Hartline, Haldan Keffer (1903–1983)

US physiologist who was awarded a Nobel Prize for Physiology or Medicine in 1967 with Ragnar Granit and George Wald for their work on the physiology and chemistry of vision. Hartline measured electrical activity in the eye and determined that a decrease in the neurotransmitter (chemical messenger) signals to the brain that the eye has been stimulated by light.

Hartline's first work was to measure the electrical activity generated by light in the retina of the vertebrate eye. He went on to investigate the more simply organized eye of the horseshoe crab Limulus. This eye, like the insect eye, consists of separate ommatidia (‘micro-eyes’) each of which has its own refracting index, photosensitive cells (receptor cells), and sensory nerve cell. Study of a single ommatidium, with its single group of fibres connecting to the central nervous system, provided Hartline with an opportunity to study electrical events following light stimulation.

Hartline's research resulted in the ‘generator potential’ theory in which slow, non-propagated voltage variation develops in the receptor cell. When light hits the receptor cell it decreases the permeability of the cell membrane to sodium ions. This results in the receptor cell leaking less neurotransmitter in the light than in the dark. Thus, it is actually a decrease in a receptor cell's chemical signal to neurons that serves as the messenger that photoreceptors have been stimulated by light.