How do we make memories?


How do we make memories, where do our brains store them, and when we lose them, do we lose them forever?


I used to think of memories as single entities, like books on a shelf. Instead, memories are more like a cloth that weaves together visual images, sounds, other sensations and emotions.

Different areas of the brain process and store different aspects of a memory. For example, when you learned a song — “The Star-Spangled Banner,” say — you stored the words in a different region of your brain from where you stored the song’s melody. The memory of the words and melody might also be connected to an image of the American flag, stored in another location. Your memories are thus intricately broken down into pieces, stored in different places, but “cross-referenced” and thereby connected.

But how does a memory get made and filed away?

STAGE 1: ACQUISITION. When you learn new information, it first gets stored in short-term memory. This occurs primarily in a part of the brain called the hippocampus, deep inside the brain. Most short-term memories quickly fade.

STAGE 2: CONSOLIDATION. For short-term memory to become long-term memory, the initial neuronal pathways must be strengthened. When an event is emotionally charged, it is more likely to become part of long-term memory. Once a memory is established (consolidated), it is stored in areas of the cerebral cortex. That’s the large, domed outer layer of the brain.

The memories that endure are those that were encoded most completely in the first place — the information that you paid the closest attention to when you learned it. Memories that involve multiple senses as well as emotions are more likely to be retained.

For example, like me, you probably remember exactly what you were doing when you heard about the 9/11 terrorist attacks. You probably remember who else was with you, where you were, and the first picture you saw of the twin towers. That’s because the emotional charge of learning that our country had been attacked caused you to store everything about that moment in your memory.

But, like me, you probably have no idea what you were doing exactly 24 hours before and 24 hours after you heard about the 9/11 attacks.

STAGE 3: RETRIEVAL. Memories are stored in the brain inside certain small groups of nerve cells (neurons). Each cell has little projections that, microscopically, look like tiny trees with branches and twigs. Connections between the “trees” of two cells are called synapses. Two nerve cells talk to each other through the synapse. When you’re not thinking about a memory, the synapses are inactive. When you want to recall a memory, your brain must reactivate the synapses.

You asked if a person who loses memories loses them forever. Until recently, we thought that was probably the case. However, exciting new research indicates that “lost” memories may not have been destroyed. They may just have been made inaccessible — and potentially recoverable. In tomorrow’s column I’ll discuss that research.

(This column is an update of one that originally ran in September 2013.)