method, we present a study demonstrating that individuals do not rely uniquely on non-numerical information in temporal numerosity discrimination, but they can be significantly biased by temporal cues both in the visual and auditory modalities. Moreover, a second study shows that temporal biases emerge even in absence of an explicit conflict between magnitudes at the response-selection level, such as in numerosity estimation tasks, in support of theoretical accounts that assume a partial representational overlap between magnitudes, specifically for the temporal and numerical domains. A third study focuses on the investigation of non-numerical interference effects in developmental dyscalculia, to understand the specificity of the numerical difficulties displayed in this learning disability compared to domain-general deficits. We compared children with dyscalculia and children with average mathematical skills but similar visuospatial memory abilities in an explicit numerosity comparison task and a spontaneous categorization task assessing the saliency of numerosity and total surface area of the elements. We found in children with DD evidence of a reduced precision in discrimination without increased reliance on continuous features, suggesting a deficit in numerosity representation not necessarily linked with reduced filtering abilities. Finally, the last work introduces a new tool for generating non-symbolic numerical stimuli with precise manipulation of spatial visual features of the images. Specifically, through a user-friendly interface, the presented program aims at helping researchers with different levels of expertise to generate experimental sets with flexible and personalized characteristics, based on the specific experimental questions. Taken together, the original empirical studies and methodological innovation described in this work provide a multi-faceted contribution to understand the mechanisms of numerosity perception and the refinement of basic numerical skills.