How The Limbic System Works

 

I asked Doris, a very nice lady about to be turned fourteen, what intrigued her most about dreams.

Doris remained pensive for a moment.

"Why is it that I can't remember pleasant dreams?" she said. "The only dreams I can remember are the bad ones. You know... nightmares and stuff"

It was a good question. Not many people can remember their dreams; in fact, the vast majority of us forget our dreams. Some forget their dreams so easily that they even think they don't dream at all.

So, what makes the dream so hard to remember? And, why nightmares are easier to recall? One clue is the level of emotion involved.

When we experience a nightmare, we feel strong emotions, and emotions help us remember the dream.

"Emotions help us remember? How can that be?" asked Doris.

That's the way the brain works, I told her, or rather how the limbic system does.

"Yeah, right, the limbic system!" Doris exclaimed ironically, overacting.

The limbic system, I told her, is the name scientists give to a complex structure, made of small solid pieces that we all humans have in our head, towards the center of the brain, a little below. Each of the pieces with a specific role related with human behavior.

One of those pieces, for instance, is involved with feeling emotions, especially if they are strong, like fear or reward. Another one is responsible for converting the impressions of the day into memories.

"Now that you mention it, I think I read something like: dreams being used to store memories".

"You're right! The limbic system works while you sleep, encoding the memories of the things that you experienced during the day - that is called short-term memory-, relating them to feelings and sensations, and turning them into long-lasting memories - these are called long-term memories.

"You know, I always wondered... there are some odors and some songs that take me back, as if by magic, to a very specific moment in the past of my life. Do you think that could have something to do with that mechanism you are saying?"

"That is very possible. The limbic system is also known to be related to our sense of smell. Everything is coded into the long-term memory. Encoding mental memories with sensations helps the mind to retrieve those memories after a very long time. That is called indexing".

"That's why the nightmares are so easy to remember. Once you are awake, it's easy to feel back the emotions you experienced, and those emotions allow you to remember what you dreamed".

"I don't think I have heard about this limbic system you say."

"That might be because scientist did not know what its function was until recently. It was discovered accidentally in the nineteen fifties. Do you know how they discovered it?"

"I guess you will make me know".

"A patient who suffered from terrible seizures had removed one of the pieces that structured the limbic system. This man became famous by the letters H.M."

"H.M.?"

"Those were the initials of his name and last name."

"I figured that. What happened to Mr. H.M.?

"After the surgery, H.M. could not remember the things that had happened recently but, strangely, he could remember very well the facts of his distant past; more accurately, he remembered facts stored in his long-term memory before surgery, but he could not remember recent facts."

"The things he could remember were those which his limbic system had coded as a memory before the extirpation of that small structure. The things he experienced after were remembered for a while but, as they could no longer be encoded, they were forgotten the morning after.

"Poor man."

"Do you know how that little piece is called? You will like its name".

"What's its name?"

It's called the hippocampus.

Doris didn't show the least excitement. Her expression was apathetic.

"And... why should I like that name?"

"Was not the nymph Doris who traveled mounted on a hippocampus?"

"That was the oceanid Doris. The nymph Doris was her granddaughter, I think."

"Well, anyway,indexing mental images with feelings and sensations is the most advanced known mechanism for storing thousands of simultaneous details into memory. Not even the most sophisticated computer is capable of applying that kind of coding, at least not yet".

"Not yet?"

"Trust me, with all the information being created every second on the web, there is going to be a need for a better indexing system in the future.

"That sound great, really, but it doesn't explain why do we forget our dreams"

"You're right. I owe you that. I will try an explanation next time we talk."