4.6: What Ambiguous Figures Show- Expectations and Set

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Ambiguous figures are intriguing. They aren’t typical of the things we normally see, though, so why spend so much time on them? The answer is that they tell us something very important about the causes of our visual experiences.

Let’s begin with a simple example of causation. Suppose that you enter the same sequence of numbers on your computer, which you have programmed to calculate averages. Then you are asked to enter the same numbers on my computer, which I tried to program to work the same way that yours does. But the two computers come up with different answers. How might we explain this?

One hypothesis is that you mistakenly entered different numbers in the two computers. But you double check and find that you gave the same input to each computer. Now what could explain the different results? Since the input is the same—it is held constant—in the two cases, the difference must have something to do with what goes on inside each computer.

So far, so good. But what is it in the computers that accounts for the different outputs? Perhaps one of the programs has a bug in it. You could test to see whether this was the case by working through each program. If they are the same, then the difference in the two computers’ behavior must have something to do with the hardware or the other programs in the computer. (At this point you might want to call in your friendly hacker, Wilbur.)

We often reason about causation in this way. In a later chapter, we will examine the intricacies of such reasoning, but here it is enough to note that ambiguous figures allow us to learn something about the causes of perception in the same way that we learned something about the causes of the different outputs of the two computers.

When you look at the Necker cube, or any of the other ambiguous figures, the input to your visual system is the same, no matter how you perceive the figure. It is the same when you see the two faces as it is when you see the vase. Yet your visual experience is different.

Since the input is the same in the two cases, the difference must have something to do with what happens inside you after the image is formed. It involves the way the input is processed. Moreover, if we can manage to hold further factors constant, we may be able to zero in on the factors that affect the internal processing.

In short, perceptual constancies suggest that we can have different inputs while having the same output (the same visual experience). This suggests that sameness of input is not a necessary condition for having the same experience. And our ambiguous figures show that we can have the same input while having different output (different visual experiences). This shows that sameness of input is not a sufficient condition for having the same experience.

Perceptual constancies suggest that having the same sensory input is not necessary for seeing the same thing.
Ambiguous figures show that having the same sensory input is not sufficient for seeing the same thing.

The moral is that the mind is active. But what determines the nature of the active role it plays? The next few examples give us part of the story.