Lateral Inhibition

1. Take a full sheet of paper and roll it lengthwise into a tube that is about 1/2 inch (13 millimeters) in diameter. (If you are using 8 1/2 x 11-inch paper, the tube length will be 11 inches; if you are using A4 paper, it will be about 30 centimeters.) Use tape to keep the tube from unrolling.
2. Take the other sheet of paper and cut a lengthwise strip that’s about 2 1/2 inches (6.4 cm) wide (if you are using 8 1/2 x 11-inch paper, the tube length will be 11 inches; if you are using A4 paper, it will measure about 30 cm). Roll this strip lengthwise into a tube that’s also about 1/2 inch (13 mm) in diameter and tape it as well.

With both eyes open, look at the white screen, wall, or paper through the tube you made from a full sheet of paper (the wall of this tube will be thicker than the other tube). Notice that the spot of light you see through the tube appears brighter than the wall of the tube.

Now do the same thing using the tube you made from the narrow strip of paper. Notice that the spot you see through the tube appears darker than the wall of the tube.

When light hits your eyes, receptors in your eyes send a signal to your brain. Receptors that receive light also send signals to neighboring receptors that tell them to turn down, or inhibit, their sensitivity to light.

When you look at the white wall without a tube, you see a uniform field of brightness because all the receptors are equally inhibited. When you look through the tube that you made from a full sheet of paper, the spot of light is surrounded by the dark ring of the tube. The spot appears brighter because the receptors in the center of your retina are not inhibited by signals from the surrounding dark ring.

In contrast, light shines through the walls of the tube that you made from the thin strip of paper. When you look through this thin-walled tube, the spot appears darker because light comes through the wall of the tube, causing the receptors at the center of your retina to be inhibited. This is known as lateral inhibition.