The Neural Control of Vision
G. The Function of the On and Off Channels in Vision

To test the ability of monkeys to detect stimuli made visible by virtue of light increment or decrement monkeys were trained to make saccadic eye movements after central fixation, to either a light or a dark spot as depicted on the monitors shown in Figure 26. Under normal condition monkeys do this very well and quite quickly as shown by the saccadic reaction times. After APB blocks the ON cells in the retina, monkeys have difficulty seeing the bright spot and when they do, it takes them a long time to respond. They continue seeing the dark spot unabated.