ILLUSIONS ARE ANOMALIES that can reveal clues about the mysterious workings of the brain to neuroscientists in much the same way as the fictional Sherlock Holmes can solve a crime puzzle by homing in on a single out-of-the-ordinary fact. Think of the phrase “the dog that did not bark” (in Sir Arthur Conan Doyle’s short story “Silver Blaze”) or of the missing dumbbell (in Conan Doyle’s novel Valley of Fear).
Perhaps the most famous examples of such visual tricks are the geometric optical illusions. In the Ponzo illusion (a), first demonstrated by Italian psychologist Mario Ponzo in 1913, one horizontal line looks shorter than the other one, although they are identical. In the Müller-Lyer illusion (b, on page 46), created by German psychiatrist Franz Müller-Lyer in 1889, the line bounded by the diverging arrowheads looks shorter than the one with converging arrowheads—although they, too, are identical.
These illusions are very familiar yet powerful; knowledge of true line length does not stop or diminish their effect. Do we have any idea what causes them? Why would the visual system persist in committing an error, in perceiving incorrectly something so simple even when we consciously know it is a trick? Before we explore those questions, let us introduce two more eye puzzles.
In d, on page 47, we have a field of shaded disks that are seen as eggs dispersed among cavities. The disks that are light on top look like bumps or eggs, the others like cavities. This sense of depth comes from a built-in tendency for your visual system to assume that light shines from above (after all, we evolved on a planet with a single sun overhead). So the brain interprets the disks that are lighter on top as rounded like eggs and the light-on-bottom ones as cavities (because a hollow would be light on its bottom if lit from above). In e, on page 47, the shading gradient changes from left to right, and the depth is far less compelling (the tokens seem flatter) and more “bistable” (individual disks are equally likely to be seen as convex or concave, and the light source can be seen as arising from either side).
So far so good. But we also noticed that the perceived gradient of lightness—the apparent difference in brightness between the lightest and darkest parts of each disk—seems shallower for the spheres than for the craters. The brightness gradient also appears less steep for the light-on-top disks than for the light-on-side disks. Why? The physical gradient is exactly the same for each of the shaded disks (to convince yourself, rotate the paper).