After attending the universities of
Leipzig and
Munich as an Eldridge Johnson traveling research scholar from the
University of Pennsylvania, he returned to the US to take a position in the Eldridge Reeves Johnson Foundation for Medical Physics at Penn, which was under the directorship of
Detlev W. Bronk at that time. In 1940–1941, he was Associate Professor of Physiology at Cornell Medical College in New York City but returned to Penn and stayed until 1949. Then he became professor of
biophysics and chairman of the Jenkins Department of Biophysics at
Johns Hopkins University in 1949. One of Hartline's graduate students at Johns Hopkins,
Paul Greengard, also won the Nobel Prize. Hartline joined the staff of
Rockefeller University, New York City, in 1953 as professor of neurophysiology. 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 concentrated his studies on the eye of the
horseshoe crab (
Limulus polyphemus). Using minute electrodes, he obtained the first record of the electrical impulses sent by a single
optic nerve fibre when the receptors connected to it are stimulated by light. 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. ==Awards and honors==