The Neural Control of Vision
I. Adaptation and Afterimages

That the change in the ratio of the photopigment is the most important signal for ganglion cells can readily by demonstrated. When the rate at which photons are delivered to the eye is kept constant, within a very short time, measured in seconds, we lose the percept because a steady state is reached in the ratio of the pigment molecules. The perceptual effect is demonstrated in Figure 40 using gaussian stimuli to minimize the effects of eye tremor. This figure has two little sharp fixation spots and two blobs, one light and one dark. Fixate on the upper fixation spot for about 20 seconds and you will notice that the blobs disappear. Normally this does not happen because our eyes are constantly on the move.

Now one can consider another phenomenon related to adaptation using this same figure. This is the phenomenon of afterimages that are most common when we are exposed to bright light. Once again, fixate on the upper fixation spot in Figure 40. After the blobs disappear shift your gaze to the lower fixation spot. You will now see two distinct negative afterimages. Where the dark blob had been you now see a light spot; the reverse is the case on the other side.