Individual differences distinguish mechanisms underlying contextual size illusions

Abstract

AbstractPrevious research suggests the magnitudes of the Ebbinghaus, Delboeuf, Ponzo, and tilt illusion all depend on the cortical distance between the neural representations of target stimuli and the surrounding context. However, several psychophysical studies found no compelling association between these illusions, calling this hypothesis into question. Here we ask if these discrepant reports could arise from methodological differences between these studies. We ran a battery of visual size illusion and basic discrimination tasks with carefully matched geometric properties, using a classical forced choice design. Ebbinghaus and Delboeuf illusion magnitudes are strongly correlated, consistent with the idea that they reflect the same underlying mechanism. Ponzo illusion magnitude also correlates with these two illusions, despite to a lesser extent, indicative that both shared and independent factors govern these illusions. Interestingly, the classical arrowhead version of the Mueller-Lyer illusion does not correlate with any of the other illusions or the ability to discriminate line length. This suggests that an altogether separate process underlies this perceptual effect. We further demonstrate that presenting stimuli briefly with central fixation critically affects measurements of the Ebbinghaus illusion. Taken together, our findings therefore highlight the importance of experimental parameters when probing relationships between perceptual effects and their links to neural processing.