Haldan Keffer Hartline ForMemRS[1] (December 22, 1903 – March 17, 1983) was an American physiologist who was a co-recipient (with George Wald and Ragnar Granit) of the 1967 Nobel Prize in Physiology or Medicine[2][3][4][5][6][7][8] for his work in analyzing the neurophysiological mechanisms of vision.

[10] He began his study of retinal electrophysiology as a National Research Council Fellow at Johns Hopkins School of Medicine, receiving his medical degree in 1927.

[citation needed] Hartline investigated the electrical responses of the retinas of certain arthropods, vertebrates, and mollusks, because their visual systems are much simpler than those of humans and thus easier to study.

He found that the photoreceptor cells in the eye are interconnected in such a way that when one is stimulated, others nearby are depressed, thus enhancing the contrast in light patterns and sharpening the perception of shapes.

Hartline thus built up a detailed understanding of the workings of individual photoreceptors and nerve fibres in the retina, and he showed how simple retinal mechanisms constitute vital steps in the integration of visual information.