Hartline, Haldan Keffer (1903-1983) was an American physiologist. He shared the 1967 Nobel Prize in physiology or medicine for his work on the neuro-physiology of vision. He shared the prize with American biochemist George Wald and Finnish-born Swedish researcher Ragner Arthur Granit .

Hartline was born on Dec. 22, 1903, in Bloomsburg, Pennsylvania. In 1923, he earned a bachelor's degree from Lafayette College and learned quantitative experimental biology at the marine laboratory at Woods Hole, Massachusetts. Hart-line received a medical degree from Johns Hopkins University in 1927 but never worked as a doctor. Instead, he spent his career doing research.

Hartline studied physiology at Johns Hopkins from 1927 to 1929 and in Germany from 1929 to 1931. In 1931, he was appointed a fellow in medical physics at the University of Pennsylvania's Eldridge Reeves Johnson Foundation.

Hartline's research on the neurophysiology of vision took place over 40 years, starting in the early 1930's. He conducted experiments on horseshoe crabs and frogs to determine how the retina of the eye interprets visual information and passes it along to the brain through the optic nerve. He applied tiny electrodes to cells in the eyes of these animals to observe how the eye responds to patterns of light and shade on the retina in distinguishing shapes. He studied the kinds of signals that are transmitted along the optic nerve. He discovered that stimulation in one region of the retina causes a decrease of activity in surrounding regions, a principle he called lateral inhibition. He and an associate, Floyd Ratliff, produced a mathematical model demonstrating this activity.

In 1949, Hartline became professor and head of the department of biophysics at Johns Hopkins. He worked at the Rockefeller Institute for Medical Research (now Rockefeller University) from 1953 to 1974 and became a professor there in 1965. Hartline died on March 18, 1983, in Faliston, Maryland.