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

My favorite example of how we can use illusions to discover the hidden assumptions underlying perception is shape-from-shading (Figure 2.3). Although artists have long used shading to enhance the impression of depth in their pictures, it’s only recently that scientists have begun to investigate it carefully. For example, in 1987 I created several computerized displays like the one shown in Figure 2.3—arrays of randomly scattered disks in a field of gray. Each disk contains a smooth gradient from white at one end to black on the other, and the background is the exact “middle gray” between black and white. These experiments were inspired, in part, by the observations of the Victorian physicist David Brewster. If you inspect the disks in Figure 2.3, they will initially look like a set of eggs lit from the right side. With some effort you can also see them as cavities lit from the left side. But you cannot simultaneously see some as eggs and some as cavities even if you try hard. Why? One possibility is that the brain picks the simplest interpretation by default, seeing all of the disks the same way. It occurred to me that another possibility is that your visual system assumes that there is only a single light source illuminating the entire scene or large chunks of it. This isn’t strictly true of an artificially lit environment with many lightbulbs, but it is largely true of the natural world, given that our planetary system has only one sun. If you ever catch hold of an alien, be sure to show her this display to find out if her solar system had a single sun like ours. A creature from a binary star system might be immune to the illusion.

FIGURE 2.3 Eggs or cavities? You can flip between the two depending on which direction you decide the light is shining from, right or left. They always all flip together.

So which explanation is correct—a preference for the simpler interpretation, or an assumption of a single light source? To find out I did the obvious experiment of creating the mixed display shown in Figure 2.4 in which the top and bottom rows have different directions of shading. You will notice that in this display, if you get yourself to see the top row as eggs, then the bottom row is always seen as cavities, and vice versa, and it is impossible to see them all simultaneously as eggs or simultaneously as cavities. This proves it’s not simplicity but the assumption of a single light source.

FIGURE 2.4 Two rows of shaded disks. When the top row is seen as eggs, the bottom row looks like cavities, and vice versa. It is impossible to see them all the same way. Illustrates the “single light source” assumption built into perceptual processing.

FIGURE 2.5 Sunny side up. Half the disks (light on top) are seen as eggs and half as cavities. This illusion shows that the visual system automatically assumes that light shines from above. View the page upside down, and the eggs and cavities will switch.

It gets better. In Figure 2.5 the shaded disks have been shaded vertically rather than horizontally. You will notice that the ones that are light on top are nearly always seen as eggs bulging toward you, whereas the ones that are dark on top are seen as cavities. We may conclude that, in addition to the single-light-source assumption revealed in Figure 2.4, there is another even stronger assumption at work, which is that the light is shining from above. Again, makes sense given the position of the sun in the natural world. Of course, this isn’t always true; the sun is sometimes on the horizon. But its true statistically—and it’s certainly never below you. If you rotate the picture so it’s upside down, you will find that all the bumps and cavities switch. On the other hand, if you rotate it exactly 90 degrees, you will find that the shaded disks are now ambiguous as in Figure 2.4, since you don’t have a built-in bias for assuming light comes from the left or the right.