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Our ability to represent temporal, spatial, and numerical information is critical for understanding the world around us. Given the prominence of quantitative representations in the natural world, numerous cognitive, neurobiological, and developmental models have been proposed as a means of describing how we track quantity. One prominent theory posits that time, space, and number are represented by a common magnitude system, or a common neural locus (i.e., ). Despite numerous similarities in representations of time, space, and number, an increasing body of literature reveals striking dissociations in how each quantity is processed, particularly later in development. These findings have led many researchers to consider the possibility that separate systems may be responsible for processing each quantity. This review will analyze evidence in favor of a common magnitude system, particularly in infancy, which will be tempered by counter evidence, the majority of which comes from experiments with children and adult participants. After reviewing the current data, we argue that although the common magnitude system may account for quantity representations in infancy, the data do not provide support for this system throughout the life span. We also identify future directions for the field and discuss the likelihood of the developmental divergence model of quantity representation, like that of Newcombe (Ecological Psychology, 2, 147-157, 2014), as a more plausible account of quantity development.Keywords Nonsymbolic quantity processing . Temporal precision . Spatial processing . Numerical acuity Our lives are inundated with temporal, spatial, and numerical information. Given the fundamental nature of quantity processing, it may not be surprising that infants, children, adults, and even nonhuman species rely on quantity information to navigate the world. The ability to track quantities is vital for basic learning processes such as foraging (e.g.

Prominent theories suggest that time and number are processed by a single neural locus or a common magnitude system (e.g., Meck and Church, 1983; Walsh, 2003). However, a growing body of literature has identified numerous inconsistencies between temporal and numerical processing, casting doubt on the presence of such a singular system. Findings of distinct temporal and numerical biases in the presence of emotional content (Baker et al., 2013; Young and Cordes, 2013) are particularly relevant to this debate. Specifically, emotional stimuli lead to temporal overestimation, yet identical stimuli result in numerical underestimation. In the current study, we tested adults’ temporal and numerical processing under cognitive load, a task that compromises attention. Under the premise of a common magnitude system, one would predict cognitive load to have an identical impact on temporal and numerical judgments. Inconsistent with the common magnitude account, results revealed baseline performance on the temporal and numerical task was not correlated and importantly, cognitive load resulted in distinct and opposing quantity biases: numerical underestimation and marginal temporal overestimation. Together, our data call into question the common magnitude account, while also providing support for the role of attentional processes involved in numerical underestimation.

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Throughout the life span, we are capable of representing quantities in the absence of language, or nonsymbolically. Additionally, over the course of development, we learn many symbolic measurement systems for representing quantities such as time and number. Despite substantial evidence of a relation between the acquisition of symbolic and nonsymbolic numerical acuity (see Halberda, Mazzocco, & Feigenson, 2008), no work has explored whether a similar relation exists between understanding temporal units of measurement and timing precision. That is, does a child's understanding of words like "second," "minute," and "hour" have any relation to their ability to tell which of two events lasted longer? Six-and 7-year-old children (n ϭ 102, M age ϭ 83.44 months, 52 females), who are in the process of learning temporal units of measurement, completed a temporal discrimination task (assessing nonsymbolic temporal acuity) and a symbolic timing assessment. Results revealed a positive correlation between children's nonsymbolic temporal acuity and their understanding of temporal units of measurement. Importantly, this correlation held when controlling for age and numerical acuity, suggesting a unique relation between children's temporal acuity and their understanding of temporal units of measurement. This study is the first to show a relation between symbolic and nonsymbolic representations of time.