Figure 3. Complex Influences of Spatial Context on Brightness Perception.

(A) Simultaneous contrast enhancement. The intensities of the two central squares are identical, but the central square on the left appears darker. This brightness illusion, which has been known for centuries, can be accounted for by the differences in spatial context (light on left and dark on right).

(B) The “corrugated plaid” illusion devised by vision scientist Edward Adelson shows the pronounced dependence of brightness perception on image cues for depth, form, and shading. Patches b1 and b2 are identical in intensity, but b2 appears brighter.

(C) By changing cues for depth, form, and shading, yet leaving the local luminance configuration intact, the brightness illusion present in (B) is greatly reduced. Patches c1 and c2 are identical in intensity and they appear approximately so.

The illusion can be accounted for by the use of contextual cues to identify the visual scene that produced the image. One important visual scene property is the reflectance of surfaces, since that property enables object recognition. The brightness values perceived in this illusion correspond to probabilistic inferences about surface reflectance that are driven by contextual cues that suggest a specific visual scene interpretation.

Source: Edward H. Adelson, “Perceptual Organization and the Judgment of Brightness,” Science 262 (1993): 2042–2044; and Thomas D. Albright, “Why Do Things Look as They Do?” Trends in Neuroscience 17 (1994): 175–177.