From whole-eye responses to single fibers
Hartline received his medical degree from Johns Hopkins in 1927 and subsequently studied mathematics and physics. At Pennsylvania he recorded responses from individual optic-nerve fibers in horseshoe crabs and investigated vertebrate retinas, especially frogs. These methods made the signals of individual receptor units accessible.
References: [1]
What neighboring illumination changes
A 1956 study with H. G. Wagner and Floyd Ratliff described how illuminating neighboring ommatidia reduced the impulse frequency of a target unit. Cutting lateral connections abolished the inhibition. Changing illumination and connections made interactions within the visual network experimentally testable.
References: [3]
Lateral inhibition and visual contrast
These experiments showed that visual receptor units do not simply report light intensity in isolation. Inhibition from neighboring units can exaggerate response differences between differently illuminated regions, helping explain contrast. This connected initial receptor responses with processing by retinal networks.
Continuing quantitative vision research
Hartline became head of biophysics at Johns Hopkins in 1949 and moved to Rockefeller in 1953; Ratliff joined him the following year. They continued investigating receptor properties and inhibitory interactions in the horseshoe-crab eye. The shared 1967 prize recognized research on the early physiological and chemical processes of vision.