Biology of Memory: The Hippocampus

In this video I introduce the biology of memory and the role of the hippocampus on long-term memory formation. I discuss several cases of severe memory loss including patients H.M. and E.P., who suffered anterograde amnesia and cannot form new memories, and Clive Wearing, who also suffers from retrograde amnesia and is unable to recall his past. These case studies reveal the role of the hippocampus in explicit memory formation and also demonstrate that procedural memory does not rely on the the hippocampus.

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E.P. Case Study (Scientific American Frontiers)
https://www.youtube.com/watch?v=CL-0gfpf_6g

Clive Wearing Documentary
https://www.youtube.com/watch?v=k_P7Y0-wgos&t

 

Video Transcript:
Hi, I’m Michael Corayer and this is Psych Exam Review. In this video we’re going to begin looking at the biology of memory.

Now in a previous video I mentioned that memory is really complex and the models that we have are very simplified and so when we talk about biology here, remember that we’re just scratching the surface. There’s a lot more detail and in fact there’s a lot that’s still not well understood.

In the previous video we talked about the relationship between emotion and memory and this makes sense because we have strong emotional experiences, it makes sense that we would want to remember these. We want to learn from them. So if you have a strong negative emotional reaction like fear you probably want to form a memory of this so you can hopefully avoid a similar situation in the future.

Based on this it shouldn’t be a surprise that the parts of the brain involved in emotion are also involved in the formation of memory. This brings us to the limbic system and the part of the limbic system that seems to be especially important for forming new long-term memories is the hippocampus. You might recall that hippocampus comes from the Greek for “seahorse” and it’s this curled structure. You have two hippocampi, you have one on each side of your brain.

Some of the first evidence for the role of the hippocampus on memory formation comes from a famous patient known as patient H.M., whose real name was Henry Molaison. What happened with patient H.M. was that he had surgery to stop seizures and this involved removing the hippocampus from both sides of his brain, a hippocampectomy and the result of this surgery was that the seizures were reduced but H.M. couldn’t form new long-term memories. His sensory memory was intact and his short-term memory was intact, so he could repeat a list of words for instance but he couldn’t move those words to long-term memory. He couldn’t remember them a few minutes later. In fact he couldn’t remember that you had asked him to remember a list of words.

This shows us that the hippocampus seems to be crucial for forming new memories. Now what HM suffered from is referred to as anterograde amnesia and this refers specifically to inability to form new memories but H.M.’S previous memories were intact. He could remember things from before the surgery and so this shows that the hippocampus might be crucial for forming new memories but it’s not where memories are stored.

It’s not the case that if you remove the hippocampus you forget everything that you had previously experienced. Another interesting thing is that the hippocampus doesn’t seem to be involved in procedural memories because HM could still form new procedural memories. If you give him a physical task and he practiced it, he could get better at it.

For instance you could have a task like trying to draw a picture, copy a picture, looking into a mirror. This is very hard to do, to look into a mirror while you draw, but with practice you can get better at it. So if HM were to practice this task, he would get better at it but if you asked him he wouldn’t remember practicing it.

Another patient with similar damage is patient E.P. and I’ll post a link in the video description where you can find an interview with E.P. We can see him interacting with these interviewers. Now these are people that he has met hundreds of times and yet he can’t remember their names but he does show some sort of emotional reaction to them. He seems like the previous experiences have been positive right? He seems warm and friendly to them even though he thinks he doesn’t remember meeting them before so this shows us this separation in long-term memory between our explicit memory, declarative memory, and our implicit memory or non declarative memory.

Another famous case of memory loss is a man named Clive Wearing and Clive Wearing is considered to have one of the worst cases of memory loss that’s ever been recorded. This is because Wearing suffers from anterograde amnesia but he also suffers from retrograde amnesia and this refers to loss of previous memories. In Clive Wearing’s case, he can’t form new memories but he also can’t remember his past. He doesn’t remember things from his childhood or really much of anything about his previous life. He has some memories, he can recognize his wife, in fact he shows a strong emotional reaction to her each time he sees her because he doesn’t remember that he’s just seen her a few minutes before.

I’ll post a link in the description where you can see some video of Clive Wearing but an interesting thing here is his procedural memories are intact. He’s lost his old episodic memories and his old semantic memories but he hasn’t lost his procedural memory. He can still play the piano, obviously he can still walk and he can still talk, so actually he still has his memory for word meanings, and things.

He can understand and carry on a conversation but within, you know, 30 seconds he’s forgotten what you’re talking about, what you’ve been talking about for the past few minutes, right? So it’s a very severe case of memory loss and it helps to demonstrate these different types of memory; that things are not, you know, there’s not one memory area of the brain that stores everything but the things are distributed and certain types of damage can affect some types of memory and not others.

Ok, in the next video we’ll take a look at the neural basis of memory. We’ll look at how chemical signaling between neurons might play a role in the formation of memories. Ok, I hope you found this helpful, if so, please like the video and subscribe to the channel for more.

Thanks for watching!