Incognito (7 page)

Motion can be seen even when there is no change in position. (a) High-contrast figures like these stimulate motion detectors, giving the impression of constant movement around the rings. (b) Similarly, the zigzag wheels here appear to turn slowly.

There are many illusions of motion with no change of position. The figure below demonstrates that static images can appear to move if they happen to tickle motion detectors in the right way. These illusions exist because the exact shading in the pictures stimulates motion detectors in the visual system—and the activity
of these receptors is
equivalent
to the perception of
motion. If your motion detectors declare that something is moving out there, the conscious you believes it without question. And not merely believes it but
experiences
it.

A striking example of this principle comes from a woman who in 1978 suffered carbon monoxide poisoning.
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Fortunately, she lived; unfortunately, she suffered irreversible brain damage to parts of her visual system—specifically, the regions involved in representing motion. Because the rest of her visual system was intact, she was able to see stationary objects with no problem. She could tell you there was a ball over there and a telephone over here. But she could no longer see motion. If she stood on a sidewalk trying to cross the street, she could see the red truck over there, and then here a moment later, and finally over there, past her, another moment later—but the truck had no sense of
movement
to it. If she tried to pour water out of a pitcher, she would see a tilted pitcher, then a gleaming column of water hanging from the pitcher, and finally a puddle of water around the glass as it overflowed—but she couldn’t see the liquid move. Her life was a series of snapshots. Just as with the waterfall effect, her condition of motion
blindness tells us that position and motion are separable in the brain. Motion is “painted on” our views
of the world, just as it is erroneously painted on the images above.

A physicist thinks about motion as change in position through time. But the brain has its own logic, and this is why thinking about motion like a physicist rather than like a neuroscientist will lead to wrong predictions about how people operate. Consider baseball outfielders catching fly balls. How do they decide where to run to intercept the ball? Probably their brains represent where the ball is from moment to moment: now it’s over there, now it’s a little closer, now it’s even closer. Right? Wrong.

So perhaps the outfielder’s brain calculates the ball’s velocity, right? Wrong.

Acceleration? Wrong.

Scientist and baseball fan
Mike McBeath set out to understand the hidden neural computations behind catching fly balls.
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He discovered that outfielders use an unconscious program that tells them not where to end up but simply how to keep running. They move in such a way that the parabolic path of the ball always progresses in a straight line from their point of view. If the ball’s path looks like its deviating from a straight line, they modify their running path.

This simple program makes the strange prediction that the outfielders will not dash directly to the landing point of the ball but will instead take a peculiarly curved running path to get there. And that’s exactly what players do, as verified by McBeath and his colleagues by aerial video.
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And because this running strategy gives no information about where the point of intersection will be, only how to keep moving to get there, the program explains why outfielders crash into walls while chasing uncatchable fly balls.

So we see that the system does not need to explicitly represent position, velocity, or acceleration in order for the player to succeed in catching or interception. This is probably not what a physicist would have predicted. And this drives home the point that
introspection has little meaningful insight into what is happening behind the scenes. Outfielding greats such as Ryan Braun and Matt Kemp have no idea that they’re running these programs; they simply enjoy the consequences and cash the resulting paychecks.

When Mike May was three years old, a chemical explosion rendered him completely blind. This did not stop him from becoming the best blind downhill speed skier in the world, as well as a businessman and family man. Then, forty-three years after the explosion robbed him of his vision, he heard about a new surgical development that might be able to restore it. Although he was successful in his life as a blind man, he decided to undergo the surgery.

After the operation, the bandages were removed from around his eyes. Accompanied by a photographer, Mike sat on a chair while his two children were brought in. This was a big moment. It would be the first time he would ever gaze into their faces with his newly cleared eyes. In the resulting photograph, Mike has a pleasant but awkward smile on his face as his children beam at him.

The scene was supposed to be touching, but it wasn’t. There was a problem. Mike’s eyes were now working perfectly, but he stared with utter puzzlement at the objects in front of him. His brain didn’t know what to make of the barrage of inputs. He wasn’t experiencing his sons’ faces; he was experiencing only uninterpretable sensations of edges and colors and lights. Although his eyes were functioning, he didn’t have
vision
.
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And this is because the brain has to
learn
how to see. The strange electrical storms inside the pitch-black skull get turned into conscious summaries after a long haul of figuring out how objects in the world match up across the senses. Consider the experience of walking down a hallway. Mike knew from a lifetime of moving down corridors that walls remain parallel, at arm’s length, the whole way down. So when his vision was restored, the concept of converging perspective lines was beyond his capacity to understand. It made no sense to his brain.

