In 1956, Hartline revisited this concept of lateral inhibition in horseshoe crab (Limulus polyphemus) eyes, during an experiment conducted with the aid of Henry G Wagner and Floyd Ratliff. Hartline explored the anatomy of ommatidia in the horseshoe crab because of their similar function and physiological anatomy to photoreceptors in the human eye. Also, they are much larger than photoreceptors in humans, which would make them much easier to observe and record. Hartline contrasted the response signal of the ommatidium when a single concentrated beam of light was directed at one receptor unit as opposed to three surrounding units. He further supported his theory of lateral inhibition as the response signal of one unit was stronger when the surrounding units were not exposed to light.