Abstract
Measuring perceptual bias is method-dependent. When judging perceived stimulus size, we previously found a traditional two alternative forced choice (2AFC) task produced bias estimates that were significantly greater than a novel perceptual matching (PM) task, a variant of a comparison-of-comparisons (CoC) task (e.g. Finlayson, Papageorgiou, & Schwarzkopf, 2017). CoC tasks are thought to better control for decisional criterion issues than 2AFC because it constrains stimuli to differ only in the stimulus parameter of interest (Morgan, Melmoth, & Solomon, 2013; Jogan & Stocker, 2014; Patten & Clifford, 2015). Using the Ebbinghaus illusion, here we aimed to test if decisional bias could be driving the difference in bias estimation between 2AFC and PM. In Experiment 1 (n=7), we collected confidence reports alongside each 2AFC judgment and estimated perceptual bias as the point of maximum uncertainty, because metacognitive bias estimates have also been suggested to control for criterion shift (Gallagher, Suddendorf, & Arnold, 2019). We found no difference between 2AFC and metacognitive estimates, while PM estimates were significantly lower than both. In Experiment 2 (n=22), we tested if PM or 2AFC tasks were better at capturing true perceptual bias. We varied test target sizes using idiosyncratic bias estimates from PM and 2AFC and found that 2AFC estimates were superior in nulling the Ebbinghaus illusion. Subsequent simulations revealed that CoC tasks skew bias estimates because 2AFC requires judgment against the position of the true bias, while CoC involves judging the distances from the true bias. In Experiment 3 (n=32), we showed that this dependency can be corrected via adaptive stimulus sampling. On average, adaptive PM produced comparable bias estimates to 2AFC and both methods were equally good at nulling the Ebbinghaus illusion.