How memory engrams are like notebooks in your brain

Moushumi Nath
6 February 2018

Above: Image © adventtr,iStockPhoto.com

Do you remember your first-ever day of school?

Do you remember the capital of Germany? (Hint: it starts with a B!)

Both of these are examples of memories, pieces of information that you can recall from your past.

Scientists often classify memories as short-term memories (things you remember from the past twenty seconds) or long-term memories (things you remember from longer ago - from a week ago, or even years ago).

Long-term memories can be further subdivided into different types, such as:

Type of memory What does it mean? Examples:

Declarative memories

Memories you are consciously aware of

Episodic memories: memories that you have experienced.

Remember your first day of school? That’s an episodic memory.

Semantic memories: memories about facts.

Remember the capital of Germany? That’s a semantic memory.

(Did you get the answer? It’s Berlin!)

Non-declarative memories

Memories you’re not consciously aware of

Motor memories: memories of how to move in a specific way.

Remember how to ride a bike? That’s a motor memory.

In addition to the above categories, memories can be divided based on the type of content they carry:

Type of Memory What does it mean? Examples:

Spatial memories

Memories about an environment.

Spatial memories: memories about the environment.

Remember the layout of your bedroom? That’s a spatial memory.

Emotional memories

Memories with emotional content, such as fear.

Emotional memories: memory characterized by emotions, such as fear.

Have you ever lost a loved one? That’s an emotional memory.

See how many different types of memories you can have? Lots!

Memories have intrigued scientists for centuries! Today, let’s look at memory engrams. These are the sites in your brain where you store memories.

The 3 Stages of Memory

When storing a long term memory, you can think of your brain going through three stages:

  1. Encoding
  2. Storage
  3. Retrieval

A simple way to understand these three stages is to think of a memory as a notebook.

During class, your teacher talks, and you write down a few notes here and there into your notebook. In other words, you are encoding the information you hear.

Once class is finished, you take your notebook and store it in your bookshelf in your locker until the next time you need it. This is storage.

Finally, when you need the information again, you go back to your locker and get your notebook. This is retrieval.

Your decision on when to retrieve your notebook depends on what cues are available to you. For example, if it’s nearing exam time, then that’s a pretty good cue for you to go back to your locker, retrieve your notebook, study and ace that exam!

Similarly, your brain looks for cues for when you might “need” a memory. If you hear the word “Germany,” that might be its cue to retrieve “Berlin.” If you smell crayons, that might be its cue to remember your first day at school.

So where are the notebooks, or memories, stored in humans?

Memory Engrams

Your brain is a collection of notebooks! There’s a specific term scientists use to describe the areas in which memories are stored in the brain: memory engrams.

Scientists believe that different types of memories are stored in different parts of the brain. For example, spatial memories may be stored in the hippocampus. The memory of your warm, comfortable bedroom may be nestled here.

Did you know? An epilepsy patient known as H.M. had his hippocampi removed, hoping to treat his seizures. After this, he couldn’t form any new episodic memories. This observation led scientists to believe that the hippocampus was important for the encoding of episodic memories.

On the other hand, fear-based memories may be stored in the amygdalae. For example, your memory of getting a dog bite when you were a child might get activated here every time you see a dog. And every time you saw a dog, you’d feel fear.

Did you know? A famous patient known as S.M. was unable to experience fearful emotions. For example, she would show no fear when exposed to tarantulas or while watching certain horror films. Scientists associated this with damage to her amygdalae, caused by a very rare genetic disease known as Urbach-Wiethe disease.

The Hunt for Engrams

Scientists are still doing experiments to figure out which engrams are important for which types of memories.

For example, scientists have looked at how well an animal can remember something after researchers lesion, or damage, a specific brain region. In one study, rats that were trained in learning different mazes had trouble completing the mazes after their hippocampi were lesioned. This suggests that the hippocampus may be important for spatial memory.

Summing up

While scientists are hard at work trying to understand how memories are encoded, stored, and retrieved, you are continuously forming memories every day. Memories are essentially notebooks stored in your brain, containing information about everything that you have ever experienced. Perhaps your brain has encoded some of what you’ve just read in an engram. So if ever you get tested on memory engrams, you’ll be able to go retrieve this information!

Learn more

What is memory? The present state of the engram (2016)
M. Poo et al., BMC Biology 14

The human amygdala and the induction and experience of fear (2011)
J.S. Feinstein, R. Adolphs, A.R. Damasio & D. Tranel, Current Biology 21

The legacy of patient H.M. for neuroscience (2009)
L. R.Squire, Neuron 61

Learning and Memory
American Psychological Association

Moushumi Nath

 I am a graduate student at the University of Toronto in the Department of Physiology. I study how the brain functions in learning and memory! This means I get to play with mice, visualize the brain, and listen in on how neurons communicate with each other. My interests in learning and memory began during my undergraduate degree at McGill University, where I completed a BSc in Honours Neuroscience. I look forward to continuing to engage in the fields of science communication and science policy. Sidenote: I am an avid free-food scavenger.