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Having a motion detector is necessary to see a moving object, but it is not sufficient. Our baby must also develop a higher-level understanding of the very concept of motion. She builds up that understanding through repeated experimentation. She rolls the ball back and forth, picks it up, puts it in her mouth, rolls it some more, picks it up again, realizes that it is the same ball before and after she has rolled it. Instinctively she begins to comprehend that the ball is a discrete object that can move in space.

Critically, a linkage is formed between her motion detector in her visual cortex and her higher-level cognitive understanding of motion. To experience seeing an object in motion she relies upon both—and her brain's ability to associate the two. She cannot see motion in any meaningful way if she does not understand it.

A striking illustration of this point was chronicled by anthropologist Colin Turnbull when he spent time with the Mbuti pygmies in the Ituri Forest of the Democratic Republic of Congo. The Ituri Forest is extremely dense. A life lived entirely within its borders would not afford you experience with vast open spaces or sight across long distances. Such was the life of Kenge, a twenty- two-year-old Pygmy. That is, until Turnbull showed up and enlisted Kenge to be his assistant and guide.

On an excursion they drove into a rare clearing. Two miles across the plain a herd of buffalo were grazing. Kenge had never had any experience perceiving such large objects at such a great distance. He thus did not see buffalo in the distance. His brain wasn't wired to produce that experience. Instead, he saw insects nearby. When he asked Turnbull to name the strange insects, Turnbull drove in their direction to demonstrate their size. The buffalo insects appeared larger and larger to Kenge as they grew closer, another new experience for him. Kenge grew uncomfortable and asked Turnbull if witchcraft was at work.

More dramatic examples are common in people who regain sight later in life—more precisely, people who regain eyesight, or functioning of their eyes. Robert Kurson brilliantly recounted the story of Michael May in his book 'Crashing Through'. In 1957, when May was three years old, his corneas (the very front of the eye) were destroyed in a chemical explosion. Forty-three years later they were repaired by a pioneering stem cell treatment. The treatment restored normal function to his eyes, but it left him with far less than normal sight. For example, he does not experience depth perception in any meaningful way. Walk away from May and he perceives you shrinking in size. He can conceptually understand what is going on, but his visual cortex never developed the perceptual analog to that conceptual understanding.

Your experience of sight is inextricably intertwined with your conceptual understanding of the world. In our cocktail party example, this is how a pattern of firing photoreceptor cells in your retinas became the muscle movements necessary for you to climb those stairs—without so much as a single conscious thought on your part. You did far more than simply perceive those stairs with your eyes.

Beyond conceptual understanding, the experience of sight implicates your knowledge, memories, opinions, emotions, and mental attention. All of these things, and far more, are linked in your brain to your sight. The linkage works both ways. For example, your experience of sight can impact the way you feel, and the way you feel can impact what you see. Making matters even more complicated, these linkages most often play out subconsciously—you are unaware of them.

Are you growing doubtful? Return to our false Carol sighting. If you had just spoken to Carol and knew she was out of town, would you still have seen her? Did you falsely spot Carol because you unconsciously recognized the stranger's dress as one that Carol owns?

What about that yellow hat? Suppose that the woman wearing it is a dear friend of yours who is courageously battling cancer. She recently began a course of chemotherapy and as a result has lost her hair. Do you still see a hideously ugly hat, or does it now look beautiful to you, a bold image of strength? Similarly, what would the hat look like if earlier in the day you had seen a photograph of your favorite celebrity wearing it? Could the magazine photo change your perception of the hat—even if you don't consciously make the connection?

Consider your mood and your memories. If you begrudgingly arrived at the party ravenously hungry and you think shellfish of all varieties are disgusting, what happened when you saw that one meager shrimp on that vast tray? Did it worsen your mood? If you were feeling miffed to be at a lousy party with lousy food and not enough of it to boot, what details did you take in about your surroundings—consciously and unconsciously? Did the stains on the floor jump out at you and stick in your mind all night? Were you more inclined to see shabbiness and tune out cocktail party chic?

Instead imagine that years ago you met the love of your life when your hands accidentally touched as you both reached for the last shrimp on a tray at a New Year's party. You and your beloved adore shrimp. Near midnight that New Year's Eve a little boy dressed in an adorable suit raced by the two of you and jostled your glass, spilling your wine on your pants. Your future spouse produced a napkin and tended to the stain. Embarrassed, you averted your eyes and stared down at the floor, focusing on the stain your spilled wine made on the carpet. The crowd began to count down the New Year, and your heart skipped a beat.

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