Proportional reasoning is a ubiquitous part of the humanexperience. We engage in proportional reasoning to meetboth informal and specialized goals across a range ofdomains, such as medicine (e.g., disease rates, drug dosages),finance and commerce (e.g., interest rates, discounts),cooking and baking (e.g., scaling ingredient amounts), andmany others. Given this variation in usage, it may not besurprising that proportional reasoning does not have asingular definition or interpretation, but instead is a complextopic with many interconnected concepts. The central goal ofthis symposium is to shed light on this complexity bydiscussing diverse perspectives of proportional reasoning.

A growing number of studies has revealed that humans and nonhuman animals have the ability to process magnitudes ofnonsymbolic ratios. Lewis, Mathews & Hubbard (2015) hypothesized that this ability may depend on a ratio processingsystem (RPS) that may help acquire symbolic fractions knowledge. The present study investigated ratio processing in2nd and 5th graders using functional MRI. In the scanner, children decided which of two ratios was numerically larger.The stimuli were constructed as pairs of nonsymbolic line ratios, symbolic fractions, and mixed notations. Both 2ndand 5th graders showed the distance effect the behavioral performance and the neural activation were modulated by thenumerical distance between two ratios. Notably, 5th graders showed greater neural distance effect and more overlapsin activation across notations when compared to 2nd graders. These results suggest that educational experience mightpromote recruitment of the RPS for processing symbolic fractions as well.