Grace et al. (2018) showed that observers quickly learned to respond based on differences and ratios of stimulus magnitudes in a nonsymbolic task without explicit instruction. But were observers really responding based on differences and ratios, suggesting the perceptual system effectively computes these operations? As a stronger test of relational versus exemplar responding, we studied a version of this task with noisy feedback. In three experiments, observers viewed pairs of stimuli that varied in area, brightness, or length, and responded by clicking on a horizontal bar. For different groups, feedback was based on the ratio or difference of stimulus magnitudes, linearly scaled to the response bar, plus a Gaussian noise component that was constant for repetitions of each pair. Multiple regressions showed that for every observer across experiments, coefficients for the added noise were negative when entered in a model with trained values, and were statistically significant in 76% of individual cases. These results indicate that observers responded based on the differences or ratios of stimulus magnitudes, and effectively ignored or suppressed the added noise. Regressions also showed that differences and ratios described the data well, with the untrained relation accounting for significant variance in approximately half of individual cases, similar to Grace et al.’s (2018) results. Overall, these results suggest that the perceptual system automatically computes two operations, corresponding to differences and ratios, when comparing stimulus magnitudes.

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