brain

How does the brain remember things?

Memory is one of the brain’s building blocks. It can help keep us safe — that red stove burner is hot, don’t touch it! — and it serves as the foundation for our identities and life narratives.

So, how does the brain store and retrieve memories?

brain
A laser microscope image of the rat’s hippocampus. The hippocampus is an important brain area for memory formation.

 

The simplest explanation is that each new memory causes the human brain to reshape itself. This is accomplished through the actions of synapses, which are the tiny gaps between brain cells. Neurons, or brain cells, communicate with one another via an elegant electrochemical system. A change in one cell’s electrical charge causes the release of chemicals known as neurotransmitters across synapses.

The neurotransmitters are then picked up by the neuron on the other side of the gap, where they cause electrical changes.
“Memories are ultimately encoded in circuits, and synapses are just a means of etching out these circuits,” said Don Arnold, a neuroscientist at the University of Southern California. “When you make a memory, you have this new circuit in the brain that encodes the memory.”

When one neuron continuously stimulates another, their connection strengthens, making it easier for them to stimulate each other over time. When they communicate infrequently, their bond weakens, and sometimes they stop communicating entirely.

At the most fundamental level, the brain can store memories by strengthening connections between neural networks.

Where do memories reside in the brain?
Human memories are stored in various areas of the brain. The hippocampus, which is actually a pair of regions tucked deep within the brain and curled into themselves like seahorses, is the most important. These paired regions are critical for initial memory formation and for the transfer of memories from short-term to long-term storage.

Short-term memory lasts only 20 or 30 seconds before it fades. For example, you may remember a new phone number for the time it takes to dial it, but unless you rehearse the number repeatedly, the neural circuits that formed that short-term memory will cease to activate together, and the memory will fade.

When you practice or try to remember something, the hippocampus kicks in to strengthen the circuits. Longer-term memories are transferred to the neocortex, the wrinkly outer part of the brain that is responsible for much of our conscious experience, over time.

(However, because nothing in the brain is simple, a 2017 study published in the journal Science discovered that some of these long-term memories remain in the hippocampus as well.)

The amygdala, an almond-shaped region of the human brain that aids in the processing of emotions such as fear, is also involved in memory. Arnold and colleagues discovered that when fish learned to associate light with a painful sensation, they developed new synapses in one part of a brain region called the pallium and lost synapses in another part of the pallium, according to a study published in the journal Proceedings of the National Academy of Sciences in March.

According to Arnold, the pallium is similar to the amygdala, and the part of the fish pallium where synapses were strengthened in the study is full of neurons involved in processing painful stimuli, whereas the fish lost synapses among neurons that process positive or neutral stimuli.

Avishek Adhikari, a neuroscientist at the University of California, Los Angeles, believes that emotion plays an important role in memory formation. Positive and negative emotional situations are better remembered than neutral events, most likely for survival reasons: It’s probably important to remember things that were either extremely beneficial or extremely detrimental to your health.

According to Adhikari, the brain releases higher concentrations of certain neurotransmitters in high-emotion scenarios, and the presence of these neurotransmitters can strengthen memory circuits in the hippocampus.

Other memory-related regions include the basal ganglia and cerebellum, which handle motor memory required to play a piano piece, and the prefrontal cortex, which aids in “working memory,” which is required when you need to hold information in your head long enough to manipulate it, such as when solving a math problem.

brain
An engram is a physical representation of a memory that is made up of a network of neurons that all fire at the same time. This engram is found in the hippocampus of a mouse.

Memory’s Unsolved Mysteries
Even in adult brains, the formation of new neurons plays an important role in memory storage. Scientists used to believe that the brain stopped producing new neurons after adolescence, but research in the last two decades has shown that adult brains not only produce new neurons, but that these neurons are essential for learning and memory. According to a 2019 study published in the journal Cell Stem Cell, the hippocampus continues to generate new neurons even in people in their 80s and 90s.

It is difficult to observe memory formation and processing in a functioning brain. Synapses are tiny and numerous (around a trillion in an adult human’s brain), and imaging beyond the brain surface is difficult, according to Arnold. Imaging methods must also be capable of not interfering with brain function. However, new technology is enabling new discoveries. To peer into the zebrafish brain as it learns to associate a flashing light with an unpleasant sensation, Arnold and his colleagues modify the fish genome so that fluorescent proteins appear on synapses. The researchers can then use a specialized microscope to photograph these synapses and track their evolution.

Understanding how memory works is critical for progressing toward treatment of diseases that cause memory loss, such as Alzheimer’s. Understanding some of memory’s quirks can also help you improve your memory. For example, the hippocampus is involved not only in memory consolidation, but also in navigation – which makes sense given the importance of remembering where you are and where you’ve been when trying to get around. People who achieve incredible feats of memorization, such as remembering pi to tens of thousands of digits, frequently rely on the hippocampus’ spatial memory abilities. They’ll mentally associate each item to remember with a location in an imaginary place, a technique known as a memory palace.

A person who has mastered this technique can recall vast amounts of information simply by picturing the location in their mind.

“It’s a very strange thing to do,” Adhikari explained, “but it works because the hippocampus is especially good at and prone to mapping spatial routes.”

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