Afterimages

Visual Persistence

Visual persistence results from the inability of the nerve cells in the visual system, especially the photoreceptors, to follow changes of stimulation instantaneously: A finite amount of time is required for them to respond to the onset of the stimulus and, more important for present purposes, to respond to its cessation as well. As long as the nerve cells are responding to and therefore signaling the presence of a stimulus, one continues to see it, just as one continues to see the flash of the camera if one's surroundings are otherwise dark or if one closes one's eyes. One can also observe such persistence by placing a white spot on a dark disk and spinning the disk. The spot appears to have a tail, like that of a comet. The faster the disk spins, the longer the tail. If the tail just barely extends all the way around the disk, the time required to complete one full rotation is a measure of the duration of the visual persistence. Visual persistence produces positive afterimages, and this allows us to see the images in movies and television as though they were stable, moving objects instead of a succession of still images.

Adaptation

An animal's survival depends on sensing changes in its environment, and sensitivity to such changes is fostered by keeping sensory excitation at intermediate levels and by damping the response to stimuli that do not change. Negative afterimages [Page 17]are a by-product of the processes that accomplish this.

The process of light adaptation reduces the effect of light much as light-sensitive sunglasses do. Suppose that the light level increases tenfold, and that as a consequence, the sunglasses darken so that only 30% of the light passes through. Then a tenfold increase in light ends up increasing excitation only threefold. Light adaptation accomplishes the same thing as darkening the sunglasses, except that the change in effectiveness of the light occurs not by absorption but by changes in the physiology of the cells of the retina, the thin layer of light sensitive receptor cells, and associated nerve cells on the inside surface of the eye.

...