Neuroscientists understand a great deal about the brain, and new studies continue to unlock the science of memory. How does memory form in the brain, and what exactly happens when we recall something?
Memory in the Brain
Memories aren’t locked away in one particular place in the brain. Different areas of the brain are responsible for various aspects of memory, according to the Rochester Institute of Technology (RIT). The amygdala is associated with emotional responses, such as fear. The striatum is associated with memories of skills, and the hippocampus and temporal lobes are both essential for forming and recalling memories.
When you experience an event, it doesn’t just instantly become a memory that is stored in your brain forever. There are three main steps to forming memories.
Encoding, Storing, Retrieving and Forgetting
First, memories are encoded in your brain. This happens at synapses, which are the connections between brain cells called neurons.
“These connections become either stronger or weaker,” says Christine Smith, Ph.D. She is a memory researcher at Research Service, Veterans Affairs, San Diego Healthcare System and teaches psychiatry at the University of California, San Diego.
She adds, “The cells that are activated at the time that the event happens are the same ones that will be involved later when you remember that event.”
The connections between neurons start out weak and become stronger the more often you are exposed to the event. This explains why, for example, you still remember your childhood phone number but not the name of a person you met once.
Dr. Smith explains that when you recall a memory, the brain cells that were active when the memory was formed are reactivated. If the memory is a few years old, a structure called the hippocampus is involved with reactivating those cells and pulling the memory together so that you experience that as a remembered event.
Of course, we don’t remember every moment of our lives. We simply don’t need to access all that excessive information. Instead, we tend to remember the important things, or those memories that we have accessed the most. Sometimes a cue can help reactivate dusty old memories, such as when a specific scent reminds you of a person, place or time in your life.
As time passes, the connections become solidified or consolidated. The synapses for memories you retrieve often become stronger and therefore part of your long-term memories. Other connections between neurons weaken with time, causing you to forget those moments.
According to RIT, this normal process of losing memory over time is called memory trace decay. Sometimes, forgetting is due to encoding failure, which is when you don’t process the information and therefore never had the memory in the first place. Lastly, there is a type of forgetting called interference theory, which is when other information interferes with your ability to retain new memories. For example, when you know the word for “chair” in one language, it can block your ability to remember the word for “chair” in another language.
What Are the Four Types of Memory?
There are two main types of memory: short-term and long-term. But within long-term memory, there are two sub-categories: explicit, or conscious memory, and implicit, or unconscious memory.
Your working memory, or short-term memory, can only hold a few items and lasts for about 20 seconds, according to RIT. It’s helpful for when we need to hold information in our head for just a moment, and then can move on to other more important things. For example, if someone gives you an instruction, such as signing a piece of paper, you only need to remember that bit of information long enough to do the action that is required. After that, it’s useless. According to the Queensland Brain Institute (QBI), working memory is one of the best predictors of general intelligence, as measured by standard psychological tests.
If you practice enough, such as memorizing the spelling of words, or frequently discussing a favorite event, short-term memories can be strengthened enough to become long-term memories, through the consolidation process that we previously described.
Most of the memories that we think of in a conversational sense are part of your long-term memory. When you remember something that happened in your life, that is a type of long-term memory called an explicit memory. According to QBI, explicit memories can be episodic, such as when you remember a favorite childhood birthday party, or semantic, relating to facts or general knowledge.
Implicit, or unconscious memories are less obvious, but they are embedded in our minds as part of our long-term memory bank. They can be procedural, such as motor skills that you won’t forget once you’ve learned them, like riding a bike or tying your shoes. Another type of implicit memory is caused by priming, when exposure to one stimulus influences your brain’s response to another, according to QBI. These less obvious memories can influence our habits or phobias.
“We aren’t always aware when we’re operating based on these prior memories,” says Dr. Smith. “So we think of them as being predispositions about our personality.”
She explains that events that happen to you can become part of your memory repository, although you may not have conscious access to them. “So you’re doing all sorts of behaviors during the day that you probably don’t even realize are a reflection of memory,” she says.
We’ve explained how memory is supposed to work, but there are many issues that can cause memory loss. Amnesia, for example, can be caused by brain trauma such as a head injury, a stroke or tumor, or chronic alcoholism, according to National Geographic.
A traumatic experience can damage the communication between the hippocampus and the prefrontal cortex, according to Scientific American. This is evident, for example, when you are safe but your body’s fear response is triggered by a cue that reminds you of a previously dangerous experience, such as people with post-traumatic stress disorder (PTSD).
Memory loss is a normal part of aging, but some people experience severe memory loss with various types of dementia, such as Alzheimer’s disease. It’s a degenerative disease in which brain cells stop working. Dr. Smith explains that in the early stages of the disease, the memory structures of the brain are affected. At first, patients complain of memory loss. Then as the disease progresses, it also damages other parts of the brain, which causes cognitive problems, attention issues and executive functions, such as loss of language.
“By the time someone is diagnosed with Alzheimer’s disease their brain is very badly damaged,” she says. “You could show a healthy brain scan and Alzheimer’s brain scan to a child and they would see a difference. It’s very apparent.”
When considering what are the four types of memory, we can’t assume every person has each type. Some brain disorders affect one part of the memory but not another. For example, Smith describes that someone can have a lesion in the brain that affects their conscious memories, so they have trouble learning new things. But their unconscious memories are fine.
She also describes a condition called cortical blindness, in which people can see and have avoidance reflexes, but they can’t recognize what they’re looking at, so they are effectively blind.
“They can’t see, but if you were to throw a ball at them, they would be able to dodge it.”
How to Protect Your Memories
There is no cure for Alzheimer’s disease yet. However, people who are prone to memory loss can delay it by living healthy lifestyles. Some people practice brain exercises to fend off memory loss. Smith suggests that there are more interesting ways to keep your brain strong, backed by the science of memory. She advises her patients to focus on a healthy lifestyle with cardiovascular exercise, a nutritious diet and enough sleep. People with higher education have a lower risk of Alzheimer’s disease, she says, and the idea is that education may build up the brain and delay the onset of the disease. Plus, any kind of personal interaction helps stimulate the brain. Getting out into the world, having conversations and trying new things are all excellent brain exercises.
“You have more cognitive reserves so you can take more hits before you fail,” she says.