Explorers

In December 2008, the Explorers spent some time investigating the sense of vision. In particular, they discovered that the combination of their eyes and brain can sometimes be fooled into seeing something that isn't really there, or into being unable to see something that IS there.

Their first activity involved a concept known as the "blind spot." The fact that we have two eyes working together makes us oblivious to this phenomenon under most circumstances, but under carefully controlled conditions it can be seen—or NOT seen, depending on your point of view. ("Point of view," ha.)

The blind spot results from the physical architecture of our eyes. An eye has two types of light-sensing cells. The cones respond to colors and are concentrated in the center of the eye's rear surface. The rods respond only to shades of light and dark, and are found surrounding the central cones. (The rods, incidentally, are much more sensitive to light than the cones are, which is why our night-vision is nearly completely in black and white.)

The blind spot happens because the eyes do not truly "see" anything, but have to send signals about the light that they perceive to the brain, which then makes sense of the signals and turn them into a picture. To get the signals from the eye to the brain requires a connection, and this connection is a bundle of wires called the optic nerve. This nerve connects to the rear of the eye, and where it connects there are no rods or cones—and therefore no light perception.

So why don't we normally "see" a blank spot in our vision? Because each of our eyes "fills in" the blind spot from the other eye, and the brain also does a nice job of faking things as well. Closing one eye and slowly moving in and out from a spot, while not looking directly at the spot, can reveal the blind spot.

The other main activity the Explorers did involved colored after-images. Staring at an image of a particular color makes the cones that respond to that color "fire," but after a while (say, 30 seconds or so) the cones essentially say "Hey, I know this already, I don't need to fire any more." Switching your gaze to a white surface will then demonstrate a curious effect—a perfectly shaped recreation of the original picture, but in the "opposite" (or complementary) color to the original color.

This effect happens for two reasons. First, remember that the cones responding to a particular color got tired and basically quit doing their job for a bit. Second, recall that what we perceive as "white" is really a combination of all colors at once. Now, the after-image that we see makes sense. We should be seeing white in the place of that after-image, but to see white ALL of the cones need to be firing. Since some of them aren't, we perceive a color opposite to the one we stared at, because those cones are firing but their complementary ones are not. Removing some of the cones (and therefore a color) from "white" results in seeing a color where none exists.

The Explorers created several optical illusions of this type, including birds in cages, fish in bowls, and butterflies in nets—making their colors flip back and forth. They then fooled their eyes (and brains) into "seeing" a red, white, and blue American flag that started off with black and green stripes and a yellow field.

By the time they were done with the activities, the Explorers had a better understanding of the wonderful phenomenon known as "seeing." Sight may be one of our most precious senses, but it cannot always be trusted to show the truth!

Click on the links below to sample a variety of other optical illusions.

Optical web site 1

Optical web site 2