Back during the summer, an early copy of Brainworks showed up in our office. National Geographic was planning to advertise the book in mental_floss magazine (and this was even before we kept popping up on Curb Your Enthusiasm). I got sucked into the optical illusions featured throughout the book and asked if I could reprint a few of the explanations behind them. They agreed. Here you go!
The horizontal/vertical illusion dates to its description in German physiologist Adolf Fick's 1851 doctoral thesis. He demonstrated differences among simple geometrical properties and how they are perceived. These kinds of disparities are called geometrical-optical illusions. Fick observed that a vertical line looks longer than a horizontal line of the same length. This is easily seen in the letter T when the horizontal and vertical strokes measure precisely the same length, or when two line segments of exactly the same dimension form a right angle with one segment horizontal and the other vertical. * * * Another explanation rests on an illusion of perspective. The brain chooses to interpret the drawing as two tables. Applying the rules of perspective it has formed through experience, the brain views the table on the left as receding farther, and being longer, than the one on the right.
The Arrow Illusion
Compare the tables with the Müller-Lyer Illusion (or Arrow Illusion). It is named for Franz Carl Müller-Lyer, a 19th-century German psychiatrist and sociologist. He began his illusion by drawing two parallel lines of the same length. At the ends of one line, he placed two arrowhead shapes with their open ends pointing outward. At the ends of the other line, he placed the same two arrowhead shapes with their open ends pointing inward. The line segment with the arrowheads pointing inward and the ends open to the outside looks significantly longer than its mate. The illusion holds true with the line segments in any orientation.
The Ames Room
American psychologist Adelbert Ames used his background as a painter to create an elaborate trick on the brain: the construction of a distorted room that looks normal when viewed from front and center. The back wall of the room slants away from the viewer instead of lying perpendicular to the viewer's line of sight, but Ames compensated for this by using perspective cues to make the room appear normal. A person who stands at the most distant corner of the sloping wall appears tiny, with plenty of space overhead; the same person standing in the nerer corner crowds the ceiling like a giant. An adult on one side is dwarfed by a child on the other because both appear equidistant from the viewer. The key to this illusion is that perspective and one particular angle make the room appear perfectly rectangular and normal.
If you like this kind of thing, check out the book's site. Here's the description: "You'll see for yourself why these visual illusions and experiments hoodwink the brain. You'll find out how the structure of the eye influences what you see. And you'll think of events that may not have actually happened, in order to learn how the mind can create a false memory."