title: Contextual illusions reveal the limit of unconscious visual processing.
abstract: The perception of even the most elementary features of the visual environment depends strongly on their spatial context. In the study reported here, we asked at what level of abstraction such effects require conscious processing of the context. We compared two visual illusions that alter subjective judgments of brightness: the simultaneous brightness contrast illusion, in which two circles of identical physical brightness appear different because of different surround luminance, and the Kanizsa triangle illusion, which occurs when the visual system extrapolates a surface without actual physical stimulation. We used a novel interocular masking technique that allowed us to selectively render only the context invisible. Simultaneous brightness contrast persisted even when the surround was masked from awareness. In contrast, participants did not experience illusory contours when the inducing context was masked. Our findings show that invisible context is resolvable by low-level processes involved in surface-brightness perception, but not by high-level processes that assign surface borders through perceptual completion.
authors.affiliation: Department of Neuroscience, University College London, Institute of Cognitive Neuroscience, London WC1N 3BG, United Kingdom.
authors.forename: Julia J
authors.initials: JJ
authors.lastname: Harris
authors.forename: D Samuel
authors.initials: DS
authors.lastname: Schwarzkopf
authors.forename: Chen
authors.initials: C
authors.lastname: Song
authors.forename: Bahador
authors.initials: B
authors.lastname: Bahrami
authors.forename: Geraint
authors.initials: G
authors.lastname: Rees
mesh_terms: D000328:Adult; D001364:Awareness; D003118:Color Perception; D015350:Contrast Sensitivity; D004192:Discrimination, Psychological; D005260:Female; D005364:Field Dependence-Independence; D006801:Humans; D008297:Male; D009903:Optical Illusions; D009949:Orientation; D010364:Pattern Recognition, Visual; D010470:Perceptual Masking; D011601:Psychophysics; D015349:Vision, Monocular; D055815:Young Adult