The study of simple visual discriminations reveals fundamental properties of learning and memory in the nervous system. Physical measures of the light source include energy emitted (flux) or reflected (reflectance) from the stimulus per unit area. Heinrich Klüver (1942) called it brightness if the total amount of light was measured over the whole stimulus source (including contours and edges), and density of luminous flux if measured over a unit area of the stimulus. He concluded that visually decorticated monkeys could solve a luminous-flux problem but not a brightness problem.

Using this definition, a brightness discrimination includes both light intensity and light contrast on edges that contribute to pattern perception. A flux discrimination, on the other hand, pertains to differences in light intensity per unit area. This intensity can be for the whole stimulus (total flux) or for parts...