Fun with Vision, Part 1: Scotomas, Fill-In and Adaptation

The visual system calculates visual information through both brute force with millions of neurons, and through a number of short cuts, extrapolations, and self regulating mechanisms. These non-linearities in the visual system can be illustrated with visual illusions. Here, we will illustrate some of the features of the visual system that are related to those which we are currently investigating.

Perceptual Fill-In:

The visual system attempts to extrapolate information based upon what is known. A particularly prominent example of this is the Filling-in of missing information at the blind spot. The blind spot is where the ganglion cell axons gather together and leave the the retina. Since there are no photoreceptors here, it forms a spot of no visual input. This spot is actually rather huge - about 4 X 6 degrees. At a distance of 57 cm (about 22 inches), one degree of angle is 1 cm. That means that if you close one eye, there is a hole in your visual world that is an inch or two across at arms length. I don't know about you, but when I try this with my computer monitor, I would swear that there is no such "hole" in the visual world. That is because, the visual system is very good at "repairing" itself, and has compensated for this blind spot since early development. Here are some demonstrations of this amazing feat, as well as demonstrations on other regulatory and compensatory mechanisms.