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My lack of real physical injury notwithstanding, I felt substantial injury of the emotional kind. I was intensely embarrassed. I was frustrated by my failing eyes. I was angry that I was stuck with them.

Worst of all, I was forced once again to contemplate the big questions, the ones that were never far from my mind in those days, the questions of my fears. How bad is this going to get? How quickly? What will my life become as I go blind? I didn't feel hungry anymore. I felt scared, sad, and lonely. I walked back to my dorm room in Lowell House and crawled back into my bed.

In the months and years that followed, my retinas progressively deteriorated. My world blurred and morphed, the familiar and passive experience of sight becoming an arduous struggle to make sense of a bizarre carnival fun house hall of mirrors and illusions. This is when my answers to our two questions changed, how I learned: (1) You do not see with your eyes, you see with your brain. (2) The experience of sight is far more complex than perceiving what is around you. The flying tree branch I saw that day in Harvard Square came to symbolize these insights for me.

Your eyes capture a breathtaking amount of information, but that's only where sight begins. A full third of your brain is devoted to processing that information. Known as the "visual cortex," this third of your brain receives every second up to a billion signals from each of your retinas, two billion total. (The rest of your body can send your brain only an additional billion.) Your visual cortex creates from this torrent of two- dimensional data the three- dimensional experience we call sight, a truly miraculous feat. You see with your brain, not your eyes.

Consider an unremarkable scenario. You see your friend Carol across a crowded room at a cocktail party and walk over to her. You navigate myriad obstacles along the way without effort or thought. For example, you dodge a waiter's tray of hors d'oeuvres while mounting two small steps and hardly notice.

As you keep walking to Carol, you catalog countless details about your environment, consciously and unconsciously. You notice several stains on the carpet. You glance at the buffet table and see a large tray with a single shrimp lying lonely at its center. A woman steps into view and your eyes are drawn to her bright yellow hat. Curious, you take a closer look at the hat and conclude it is hideously ugly.

As you approach Carol, she stands with her back to you, engaged in a conversation. Eager to make a joke about the questionable headwear in the crowd, you tap her on the shoulder. A complete stranger turns to you, confused and slightly irritated. You respond with embarrassment, "I'm sorry, I thought you were someone else."

But that's not true. You didn't think she was someone else. You knew she was someone else. You knew she was Carol. You never questioned it. Indeed, you never even thought about it. In the world you experienced, you tapped Carol on the shoulder. She was there, in your brain, not your eyes.

Carol was as real as the backward-swimming, head-wagging fish in little Dorothy's world. Your brain created a reality that included her in just the same way little Dorothy's brain created a world with those curious fish. Your experience of sight and the physical reality of the world were two very different things. The former was an imperfect creation of your brain in response to the latter.

Now consider the true story of Marvin Anderson instead of the fictitious story about Carol. In the summer of 1982, a rape victim identified Anderson as her attacker, selecting his picture from a photo array and then identifying him in a lineup at the police station. Improperly suggestive procedures were employed by the police, making it more likely that the victim would choose Anderson. Still, she was certain he was the perpetrator of the violent assault, and at Anderson's trial she testified to that effect. In her reality, he was her attacker. She believed it. She knew it. Anderson was convicted on the basis of her certainty.

Anderson spent fifteen years in prison before he was paroled. After his parole, he spent another five years fighting to prove his innocence. Finally, in 2001 he gained access to DNA evidence that conclusively proved it. In 2002 he was granted a full pardon, twenty years after his arrest and conviction.

Marvin Anderson's image was a goldfish swimming backward through the victim's horrific recollections of her trauma. She saw him clearly, though he simply could not have been there. Unfortunately, his story is not unique. The Innocence Project reports that of all cases in which DNA evidence has conclusively exonerated the wrongly convicted, more than 70 percent of the time, false eyewitness testimony played a part in the conviction. That's a lot of false realities with great consequences.

Those realities involve far more than the mere perception of light by eyes. Each is the creation of a visual cortex. Greatly oversimplifying matters, coded in the visual cortex are a hierarchical set of increasingly complex filters or rules that you learn as a developing child.

For example, consider a baby sitting on the floor as a red ball rolls by. The pattern of information coming from her retinas that corresponds to the image of that object—a circle of firing red cone photoreceptor cells—is continuously sent to her visual cortex. The location of that pattern, however, shifts along her retinas as the ball rolls by. As she repeatedly encounters this phenomenon—a spatially shifting pattern of information—her visual cortex literally reshapes its wiring, creating a neural network that will "fire" in response to the phenomenon. Let's call that network a "motion detector."

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