Distinct Neural Representations across Discrete and Continuous Non-symbolic Ratios

Relative magnitudes, such as ratios and proportions, are crucial to everyday life. According to the ratio processing system (RPS) theory, ratios across different formats share a common neural code. Prior neuroimaging studies have shown that ratios across different visual formats are processed by overlapping brain areas and produce similar distance effects. However, this evidence supporting the RPS theory does not necessarily imply common representations. The aim of this study was to probe the key prediction of the RPS theory by investigating the neural representation of individual ratios depicted in discrete and continuous magnitudes using multivariate fMRI analyses. Twenty participants completed a delayed ratio comparison task on discrete (sets of dots) and continuous (line lengths) magnitudes while in the MRI scanner. Using representational similarity analysis (RSA), we examined the similarity between ratio representations across high-level variations, such as format (length and numerosity), and low-level variations, such as variations in line orientation and dot size, throughout the brain. Results showed little support for a shared ratio representation across discrete and continuous magnitudes. Instead, ratios in numerosity and length seemed to be encoded in qualitatively distinct ways. Altogether, results suggest that ratios are not represented in a format-independent way in the brain, thus challenging the RPS theory.