Understanding the image cues driving the switch from brightness to lightness responses in the Adelson checker-block illusion

Adelson's checker-block illusion is an engaging demonstration of the difference between lightness and brightness. The illusory nature of the stimulus derives from participants’ experience of the discrepancy between perceived lightness of two test patches (A, B) despite their physical luminance being identical. The identical nature of the test patches becomes apparent when cues informing the viewer of the scene's illumination and 3D structure are removed. Here we explore which cues drive the transition from ‘brightness’ pixel-based responses to ‘lightness’ material-based responses. Participants (n = 123) viewed versions of the stimulus with various components deleted (top, left and right-sides, shadows, outline-edges) under four between-subjects scenarios: with lighting direction varied (from left or right) and with the scene orientation varied (upside-down or correctly oriented). Participants indicated the perceived difference between A and B by responding on a Likert scale. Generalised linear mixed effects models were used to estimate the strength of each cue in driving the change of responses from brightness towards lightness. The lightness responses were strongest for upright images illuminated from the top-left, with panels adjacent to the test patches present. The stimuli, responses and model fits are shared as a dataset that can be tested against existing models of lightness perception.