Incognito (6 page)

For a powerful illustration of the limits of
introspection, consider the eye movements you are making right now while reading this book. Your eyes are jumping from spot to spot. To appreciate how rapid, deliberate, and precise these eye movements are, just observe someone else while they read. Yet we have no awareness of this active examination of the page. Instead it seems as though ideas simply flow into the head from a stable world.

Because vision appears so effortless, we are like fish challenged to understand water: since the fish has never experienced anything else, it is almost impossible for it to see or conceive of the water. But a bubble rising past the inquisitive fish can offer a critical clue. Like bubbles,
visual
illusions can call our attention to what we normally take for granted—and in this way they are critical tools for understanding the mechanisms running behind the scenes in the brain.

You’ve doubtless seen a drawing of a cube like the one to the right. This cube is an example of a “multistable” stimulus—that is, an image that flips back and forth between different perceptions. Pick what you perceive as the “front” face of the cube. Staring at the picture
for a moment, you’ll notice that sometimes the front face appears to become the back face, and the orientation of the cube changes. If you keep watching, it will switch back again, alternating between these two perceptions of the cube’s orientation. There’s a striking point here:
nothing has changed on the page, so the change has to be taking place in your brain
. Vision is active, not passive. There is more than one way for the visual system to interpret the stimulus, and so it flips back and forth between the possibilities. The same manner of reversals can be seen in the
face–vase illusion below: sometimes you perceive the faces, and sometimes the vase, even though nothing has changed on the page. You simply can’t see both at once.

There are even more striking demonstrations of this principle of active vision. Perceptual switching happens if we present one image to your left eye (say, a cow) and a different image to your right eye (say, an airplane). You don’t see both at the same time, nor do you see a fusion of the two images—instead, you see one, then the other, then back again.
¹²
Your visual system is arbitrating a battle between the conflicting information, and you see not what is really out there, but instead only a moment-by-moment version of which perception is winning over the other. Even though the outside world has not changed, your brain dynamically presents different interpretations.

More than actively interpreting what is out there, the brain often goes beyond the call of duty to make things up. Consider the example
of the retina, the specialized sheet of photoreceptor cells at the back of the eye. In 1668, the French philosopher and mathematician Edme Mariotte stumbled on something quite unexpected: there is a sizable patch in the retina where the photoreceptors are missing.
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This missing patch surprised Mariotte because the visual field appears continuous: there is no corresponding gaping hole of vision where the photoreceptors are missing.

Or isn’t there? As Mariotte delved more deeply into this issue, he realized that there
is
a hole in our vision—what has come to be known as the “
blind spot” in each eye. To demonstrate this to yourself, close your left eye and keep your right eye fixed on the plus sign.

Slowly move the page closer to and farther from your face until the black dot disappears (probably when the page is about twelve inches away). You can no longer see the dot because it is sitting in your blind spot.

Don’t assume that your blind spot is small. It’s huge. Imagine the diameter of the moon in the night sky. You can fit seventeen moons into your blind spot.

So why hadn’t anyone noticed this hole in vision before Mariotte? How could brilliant minds like Michelangelo, Shakespeare, and Galileo have lived and died without ever detecting this basic fact of vision? One reason is because there are two eyes and the blind spots are in different, nonoverlapping
locations; this means that with both eyes open you have full coverage of the scene. But more significantly, no one had noticed because the brain “fills in” the missing information 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 in
visual 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 physician
Hermann 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 a
square 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 “unconscious
inference,” 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.

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 called
the 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.
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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 in a push–pull relationship with downward-signaling neurons) allows you to see what is impossible in the outside world: motion without position change. This illusion—known as the motion aftereffect or the
waterfall illusion—has enjoyed a rich history of study dating back to
Aristotle. The illusion illustrates that vision is the product of different modules: in this case, some parts of the visual system insist (incorrectly) that the rocks are moving, while other parts insist that the rocks are not, in fact, changing position. As the philosopher
Daniel Dennett has argued, the naïve introspector usually relies on the bad metaphor of the television screen,
¹⁸
where moving-while-staying-still cannot happen. But the visual world of the brain is nothing like a television screen, and motion with no change in position is a conclusion it sometimes lands upon.