How memory works: why you can be sure of something that never happened
Where a memory lives: why one sound brings back a whole weekend
Rain on a window, and a whole weekend comes back at once: the place, the people, a voice. A memory is not kept in one spot of your brain. So where does it live?
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A month later, rain drums on your window, as it drummed on the tents that Saturday night by the lake. And there it is, all at once: the tents, the lake, Ilen and her bees.
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That weekend is kept in your brain, a mass of about 86 billion nerve cells called neurons. Each one passes signals to thousands of others.
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That night by the lake, the neurons for the rain, the tent, the water and the bees all fired at the same moment. Neurons that fire together get stronger links.
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The group of neurons tied by those strong links is the memory itself. Scientists call it an engram.
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So a month later, the rain lights up one neuron of the group. Its strong links light up the others, and the whole weekend comes back.
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Scientists showed it in mice in 2012. They marked the neurons of one fear memory so that light could switch them on, and lighting up only those neurons made a mouse freeze, in a box where nothing bad had ever happened.
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Yet a new engram is fragile, and its links are still thin. In 1953, one man lost the part of the brain that holds new ones together, and every new day slipped away.
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Quick check
Years later, a song you heard on a first date plays in a shop, and the whole evening comes back. Why the whole evening, and not only the song?
Show the answer
The neurons of the song fired together with the rest of that evening, so their links grew strong. The song lights its neurons, and they light the rest of the group.
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Go further
The idea has a famous short form, and the link and its growing have names.