Physiology also supports this distinction. Signals from the eyeballs are initially processed in the primary visual cortex at the back of the brain and then diverge into two visual pathways: the “how” pathway in the parietal lobe of the brain and the “what” pathway, linked to memories, in the temporal lobes. The former is concerned with spatial vision and navigation—reaching out to grab something, avoiding obstacles and pits, dodging missiles, and so on, none of which requires that you identify the object in question. The temporal lobes, on the other hand, enable you to recognize what an object actually is (pig, woman, table), and this process probably benefits partially from top-down, memory-based effects. There are hybrid cases in which they overlap. For example, with the faces/vase illusion there is a bias to get stuck seeing the faces. But you can switch to seeing the vase without explicitly being told “look for the vase,” if you are instead instructed to attend to the white region and see it as a foreground figure rather than as background.
Can the perception of ambiguous, bistable figures be biased in any way if they are preceded with other nonambiguous figures—a technique that is called priming? Priming has been explored extensively in linguistics (for instance, reading “foot” preceded by “leg” evokes the body part, but reading “foot” preceded by “inches” might suggest a ruler). Intriguingly, such priming can occur even if the first word appears too briefly to be seen consciously. Whether perception can be similarly primed has not been carefully studied. You might try it on friends.
Finally, as we note in one of our other articles, you can construct displays that are always ambiguous, such as the “devil’s pitchfork” or the “perpetual staircase.” Such paradoxical figures evoke wonder, delight and frustration at the same time—a microcosm of life itself.