RECALL YOUR FAVORITE memory: the big game you won; the moment you first saw your child's face; the day you realized you had fallen in love. It's not a single memory, though, is it? Reconstructing it, you remember the smells, the colors, the funny thing some other person said, and the way it all made you feel.
Your brain's ability to collect, connect, and create mosaics from these milliseconds-long impressions is the basis of every memory. By extension, it is the basis of you. This isn't just metaphysical poetics. Every sensory experience triggers changes in the molecules of your neurons, reshaping the way they connect to one another. That means your brain is literally made of memories, and memories constantly remake your brain. This framework for memory dates back decades. And a sprawling new review published today in Neuron adds an even finer point: Memory exists because your brain’s molecules, cells, and synapses can tell time.
Defining memory is about as difficult as defining time. In general terms, memory is a change to a system that alters the way that system works in the future. "A typical memory is really just a reactivation of connections between different parts of your brain that were active at some previous time," says neuroscientist Nikolay Kukushkin, coauthor of this paper. And all animals—along with many single-celled organisms—possess some sort of ability to learn from the past.