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

Area of Science:

• Visual perception
• Neuroscience
• Computational vision

Background:

• Shaded 2D images can evoke a sense of depth through shading cues.
• The perception of light source direction influences perceived shape (e.g., lighter top = convex, darker top = concave).
• Shape from Shading is the process of inferring 3D shape from 2D shaded images.

Purpose of the Study:

• To investigate if pupil size responds to the depth illusion generated by Shape from Shading.
• To determine the conditions under which pupillary responses reflect perceived depth.

Main Methods:

• Three experiments using equiluminant 2D stimuli manipulated shading to create convex or concave depth illusions.
• Pupil size was measured during tasks involving shape judgment or passive viewing with/without awareness of the illusion.
• A behavioral control experiment confirmed the "darker is deeper" heuristic.

Main Results:

• Pupil size significantly dilated more for perceived concave shapes compared to convex shapes.
• This pupillary response to perceived depth occurred when participants judged the shape or viewed it passively with awareness.
• No pupillary difference was observed during passive viewing without awareness, suggesting attention is required.

Conclusions:

• Pupil size is modulated by the perceived depth in Shape from Shading, reflecting subjective experience rather than luminance.
• The "darker is deeper" heuristic links perceived depth to a subjective perception of darkness.
• Pupillary responses can serve as a physiological indicator of subjective visual perception in depth illusions.