Our perception of the world depends, to a surprising degree, on intelligent guesswork by the brain. An oval-shaped white image exciting your retina could be produced by an egg, a perfectly circular, flat tilted disk, or an infinite number of intermediate shapes each angled to the right degree. Yet your brain "homes in" instantly on the correct answer. It does this by using certain unconscious assumptions about the statistics of the natural world--suppositions that can be revealed by visual illusions.
The manner in which the brain deals with inexplicable gaps in the retinal image--a process called filling in--provides a striking example of this principle. You can demonstrate this using the blind spot of your eye.
Examine illustration (a). With the right eye shut, look at the center of the lower white box. Hold the page about a foot away from your face and slowly move it toward you and away from you. At a certain distance the disk on the left vanishes. It has fallen on the blind spot of your left eye, a small patch of retina called the optic disk that is devoid of receptors (an imperfection caused by the optic nerve piercing the retina as it exits the eyeball).
Victorian physicist Sir David Brewster was struck by how when the disk disappears, you do not experience a dark shadow or gaping hole in its place. The region corresponding to the disk is "filled in" by the background color. He attributed this process to God, the "Divine Artificer."
Even a straight line running through your blind spot is not lopped off in the middle, as you can see by doing the same exercise but this time looking at the higher white box in (a). The missing segment of the line appears complete. It is as if the brain regards it as highly unlikely that two short lines could lie on either side of the blind spot simply by chance. So the cells in the visual centers fire just as they would if the bar had been complete, and you therefore see a continuous line. You can try coloring the two segments differently (for example, red and green) just for fun. Do you still complete the line?
The blind spot is surprisingly big, almost the size of nine full moons in the sky. Try closing your left eye and then look around the room with your right. With some practice, you should be able to "aim" your blind spot on any small object to make it disappear from the visual field. King Charles II of England used to aim his blind spot on a prisoner's head to "decapitate" him visually before an actual beheading. We often enjoy doing the same thing to rivals at faculty meetings.
How sophisticated is the filling-in process? If the middle of a cross falls on the blind spot, would it get filled in? What about repetitive wallpaperlike patterns? With just a few colored felt-tip markers and sheets of paper (or a computer graphics package), you can explore the limits of filling in and the "laws" that govern the process. I will describe a few examples here, but you can invent your own.
In (b), your blind spot falls on the center of an X made of a long green line crossing a short red one. If you are like most people, you will see that only the longer of the two lines is completed across the blind spot. (Whereas there is no difficulty filling in the missing part of the short line if it is presented on its own.) This simple exercise demonstrates that, under some conditions, filling in is based on integrating information along the whole length of the line rather than information that is spatially adjacent.
In other circumstances the brain fills in only what is immediately around the blind spot. If you aim your left eye's blind spot on the center of a yellow doughnut, you will see a yellow disk instead of a ring; the yellow fills in. Even more remarkable, the same thing happens in (c); most people see the yellow disk pop out conspicuously against a background wallpaper of yellow rings. Instead of extrapolating the repetitive ring patterns, your visual system performs a strictly local computation. It fills in just the homogeneous yellow immediately around the disk.