How Do Humans Process Magnitudes? An Examination of the Neural and Cognitive Underpinnings of Symbols, Quantities, and Size in Adults and Children

A striking way that humans differ from other species is our unique ability to represent and manipulate symbols. This ability to process numerical magnitudes symbolically (e.g., ‘three’, ‘3’) is widely thought to be supported by an ancient system that evolved to process nonsymbolic numerical magnitudes (i.e., quantities). In this thesis, I present four empirical studies to uncover whether symbolic representations are indeed supported by the system that evolved to process quantities, or if symbolic representations are sub-served by a similar but ultimately distinct system. In experiments 1 and 2, I investigate how the adult brain processes symbols and quantities using quantitative neuroimaging meta-analytic techniques (Experiment 1), and a tightly controlled fMRI paradigm (experiment 2). Results from the meta-analysis indicate that symbols and quantities are sub-served by both common and distinct brain regions along the frontal-parietal lobes. However, using a tightly controlled adaptation paradigm to isolate brain regions that underpin symbols and quantities reveal that regions supporting symbols are quite distinct from those supporting quantities, spatially and representationally. Thus, symbols might not be processed using the system that evolved to process quantities. In experiment 3, I examine whether the processing of symbols is similar to quantities under different attentional conditions. I discover that in addition to participants being more efficient at effortfully comparing symbols than quantities, embedding distracting symbols into stimuli during a quantity comparison task affected performance more than embedding quantities into a symbolic comparison task. This indicates that symbols and quantities are processed differently, under different attentional conditions, and therefore are likely sub-served by different representational systems. In experiment 4, I investigate the origin of the difference between how humans process symbols and quantities by exploring whether children’s symbolic number knowledge relates to their spontaneous attending to quantities. I find that children are more likely to attend to quantity if they know the number word that corresponds to the quantity, suggesting that learning symbols may influence how children conceptualize quantities. In summary, while there are some similarities in how humans process symbols and quantities, there are many important differences both behaviourally, and the neural level of organization. Consequently, these findings challenge the longstanding belief that the culturally acquired ability to conceptualize numbers symbolically is grounded in the ancient system that evolved to estimate quantities