The physiology of vision
The 1967 Nobel Prize of Physiology or Medicine was awarded to Ragnar Granit, Haldan Keffer Hartline and George Wald for their work on the primary physiological and chemical visual processes in the eye.
George Wald worked on the eye pigments and identified Vitamin A in the retina. This compound had just been isolated in Paul Karner’s lab. Wald showed that when the pigment rhodopsin in the eye was exposed to light, it yielded the protein opsin and a compound containing vitamin A, suggesting that vitamin A was essential to vision. He elucidated the mechanisms by which light triggers the rod-shaped sensory cells of the eye into action, the first step of vision.
Ragnar Granit identified the three types of cone-shaped sensory cells in the retina, each of which is sensitive to a different segment of the visual spectrum of light, allowing us to perceive color. He was the first to demonstrate that nerve fibers in the retina are sensitive in different ways to different wave lengths.
Haldman Keffer Hartline went into deeper anatomical studies showing how the rods and the cones of the retina are connected in the brain. He studied the neurophysiology of vision using retinal electrophysiology in arthropods, vertebrates and mollusks. He obtained the first record of electrical impulses sent by a single optic nerve fiber when stimulated by light using the horse shoe crab as a model, showing that the visual information is relayed to the brain by nerves. He also found that individual nerves were not fired in the same way, some were stimulated by steady light, others responded only when the light first lit the eye and others went into action when the light faded. Communication between the nerve cells also meant that when one nerve is stimulated, others nearby are depressed. This lateral inhibition enhances the contrast in light patterns and sharpens the perception of shapes before the information is sent to the brain.
Hartline thus showed that simple retinal mechanisms constitute vital steps in the integration of visual information. Half of what the brain interprets from what is transmitted by the eye is decided before an image even reaches the visual cortex.