Our perception of optical illusions is controlled by our brains. For example, the brain can easily flip between two different views of an object to turn something that's two-dimensional on a piece of paper into an object that we perceive as being 3-D. But how?
It's complicated. The 1981 Nobel Prize in Physiology or Medicine was awarded, in part, to David Hubel and Torsten Wiesel for their discoveries in how the brain interprets the coded communications sent to it from the eyes. (That year, the award was given to multiple winners.) They learned that there is a stepwise process in how the brain analyzes what the eye sees. Each nerve cell or neuron in the brain is responsible for a specific detail in the pattern of the retinal image. But even with Hubel and Wiesel's discoveries and our knowledge of the different parts of the brain that deal with color, form, motion and texture, scientists still don't really have a sense for how all the messages come together to produce our overall perception of an object.
Using MRI scans, scientists can analyze what is happening in our brains when we look at illusions. They've learned that neurons can actually compete with one another to see light and dark spots. The winning neurons influence the message your brain gets and, therefore, what you end up perceiving [source: Hogenboom].
One theory that researchers have put forth is that some illusions cause us to be fooled because they capitalize on the way the brain constantly tries to make predictions of what will happen next in order to compensate for the small lag time between when an event happens and our ability to perceive it. Sometimes the prediction doesn't match the reality that the illusion depicts.
Another theory tries to explain "apparent motion" illusions, such as the so-called snake illusion where the objects appear to be moving on the page. Here, scientists suggest that the small — almost imperceptible — rapid movements our eyes make (called saccades) that normally get smoothed out by the brain to give us a single picture are responsible for us perceiving motion when there is none. But others say that the illusion instead works because it's sending so many pieces of information to our retina at once, and the simultaneous messages to our visual cortex cause confusion.
Obviously, not all illusions work the same way, and some theories don't always hold up when slight modifications are made to the illusions. So, in short, we're still confused about why our brains are so confused!