From: The World In Your Head by Steven Lehar

Fig. 5.3 (A) A model of a perceived illumination source on the surface of the perceptual sphere propagates light signal throughout the volume of the perceptual sphere, illuminating all exposed opaque surfaces in that representation. (B) A reverse ray tracing from every opaque surface in the space projects back to a percept of the illuminant apparently responsible for the observed illumination of the scene.