Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology
Mechanisms of Lightness Perception
The amount of light coming from a surface (the luminance) depends on both the amount of light striking the surface (the illuminance) and the proportion that returns (the reflectance). A gray surface in sunlight may have a thousand times the luminance that it has in the shade, but it still looks gray. To achieve this feat of ``lightness constancy," the visual system must discount the illumination and estimate the reflectance. Many mechanisms have been proposed. By studying lightness illusions we can reveal that the lightness computation involves several levels of processing, ranging from low-level sensory mechanisms to higher level cognitive mechanisms. Many powerful effects can be attributed to mid-level mechanisms, involving contours, junctions, and grouping.