LATERAL INHIBITION
We tend to ignore diffuse or homogenous light. We are predisposed to sharpen images to see contrasts and borders. We deal with hue and brightness by making contextual comparisons relying on context.
The Retina is a complex of seven cell layers. It can be viewed as an extension of the brain itself. Cells are arranged in clusters called center-surround receptive fields. Adjacent cells exhibit lateral inhibition. Broadly speaking, a broad band of diffuse light will stimulate a large number of adjacent cells which will tend to inhibit each other. A sharp point of light hitting a centre cell will not be inhibited by its neighbors and will turn on underlying ganglion cells. Patterns of dots are recognized instantly and are interpreted as sharpened lines in the visual cortex.
HALDANE KEFFER HARTLINE
American animal physiologist who was Nobel Prize Laureate for Medicine in 1967 for primary physiological and chemical visual processes in the eye
[1903- 1983]
Hartline’s performed much of his landmark research on horseshoe crabs. These heavily armored, slow moving creatures resemble prehistoric trilobites. Horse shoe crabs represent an elegant and simplified model for understanding human vision. Each compound eye has only 1,000 light receptors. The cones and rods of the lateral eyes resemble those in human eyes but are 100 times bigger. According to Hartline:
The inhibitory interaction in the eye of Limulus is an integrative process that is important in determining the patterns of nervous activity in the visual system. It is analogous to the inhibitory component of the interaction that takes place in the vertebrate retina. Inhibitory interaction results in the exaggeration of differences in sensory activity from different regions of the eye illuminated at different intensities, thus enhancing visual contrast.
Hartline, H.K., Wagner, H.G., Ratcliff, Floyd, Inhibition in the Eye of Limulus, Journal of General Physiology, 1956, 39:5