Slide 21

Figure 21. First fixate on the left X for a few seconds, and then fixate on the right X. You should see a negative afterimage of the image on the left.

Stimulation of a neural pathway results in its adaptation, meaning that it is excited less over time. This adaptation can result in negative afterimages. Here, fixation on the X results in adaptation of the red-green color pathway. When looking at a blank (white) screen, the visual pathways that were strongly stimulated by the image are adapted and discharge at a reduced rate. The brain interprets this as the opposite colors. In this particular slide, most of the adaptation takes place in the retina. A good way to test for this is to examine the binocularity of the effect. Fixate the left image with your left eye only. Now shift to the right X and note the after image. If you quickly switch eyes, you will note that the right eye has only a very week after-image in comparison to the left. The vast majority of the effect is monocular, and therefore occurs before the information from the two eyes is combined in the primary visual cortex..