The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

FIGURE 2.2 This picture has not been Photoshopped! It was taken with an ordinary camera from the special viewing point that makes the Ames room work. The fun part of this illusion comes when you have two people walk to opposite ends of the room: It looks for all the world as if they are standing just a few feet apart from each other and one of them has grown giant, with his head brushing the ceiling, while the other has shrunk to the size of a fairy.

Another striking example is the famous Ames room illusion (Figure 2.2). Imagine taking a regular room like the one you are in now and stretching out one corner so the ceiling is much taller in that corner than elsewhere. Now make a small hole in any of the walls and look inside the room. From nearly any viewing perspective you see a bizarrely deformed trapezoidal room. But there is one special vantage point from which, astonishingly, the room looks completely normal! The walls, floor, and ceiling all seem to be arranged at proper right angles to each other, and the windows and floor tiles seem to be of uniform size. The usual explanation for this illusion is that from this particular vantage point the image cast on your retina by the distorted room is identical to that which would be produced by a normal room—it’s just geometric optics. But surely this begs the question. How does your visual system know what a normal room should look like from exactly this particular vantage point?

To turn the problem on its head, let’s assume you are looking through a peephole into a normal room. There is in fact an infinity of distorted trapezoidal Ames rooms that could produce exactly the same image, yet you stably perceive a normal room. Your perception doesn’t oscillate wildly between a million possibilities; it homes in instantly on the correct interpretation. The only way it can do this is by bringing in certain built-in knowledge or hidden assumptions about the world—such as walls being parallel, floor tiles being squares, and so on—to eliminate the infinity of false rooms.

The study of perception, then, is the study of these assumptions and the manner in which they are enshrined in the neural hardware of your brain. A life-size Ames room is hard to construct, but over the years psychologists have created hundreds of visual illusions that have been cunningly devised to help us explore the assumptions that drive perception. Illusions are fun to look at since they seem to violate common sense. But they have the same effect on a perceptual psychologist as the smell of burning rubber does on an engineer—an irresistible urge to discover the cause (to quote what biologist Peter Medawar said in a different context).

Take the simplest of illusions, foreshadowed by Isaac Newton and established clearly by Thomas Young (who, coincidentally, also deciphered the Egyptian hieroglyphics). If you project a red and a green circle of light to overlap on a white screen, the circle you see actually looks yellow. If you have three projectors—one shining red, another green, and another blue—with proper adjustment of each projector’s brightness you can produce any color of the rainbow—indeed, hundreds of different hues just by mixing them in the right ratio. You can even produce white. This illusion is so astonishing that people have difficulty believing it when they first see it. It’s also telling you something fundamental about vision. It illustrates the fact that even though you can distinguish thousands of colors, you have only three classes of color-sensitive cells in the eye: one for red light, one for green, and one for blue. Each of these responds optimally to just one wavelength but will continue to respond, though less well, to other wavelengths. Thus any observed color will excite the red, green, and blue receptors in different ratios, and higher brain mechanisms interpret each ratio as a different color. Yellow light, for example, falls halfway in the spectrum between red and green, so it activates red and green receptors equally and the brain has learned, or evolved to interpret, this as the color we call yellow. Using just colored lights to figure out the laws of color vision was one of the great triumphs of visual science. And it paved the way for color printing (economically using just three dyes) and color TV.