Basic mechanisms of numerical processing: cross-modal number comparisons and symbolic versus nonsymbolic numerosity in the intraparietal sulcus.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Cappelletti et al. The most basic form of numerical cogni-tion is numerosity, which is the ability to discern the quantity of objects in a set (e.g., are there more blue dots or more yellow dots?). Numerosity is considered part of an evolutionarily ingrained approximate number system, which supports estimation of quantities without the use of symbols (e.g., Arabic numerals) or language. The approximate number system has been observed in human babies, nonhuman primates, and even birds and fish (Knops and Willmes, 2014). Building on this common ground, scholars debate the extent to which a common mechanism supports numerosity and other quantitative judgments, such as estimates of time and distance (Bueti and Walsh, 2009), and the extent to which the approximate number system is separate from abstract mental arithmetic (e.g., math problems expressed as Arabic numerals). Regarding the second debate, on the one hand, there is evidence of considerable behavioral and neural overlap between approximate and abstract numerical processing (Knops and Willmes, 2014). On the other hand, multiple studies have demonstrated dissociations between symbolic and nonsymbolic numerical processing (Lyons and Beilock, 2013). In a recent article in The Journal of Neuroscience, Cappelletti et al. (2013) reported results that speak to both of the debates described above. First, the authors demonstrated that improvements in nu-merosity after training can transfer across domains, from basic quantity comparisons (trained task: are there more blue or yellow dots?) to judgments about time (untrained task: which time interval was longer?) and space (untrained task: which line is longer?). Further, they demonstrated that the skills underlying this transfer rely on the intraparietal sulcus (IPS)/posterior parietal cortex (PPC), an area that has been consistently found to support both symbolic and nonsymbolic number processing (Hauser et al., 2013). Training-based improvements transferred to untrained quantity discrimination tasks only when the training was accompanied by transcranial random noise stimulation (tRNS) applied to the IPS/PPC. Training accompanied by sham tRNS or by tRNS applied to motor regions did not transfer to untrained tasks, nor did tRNS without training improve task performance. Crucially , numerosity improvements resulting from the combination of training …