Bumps or dents? The light-from-above prior

Disk radius (px):

Grid columns:

Grid rows:

Light direction (°, 90 = from above):

Shading contrast:

Odd disk (inverted shading)

What to look for: all disks are physically identical except one, whose shading is inverted. Because the visual system assumes light comes from above, disks that are bright on top look like bumps and the odd one looks like a dent. Press Rotate 180° and the dent turns into a bump. Move the light to 0° or 180° (from the side) and the depth becomes ambiguous.

Reference: Ramachandran, V. S. (1988). Perception of shape from shading. Nature, 331(6152), 163–166. doi:10.1038/331163a0

Demo for Lecture: Basic Neuro-cognitive Psychology.
Created by Z. Shi, msense Lab.