In the vertebrate retina, all photoreceptors hyperpolarize in response to light. In the outer retina, at the bipolar cell level, a dual system is created from the cones forming the ON and OFF channels. In the rod system a similar arrangement is found, but the ON and OFF channels in many species are formed using an amacrine cell network in the inner retina. Physiological experiments in which the ON bipolar cells are pharmacologically blocked reveal that in the primate the two channels remain largely segregated in the geniculostriate system until they reach the cortex, where they converge upon single cells. Behavioral studies show that following ON channel block, notable deficits arise in the detection of light increments but not light decrements. These and related studies suggest that the ON and OFF channels optimize information transfer to the CNS by providing excitatory signals for both increases and decreases in light energy.