Top-down effect on pupillary response: Evidence from shape from shading

• Main Authors: Henik, A. (Author), Hershman, R. (Author), Sapir, A. (Author)
• Format: Article
• Language: English
• Published: Elsevier B.V. 2021
• Subjects: adult; Article; attention; binocular convergence; clinical assessment; comparative study; controlled study; darkness; decision making; depth perception; Depth perception; Depth Perception; event related potential; female; Form Perception; gaze; heuristics; human; Humans; illumination; Illusion; illusion of depth; image processing; Judgment; luminance; major clinical study; male; Optical Illusions; orientation; Orientation; outcome assessment; pattern recognition; priority journal; pupil; Pupil; pupil diameter; Pupil light reflex; pupil reflex; pupillometry; Pupillometry; shade; Shape from shading; temporal analysis; visual illusion; visual stimulation
• Online Access: View Fulltext in Publisher

 Shaded 2D images often create an illusion of depth, due to the shading information and assumptions regarding the location of the light source. Specifically, 2D images that are lighter on top usually appear convex while images that are darker on top, usually appear concave, reflecting the assumption that light is coming from above. The process of recovering the 3D shape of a shaded image is called Shape from Shading. Here we examined whether the pupil responds to the illusion of depth in a shape from shading task. In three experiments we show that pupil size is affected by the percept of depth, so that it dilates more when participants perceive the stimulus as concave, compared to when they perceive it as convex. This only happens if participants make a judgment regarding the shape of the stimulus or when they view it passively but are aware of the different shapes. No differences in pupil size were found with passive viewing if participants were not aware of the illusion, suggesting that some aspects of shape from shading require attention. All stimuli were equiluminant, and the percept of depth was created by manipulating the orientation of the shading, so that changes in pupil size could not be accounted by changes in the amount of light in the image. We posit, and confirmed it in a behavioral control experiment, that the perception of depth is translated to a subjective perception of darkness, due to the “darker is deeper” heuristic and conclude that the pupillary physiological response reflects the subjective perception of light. © 2021 Elsevier B.V.