An important objective of visual neurophysiology is to understand visual perception in terms of neuronal connections. In the case of color vision, some of the chief aspects of perception to be explained are color mixing and color constancy. Color mixing is now well understood in physiological terms; progress has been made in the area of color constancy, but there our understanding is far from complete.Anatomically our visual system consists, broadly of a series of neuronal stages; the first three of these are housed in the retina, beginning with the rods and cones and ending with retinal ganglion cells, whose axons make up the optic nerve. Each ganglion cell receives input, via bipolar cells, from a small compact aggregate of rods and cones, which constitute the receptive field of that cell. In 1950, Stephen Kuffler, working with cats, found that each retinal ganglion cell receptive field is a few mm in diameter and is subdivided into a small excitatory (“on”) center and larger inhibitory (“off”) surround or, for about half the cells, an inhibitory center and an excitatory surround. This organization gives an immediate and simple explanation for the fact that object's whiteness or blackness depends on spatial differences in brightness, and not on the absolute levels of light coming from the object.