Understanding less than nothing: neural distance effects for negative numbers.

Little work has examined how the mental number system accommodates counterintuitive quantities such as negative numbers, which seem to extend the left end of the mental number line and reverse the established relationship between digit magnitude and value; even less research has been conducted on the neural systems supporting negative number understanding. This study aimed to determine whether adult behavioral and neural responses to negative number paired comparisons were similar to those expected for positive numbers. Mixed pairs (with one positive and one negative number) were also included. Negative number responses demonstrated an increased typical distance effect relative to that for positives, with decreasing response times and intraparietal sulcus activity for comparisons farther apart than those closer together. Negative pairs also showed more activity than positive comparisons across distances in the occipital lobe, inferior and superior parietal lobule, and bilateral caudate and putamen. Mixed pair effect direction varied based on polarity sensitivity, or whether attention to the negative sign was needed for accurate responses, indicating differences in processing strategy. Adults thus draw on brain areas important in numeric processing when dealing with negatives, but also recruit further areas and strategies to support the unique features of negative numbers. The increased distance effect seen may reflect a less mature understanding of negatives. This work expands our knowledge of the flexibility of the mental number system and its ability to represent difficult quantities.