- Sherlock Holmes, the legendary detective, had a theory that the brain is like an attic where a person can only store a limited amount of memories.

Dr. Watson once told him that the earth travels around the sun, duh, to which Holmes replied now that I do know it, I shall do my best to forget it.

Holmes figured clutter your attic with random facts and trivia and you won't have room for the things that matter like identifying the tiny differences between lethal poisons.

Was Holmes right?

Is our memory limited like the storage capacity of a computer, or is our memory unlimited?

And if we did have perfect memory, what would life be like if you never forgot anything?

The animated film Inside Out depicted memories as glowing spheres stacked in the brain like books in a library, but the reality is a little more complicated.

There's no one place in the brain that serves as our memory bank.

Instead, individual memories are scattered all over the brain.

Many brain cells in several different regions work together to make one memory.

For example, a memory of eating grandma's apple pie might involve some brain cells to help you remember what the pie looked like.

Others to remember the smell of the cinnamon and even cells to remember the delicious taste just to name a few.

In reality though, a memory isn't a physical thing that we can find in any given brain cell.

It's an action, not an object.

Think of baseball fans doing the wave.

No single fan is the wave.

The magic only happens when all the fans are together doing their thing in a specific order.

In the same way, a memory only happens when many connected neurons fire in a specific pattern and because the same cells can fire in many unique patterns, one group of neurons can encode multiple memories.

This increases the memory storage capacity of the brain.

Buried deep in the middle of the brain, we find a group of cells shaped like a seahorse, which is why 18th century scientists named this bit the hippocampus.

Without your seahorse, you might never remember.

We owe a lot of our understanding of memories to one famous patient known for years only by his initials HM.

In 1953, HM underwent a surgery for epilepsy, which demolished most of his hippocampus.

For the rest of his life, he exhibited a severe type of amnesia where he was unable to form new memories of facts or events, but he was still able to remember things he had learned before the surgery.

This showed us that the hippocampus is a key to making memories, but it isn't where memories are stored.

So how do experiences become memories?

If we look inside the brain of a mouse in a maze, we could draw kind of map showing which brain cells are active as the mouse experiences something.

Later, we could see the mouse's brain cells firing in the same patterns, replaying the experience in fast forward over and over, backwards and forwards to make the connections between cells stronger.

This is called consolidation, and it's how animals, including humans, commit new memories to long term storage.

Days or weeks later, a smell might trigger the same pattern of cells firing in the mouse brain.

A recall of the maze memories just like smelling cinnamon might trigger memories of grandma for you, but the brain's way of creating memories isn't foolproof.

Sometimes our mental replay of something we only imagined can feel as vivid as a real experience.

If you picture all the sights, smells and sounds of a crime scene from someone's description, you activate similar brain networks as if you had really been there.

The more you replay the scene in your mind, the more it feels like a real memory.

That's why a detective who asks leading questions can inadvertently plant a false memory in a witness.

We are able to remember a lot, but we forget even more.

Some forgetting just happens, but our brains also forget on purpose.

We have at least three different ways of forgetting.

The first is what happens when a memory fades over time, so called passive oblivescence, a term you will probably forget.

This may happen because the connections between brain cells gradually weaken over time or perhaps the memory's still there but you might lose the triggers.

Sight, sounds, smells, you need to retrieve it.

Another idea says memories could theoretically last forever, but when the same neurons get used in other memories, this interference disrupts the original memory.

This slow fade type of forgetting happens to all of us eventually.

A second type of forgetting, targeted forgetting, happens at night while we sleep.

This is when we clear out random useless tidbits of information that we've learned during the day and erase outdated memories.

For example, if yesterday you thought the earth was say a flat disc supported by three elephants and today you learn that the earth is round, your brain needs to purge one of these contradictory ideas, hopefully the one about the elephants.

In certain stages of sleep, we trim and prune connections between cells and erase unneeded memory circuits.

The third type of forgetting is motivated forgetting, something we all wish we could do for one thing or another.

This is when a person intentionally suppresses unpleasant memories.

Forgetting on purpose is a way to regulate our emotions and to focus on what needs to be done in the present instead of getting lost in negative memories of the past.

We may need motivated forgetting to maintain our self image, to maintain confidence, to stay optimistic about the future or to be able to maintain relationships with people who have hurt us.

We don't know exactly how motivated forgetting happens, but part of our brain seems to step in and block the troubling memory from being retrieved so that even though it's still somewhere in our brain, eventually we can't find it.

Our brains have so many ways to forget because forgetting is one of the most important things we do.

Forgetting allows us to move past traumatic life events.

In fact, PTSD may be a problem where someone simply remembers too much.

Forgetting also allows us to clear out junk.

Imagine every sight, sound, smell, and piece of information your brain processes every day.

If our brains didn't sweep out the garbage every night, we would soon overflow our neural networks with random, useless trivia just like Sherlock Holmes predicted.

We also wouldn't be able to replace things that are no longer true with better information, update our mental models of the world.

Deep in the scientific literature, we find stories of a handful of people who never forget anything.

They're so rare that their unforgetfulness has a medical name, hyperthymesia.

The most famous case is Jill Price, an American woman now in her fifties.

Starting from age 14, Jill's memory of her life is nearly perfect.

For any date in the past, she remembers what she wore and had for lunch that day, key historical events that she paid attention to and detailed incidents from her life.

She describes memories playing in her mind in vivid detail like a video reel that's been enhanced with smells and emotions.

Whether the events occurred yesterday or decades ago.

This might sound like a blessing, especially if you're in school, but Jill has described being haunted by upsetting memories and by regrets because unlike the rest of us, she can clearly remember every choice she made and how it turned out.

There's probably something you truly want to forget like that extremely embarrassing moment in high school that always seems to pop up at the worst times.

Can we erase those unwanted memories somehow?

In an episode of House MD, Dr. House treated a patient suffering from painful memories by performing something called electroconvulsive therapy, controlled electric shocks to the brain.

People who undergo ECT do lose some memories only not necessarily the ones they hope.

When it comes to erasing memories in humans, our best tool still works like a hammer, not a scalpel.

It's no accident that our ability to forget like our ability to remember is a complex and finely tuned mechanism.

If humans couldn't remember and learn from important events, our species probably wouldn't have survived.

But it seems that being able to forget is just as critical, an elementary part of solving this great mystery we call life.

Stay curious.