Maybe it’s a hazy snapshot of your first time riding a bicycle. Or the ability to recite the Pythagorean theorem. It could be as simple as that phone number you scrawled on a napkin before it landed in the trash.
Whatever shape they take, our memories help define who we are — and what it means to be human. While scholars have been musing on memory since the time of Socrates, new tech has helped today’s scientists learn much more about the neural and biological machinery behind our recollections. These breakthroughs have led to the discovery that our memories reside in specific clusters of brain cells. Some scientists are exploring how people store and retrieve memories as they move through a virtual reality environment. Others are studying how emotions like fear are encoded in the brain, as well as the circuitry that controls what we’re afraid of.
This research isn’t rooted in the abstract, either. The projects are aimed at real-world applications, including possible treatments for conditions such as Alzheimer’s disease and post-traumatic stress disorder.
And while much of memory science is still a blur, the matter of how, exactly, our brains form memories is coming into sharper focus.
(Credit: Evan Oto/Science Source)
The Long and Short of It
The notion of human memory doesn’t refer to any one thing. The term is an umbrella for an array of recollections, from the names of colors to half-remembered song lyrics to your first breakup. So, what are these different types of memory?
Over a century ago, scientists partitioned memory into short-term and long-term categories. Short-term memory, sometimes called working memory, refers to our ability to retain information or events from the recent past — but only for as long as about 20 seconds ago, sometimes even less. In other words, it’s the stuff that you’re actively holding in your head while performing other tasks — for example, remembering a phone number as you scroll around to plug it into your contacts list.
In the 1990s, scientists analyzed high-resolution brain scans and found that these fleeting memories depend on neurons firing in the prefrontal cortex, the front part of the brain responsible for higher-level thinking.
“They are temporary [memories],” says neuroscientist and author Dean Burnett. “It’s not meant to be for long-term storage, because they’re constantly changing and constantly in flux.
“If you hold something in the brain long enough, you can turn it into a long-term memory,” he adds. “That’s why, if you recite something like a phone number, you can eventually remember it quite easily. But if too much stuff keeps coming in, your short-term memory gets overloaded and the first [bits of information] will get kicked out.”
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By contrast, long-term memory is the treasure trove of knowledge and past events collected throughout our lives. And while short-term memories are supported by blips of neural activity, long-term memories actually forge a physical presence in the brain. When a long-term memory is formed, the connections between neurons, known as synapses, are strengthened. In some cases, entirely new synapses are created. And the more we revisit memories, activating these neural pathways, the stronger the connections become — like trampling your way through the woods to create a well-trodden path.
Long-term memories can also take several different forms. For example, implicit memories are the basis for automatic behaviors like tying your shoes or brushing your teeth. These instinctive actions take place in the unconscious part of the brain. “This is why people with amnesia can still do these things, even if they have no memory of doing them before,” says Burnett. “The training takes hold.”
Long-term remembrances that we’re actively aware of, however, are known as explicit memories. These are split between episodic and semantic memory. The latter describes specific, conceptual knowledge, like the date on which the Declaration of Independence was signed. Episodic memory describes events and experiences from your own life. Everything from your 21st birthday party to your trip to Europe falls into this category.
“Semantic memory is [knowing] that Paris is the capital of France,” says Burnett. “Episodic memory is [remembering] that time I went to France and threw up off the Eiffel Tower.”
Lighting The Way
MIT scientists labeled the cells (highlighted in red) where memory engrams are stored in a mouse hippocampus. (Credit: Steve Ramirez and Xu Liu)
The things we do in life leave traces behind, embedded in our memories. Much like Marcel Proust biting into his much-loved madeleines, causing once-forgotten memories from his childhood to come flooding back, memory traces can conjure vivid sensory experiences of things past. Since the days of ancient Greece, scholars have speculated that these remnants might even alter the physical makeup of the brain. But it wasn’t until the turn of the 20th century that scientific models of this process began to emerge. In 1904, a German scientist named Richard Semon suggested that these traces, which he called memory engrams, are represented as physical changes in the brain after an event or experience. “At the time, there was no technology to identify those brain cells which hold the specific engram for a specific memory,” says Susumu Tonegawa, professor of biology and neuroscience at MIT and winner of the 1987 Nobel Prize in Physiology or Medicine.