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from the blind spot. Notice what you see in the location of the dot when it’s in your blind spot. When the dot disappears, you do not perceive a hole of whiteness or blackness in its place; instead your brain
invents
a patch of the background pattern. Your brain, with no information from that particular spot invisual space, fills in with the patterns around it.
    You’re not perceiving what’s out there. You’re perceiving whatever your brain tells you.

*   *   *
     
    By the mid-1800s, the German physicist and physicianHermann von Helmholtz (1821–1894) had begun to entertain the suspicion that the trickle of data moving from the eyes to the brain is too small to really account for the rich experience of vision. He concluded that the brain must make
assumptions
about the incoming data, and that these assumptions are based on our previous experience. 14 In other words, given a little information, your brain uses its best guesses to turn it into something larger.
    Consider this: based on your previous experience, your brain assumes that visual scenes are illuminated by a light source from above. 15 So a flat circle with shading that is lighter at the top and darker at the bottom will be seen as bulging out; one with shading in the opposite direction will be perceived to be dimpling in. Rotating the figure ninety degrees will remove the illusion, making it clear that these are merely flat, shaded circles—but when the figure is turned right side up again, one cannot help but feel an illusory sense of depth.

     
    As a result of the brain’s notions about lighting sources, it makes unconscious assumptions about shadows as well: if asquare casts a shadow and the shadow suddenly moves, you will believe the square has moved in depth. 16
    Take a look at the figure below: the square hasn’t moved at all; the dark square representing its shadow has merely been drawn in a slightly different place. This
could
have happened because the overhead lighting source suddenly shifted position—but because of your previous experience with the slow-moving sun and fixed electrical lighting, your perception automatically gives preference to the likelier explanation: the object has moved toward you.

     
    Helmholtz called this concept of vision “unconsciousinference,” where
inference
refers to the idea that the brain conjectures what might be out there, and
unconscious
reminds us that we have no awareness of the process. We have no access to the rapid and automatic machinery that gathers and estimates the statistics of the world. We’re merely the beneficiaries riding on top of the machinery, enjoying the play of light and shadows.
HOW CAN ROCKS DRIFT UPWARD WITHOUT CHANGING POSITION?
     
    When we begin to look closely at that machinery, we find a complex system of specialized cells and circuits in the part of your brain calledthe visual cortex. There is a division of labor among these circuits: some are specialized for color, some for motion, some for edges, and others for scores of different attributes. These circuits are densely interconnected, and they come to conclusions as a group. When necessary, they serve up a headline for what we might call the
Consciousness Post
. The headline reports only that a bus is coming or that someone has flashed a flirtatious smile—but it does not cite the varied sources. Sometimes it is tempting to think that seeing is easy
despite
the complicated neural machinery that underlies it. To the contrary, it is easy
because of
the complicated neural machinery.
    When we take a close look at the machinery, we find that vision can be deconstructed into parts. Stare at a waterfall for a few minutes; after shifting your gaze, stationary objects such as the nearby rocks will briefly appear to crawl upward. 17 Strangely, there is no change in their position over time, even though their movement is clear. Here the imbalanced activity of your motion detectors (usually upward-signaling neurons are balanced