Colour Opponent Neurones in the Human Visual System

IF two gratings with the same sinusoidal luminance profile, one vertically orientated and one horizontally orientated, are imaged by means of two projectors on a white screen, the appearance continuously changes. First one grating is seen clearly, then the other; in other words, the gratings are seen to alternate or rival. There are periods when both gratings are seen together, but there are never periods when both disappear1.