A memory's journey

Yale Medicine Magazine, 2015 - Autumn

Features
Your entire brain is memory—and your memory is who you are
A T cell’s fate
What is memory?
A memory's journey
What’s in a doctor’s brain, and what’s online
Our memories, our stories
Can ginkgo tea preserve our memories?
The first things to go
Stem cells also remember
Yale designated research center by NIH
The history of human monstrosity
The most cooperative of patients
News
People
Dialogue

Different memories reside in different parts of the brain, and neuroscientists are still trying to find out where our brains store some memories.

There are things we just remember, and the things we work at remembering, like studying for a test in microbiology or American history. What year did Lincoln deliver the Gettysburg address? What’s the difference between DNA and RNA? These learned facts and figures outside of our personal experience become part of our semantic memory, but not everything in our semantic memory comes from textbooks. These memories can be the facts common to our culture, like the name of our state capital or the current president, and include other facts we acquire over a lifetime. Trying to cram all these things into our brain the night before a test is probably the worst thing we can do. Research has shown that we need to sleep in order to consolidate our memories.

There are things in our lives, like slipping on the sidewalk and breaking an arm, that remain as episodic memories—the stories of our lives. Somehow we organize these memories intocoherent narratives with a beginning, middle, and end. Along with the broken arm, we’ll remember where we were coming from, where we were going, whether we were distracted by our cell phone, the ice on the sidewalk, the moment of slipping, the realization that we were going to fall and that it was going to hurt. But episodic memories can also be pleasant—a Labor Day barbecue at the beach with friends, the conversation, the sound of the surf, and the food. Through these memories we travel back in time, from last weekend to 30 years ago.

“Mechanical memories,” which we now call procedural memories, were first observed in 1804. This is knowing how to do things. These are the skills we acquire and never forget, like riding a bicycle. If you play an instrument it seems like the muscles in your fingers remember, too, and know exactly where to strike that key or form that chord. Without even thinking, we know, in a given song, which note comes next. Procedural memory allows the muscles in a quarterback’s or pitcher’s arm to throw the ball with accuracy, and without even thinking about it. Likewise, hammering a nail is one of those skills that stays with you. Your muscles just know.

Why do we write to-do lists? Even our working memory has its limits. We use it to hold on to something for as long as we need it and no longer—usually just a few seconds. What’s the number of that takeout Chinese place? What’s that email address? In the 1950s, it was posited that our capacity to hold on to information is limited to a handful of “chunks.” That’s why we break up long series of digits like our phone numbers or social security numbers. And, as we age, working memory is among the functions most likely to decline.

On their journey through the brain, our memories pass through the hippocampus, a temporary transit hub. Where they go next depends on whether your brain is recording the capitals of the 50 states, your reaction to the latest Seth Rogen movie, or the shock of witnessing a traffic accident.

For every memory, the brain creates a new circuit of neurons, or alters or strengthens an existing circuit. And each memory has a context‒it's encoded in the brain based on meaning and association. Long-term memories fall into two types, declarative and nondeclarative. Under declarative memories‒which are encoded by the hippocampus, entorhinal cortex, and perirhinal cortex, but consolidated and stored in other parts of the brain‒come episodic memories. Episodic memories are our personal experiences, including the emotional charge of those experiences: the day I started med school … the time I spilled coffee on my suit just before a job interview … the first time I laid eyes on the love of my life. These personal memories end up in the medial temporal lobe and neocortex. Anything emotional or traumatic involves the amygdala. Semantic memories are the things outside of our personal experience that we learn, say, from textbooks. What's the capital of Estonia? When was the Battle of Waterloo? Can you recite pi to 10 digits? These types of memory reside in the lateral and anterior temporal cortex and the prefrontal cortex.

Nondeclarative memory lets us remember how to ride a bicycle, strum a guitar, or hammer a nail. These procedural memories are encoded and stored by the cerebellum, putamen, caudate nucleus, and motor cortex.

Short-term memory, or working memory, has been likened to a scratch pad where we jot down a phone number. Unless we make an effort to retain that number, it will disappear. The central executive part of the prefrontal cortex handles these types of memories. Ever wonder why we break up phone numbers and credit card numbers? That's "chunking," breaking a long number into chunks, which allows us to increase our short-term memory capacity.

A memory's journey

Contents

Different memories reside in different parts of the brain, and neuroscientists are still trying to find out where our brains store some memories.

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