Consider the simple case of the Necker cube (c and variation in d). You can view this illusion in one of two ways—either pointing up or pointing down. With a little practice, you can flip between these alternate percepts at will (still, it is great fun when it flips spontaneously; it feels like an amusing practical joke has been played on you). In fact, the drawing is compatible not only with two interpretations, as is commonly believed; there is actually an infinite set of trapezoidal shapes that can produce exactly the same retinal image, yet the brain homes in on a cube without hesitation. Note that at any time, you see only one or the other. The visual system appears to struggle to determine which of two cubes the drawing represents, but it has already solved the much larger perceptual problem by rejecting trillions of other configurations that could give rise to the retinal pattern we call the Necker cube. Top-down attention and will, or intent, can only help you select between two percepts; you will not see any of the other possibilities no matter how hard you try.
Although the Necker cube is often used to illustrate the role of top-down influences, it, in fact, proves the very opposite—namely, that perception is generally immune to such influences. Indeed, if all perceptual computations mainly relied on top-down effects, they would be much too slow to help you in tasks related to survival and the propagation of your genes—escaping a predator, for example, or catching a meal or a mate.
It is important to recognize that ambiguity does not arise only in cleverly contrived displays such as on these two pages and in e, on the next page, in which shading could make a surface feature on Mars appear to be convex or concave. In truth, ambiguity is the rule rather than the exception in perception; it is usually resolved by other coexisting bottom-up (or sideways, if that is the right word) cues that exploit built-in statistical “knowledge” of the visual world. Such knowledge is wired into the neural circuitry of the visual system and deployed unconsciously to eliminate millions of false solutions. But the knowledge in question pertains to general properties of the world, not specific ones. The visual system has hardwired knowledge of surfaces, contours, depth, motion, illumination, and so on, but not of umbrellas, chairs or dalmatians.