Similarly, when I was a child I met a blind woman and was amazed at how intimately she knew the layout of her rooms and furniture. I asked her if she would be able to draw out the blueprints with higher accuracy than most sighted people. Her
response surprised me: she said she would
not
be able to draw the blueprints at all, because she didn’t understand how sighted people converted three dimensions (the room) into two dimensions (a flat piece of paper). The idea simply didn’t make sense to her.
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Vision does not simply
exist
when a person confronts the world with clear eyes. Instead, an interpretation of the electrochemical signals streaming along the optic nerves has to be trained up. Mike’s brain didn’t understand how his own movements changed the sensory consequences. For example, when he moves his head to the left, the scene shifts to the right. The brains of sighted people have come to expect such things and know how to ignore them. But Mike’s brain was flummoxed at these strange relationships. And this illustrates a key point: the conscious experience of vision occurs only when there is accurate prediction of sensory consequences,
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a point to which we will return shortly. So although vision
seems
like a rendition of something that’s objectively out there, it doesn’t come for free. It has to be learned.

After moving around for several weeks, staring at things, kicking chairs, examining silverware, rubbing his wife’s face, Mike came to have the experience of sight as we experience it. He now experiences vision the same way you do. He just appreciates it more.

Mike’s story shows that the brain can take a torrent of input and learn to make sense of it. But does this imply the bizarre prediction that you can
substitute
one sense for another? In other words, if you took a data stream from a video camera and converted it into an input to a different sense—taste or touch, say—would you eventually be able to see the world that way? Incredibly, the answer is yes, and the consequences run deep, as we are about to see.

In the 1960s, the neuroscientist Paul Bach-y-Rita at the University of Wisconsin began chewing on the problem of how to give vision to the blind.
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His father had recently had a miraculous recovery from a stroke, and Paul found himself enchanted by the potential for dynamically reconfiguring the brain.

A question grew in his mind: could the brain substitute one sense for another? Bach-y-Rita decided to try presenting a tactile “display” to blind people.
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Here’s the idea: attach a video camera to someone’s forehead and convert the incoming video information into an array of tiny vibrators attached to their back. Imagine putting this device on and walking around a room blindfolded. At first you’d feel a bizarre pattern of vibrations on the small of your back. Although the vibrations would change in strict relation to your own movements, it would be quite difficult to figure out what was going on. As you hit your shin against the coffee table, you’d think, “This really is nothing like vision.”

Or isn’t it? When blind subjects strap on these visual-tactile substitution glasses and walk around for a week, they become quite good at navigating a new environment. They can translate the feelings on their back into knowing the right way to move. But that’s not the stunning part. The stunning part is that they actually begin to perceive the tactile input—to
see
with it. After enough practice, the tactile input becomes more than a cognitive puzzle that needs translation; it becomes a direct sensation.
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If it seems strange that nerve signals coming from the back can represent vision, bear in mind that your own sense of vision is carried by nothing but millions of nerve signals that just happen to travel along different cables. Your brain is encased in absolute blackness in the vault of your skull. It doesn’t
see
anything. All it knows are these little signals, and nothing else. And yet you perceive the world in all shades of brightness and colors. Your brain is in the dark but your mind constructs light.

To the brain, it doesn’t matter where those pulses come from—from
the eyes, the ears, or somewhere else entirely. As long as they consistently correlate with your own movements as you push, thump, and kick things, your brain can construct the direct perception we call vision.
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Other sensory substitutions are also under active investigation.
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Consider
Eric Weihenmayer, an extreme rock climber, who scales dangerously sheer rock faces by thrusting his body upward and clinging to precariously shallow foot ledges and handholds. Adding to his feats is the fact that he is blind. He was born with a rare eye disease called retinoschisis, which rendered him blind at thirteen years old. He did not, however, let that crush his dream of being a mountaineer, and in 2001 he became the first (and so far only) blind person to climb Mount Everest. Today he climbs with a grid of over six hundred tiny electrodes in his mouth, called the
BrainPort.
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This device allows him to
see with his tongue
while he climbs. Although the tongue is normally a taste organ, its moisture and chemical environment make it an excellent brain–machine interface when a tingling electrode grid is laid on its surface.
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The grid translates a video input into patterns of electrical pulses, allowing the tongue to discern qualities usually ascribed to vision, such as distance, shape, direction of movement, and size. The apparatus reminds us that we see not with our eyes but rather with our brains. The technique was originally developed to assist the blind, like Eric, but more recent applications that feed infrared or sonar input to the tongue grid allow divers to see in murky water and soldiers to have 360-degree vision in the dark.
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