When a cuff-shaped electrode is placed on the optic nerve of the cat, X and Y axons, by virtue of their different diameters, exhibit different thresholds to electrical stimulation. Large-diameter Y axons have low thresholds, while smaller-diameter X axons have high thresholds. There is very little overlap between the two populations. Given this segregation, the strength of stimulation of the optic nerve required to evoke synaptic potentials in cortical neurons becomes a reliable indicator of the type of visual input a cortical neuron receives. Potentials with thresholds below the thresholds of X axons must be mediated by Y cells of the retina and LGN. Potentials with thresholds above the Y axons of the optic nerve must be mediated by X cells. From previous experiments, one would expect to find ample input via both types of axon to area 17 of the visual cortex. This was not the case. Of 58 neurons distributed throughout the layers of area 17 from which intracellular records were taken, in only four could substantial Y excitation be detected. Three of these four were located near the border with area 18. All four received large X inputs as well. The 24 neurons studied in area 18 all received large Y inputs but no detectable X input.