Rods Cancel Cones in Flicker

Abstract

THE retina of man is equipped with two separate receptor systems. Cones, operating best in relatively strong illumination, are the basis of daylight (photopic) vision. In dim illumination (scotopic vision) we rely on rod receptors alone, and the limitations of the rod receptor system are apparent in the character of our visual sensations: in scotopic conditions colour is absent, outlines are blurred and detail is lost. At intermediate (mesopic) levels of illumination, an interesting situation arises: the rod system and the cone system are simultaneously active, and the signals that they generate combine in the production of visual sensations. Unless these rod and cone signals interfere destructively with one another, a mesopic light stimulus must be visible if either rods alone or cones alone could signal its occurrence. Experiments on the visibility of flashes¹ have yielded results consistent with this rule; but in the following experiments with flickering light, the rule breaks down.