Illusory evidence asymmetrically impacts visual performance and metacognition

<p>Our feelings of perceptual confidence appear largely driven by the magnitude of evidence supporting our choices, dubbed a metacognitive ‘positive evidence bias’. This bias is typically studied through manipulating objective (physical) evidence, but it is unclear whether it also occurs when perceptual evidence is illusory. Here, using a spatial two-alternative forced choice task plus confidence reports, we successfully used the ‘convex hull’ numerosity illusion to manipulate decisions by making dot patches appear more numerous when their spatial extent (“convex hull”) was increased. However, confidence effects interacted with the convex hull’s “congruence” with actual numerosity: in ‘congruent’ conditions (larger convex hull accompanying the more numerous patch), convex hull strongly biased confidence, but in ‘incongruent’ conditions (accompanying the less numerous patch), it exerted little effect. These findings demonstrate that illusory evidence differentially influences objective choices versus subjective confidence, and suggest observers may treat illusory evidence no differently from veridical evidence making choices and metacognitively evaluating them.</p>