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the young are naturally passionate and enthusiastic about learning. Emotions of joy and wonder, but also of horror and dread, intensify learning. That locks memories in,often for life. (Try to remember your first hobby or your first kiss. Now try to remember the first congressman you voted for, or the make of your neighbor’s car when you were ten. Usually the one is easy and the other not so easy—unless you had an early passion for politics and cars.)
    Sometimes the wow factor that works for children also works for adults. Strong emotion is often the key. We all remember where we were when the 9/11 attacks occurred, just as older people remember where they were on April 12, 1945, when President Roosevelt died suddenly on vacation at “the little White House” in Warm Springs, Georgia. Since memory remains so uncharted, we can’t say, in terms of brain function, why intense emotions can cause highly detailed memories to be deposited. Some intense emotions may have the opposite effect: in childhood sex abuse, for example, that powerful trauma is suppressed and can be retrieved only with intensive therapy or hypnosis. These matters can’t be resolved until some basic questions are answered: What is a memory? How does the brain store a memory? What kind of physical trace, if any, does a memory leave inside a brain cell?
    Until answers arise, we believe that behavior and expectations are key. When you become passionate and excited about learning again, the way children are, new dendrites and synapses will form, and your memory can once again be as strong as it was when you were younger. As well, when you recall an old memory through active retrieval (i.e., you search your mind to recall the past accurately), you make new synapses, which strengthens old synapses, increasing the odds that you will recall the same memory again in the future. The onus is on you, the brain’s leader and user. You are not your brain; you are much more. In the end, that’s the one thing always worth remembering.
    Myth 4. The brain loses millions of cells a day, and lost brain cells cannot be replaced
    The human brain loses about 85,000 cortical neurons per day, or about one per second. But this is an infinitesimal fraction (0.0002percent) of the roughly 40 billion neurons in your cerebral cortex. At this rate, it would take more than six hundred years to lose half of the neurons in your brain! We have all grown up being told that once we lose brain cells, they are gone forever and never replaced. (In our adolescence, this warning was a standard part of parental lectures about the dangers of alcohol.) Over the past several decades, however, permanent loss has been shown not to be the case. Researcher Paul Coleman, at the University of Rochester, showed that the total number of nerve cells in your brain at age twenty does not significantly change when you reach seventy.
    The growth of new neurons is called neurogenesis. It was first observed about twenty years ago in the brains of certain birds. For example, when zebra finches are developing and learning new songs for purposes of mating, their brains grow remarkably in size—new nerve cells are produced to accelerate the learning process. After a finch learns the song, many of the new nerve cells die off, returning the brain to its original size. This process is known as programmed cell death, or apoptosis. Genes not only know when it is time for new cells to be born (say, when we grow permanent teeth to replace baby teeth or undergo the changes of puberty) but also when it is time for a cell to die as when we slough off skin cells, lose our blood corpuscles after a few months, and many other cases. Most people are surprised to learn this fact. Death exists in the service of life—you may resist the idea, but your cells understand it completely.
    In the decades following these seminal discoveries, researchers observed neurogenesis in the mammalian brain, particularly in the hippocampus, which is