How do our abilities to process number and other continuous quantities such as time and space relate to each other? Recent evidence suggests that these abilities share common magnitude processing and neural resources, although other findings also highlight the role of dimension-specific processes. To further characterize the relation between number, time, and space, we first examined them in a population with a developmental numerical dysfunction (developmental dyscalculia) and then assessed the extent to which these abilities correlated both behaviorally and anatomically in numerically normal participants. We found that (1) participants with dyscalculia showed preserved continuous quantity processing and (2) in numerically normal adults, numerical and continuous quantity abilities were at least partially dissociated both behaviorally and anatomically. Specifically, gray matter volume correlated with both measures of numerical and continuous quantity processing in the right TPJ; in contrast, individual differences in number proficiency were associated with gray matter volume in number-specific cortical regions in the right parietal lobe. Together, our new converging evidence of selective numerical impairment and of number-specific brain areas at least partially distinct from common magnitude areas suggests that the human brain is equipped with different ways of quantifying the outside world.

Several recent behavioral studies have shown that the enumeration of a small number of items (a process termed subitizing ) depends on the availability of attentional resources and is not a preattentive process as previously thought. Here we studied the neural correlates of visual enumeration under different attentional loads in a dual-task paradigm using fMRI. Relatively intact subitizing under low attentional load compared to impaired subitizing under high attentional load was associated with an increase in BOLD signal in the right temporo-parietal junction (rTPJ). Crucially, attentionally modulated response in the rTPJ was specific to small set sizes (up to 3 items) and did not occur at larger set sizes (5–7 items). This result has two implications: (1) Subitizing involves part of the fronto-parietal network for stimulus-driven attention providing neural evidence against preattentive subitizing. (2) Activity in rTPJ is set-size modulated. Together with similar evidence from studies probing visual short-term memory, this result suggests that rTPJ modulation might reflect the brain's ability to attentively handle small set sizes. Thus, the rTPJ may play an important role for the emergence of a capacity limit in both enumeration and visual short-term memory.

Mental images of number lines, Galton's “number forms” (NF), are a useful way of investigating the relation between number and space. Here we report the first neuroimaging study of number-form synesthesia, investigating 10 synesthetes with NFs going from left to right compared with matched controls. Neuroimaging with functional magnetic resonance imaging revealed no difference in brain activation during a task focused on number magnitude but, in a comparable task on number order, synesthetes showed additional activations in the left and right posterior intraparietal sulci, suggesting that NFs are essentially ordinal in nature. Our results suggest that there are separate but partially overlapping neural circuits for the processing of ordinal and cardinal numbers, irrespective of the presence of an NF, but a core region in the anterior intraparietal sulcus representing (cardinal) number meaning appears to be activated autonomously, irrespective of task. This article provides an important extension beyond previous studies that have focused on word-color or grapheme-color synesthesia.