[MUSIC] So what we talked about in the last segment is that what HM taught us is that the temporal lobe, the front part of the temporal lobes on both sides are required for declarative memories. Let's look in at what are the structures in the front of the temporal lobe. And the required structure for memory formation is a structure called the hippocampus and it sits about here. And this is called the hippocampus. It's a type of cerebral cortex. It is not neocortex, it's a three layered cortex, and neocortex is a six layered cortex. So the hippocampus is an old type of cerebral cortex. And let's just take a look at it. Here is the hippocampus in all of its glory. This is, actually let's go back. We're gonna look at a section through here, and what you see is just the front bit of the temporal lobe. And right here, this bit, is the hippocampus. Okay, so what does the hippocampus do for memory? Well, remember that we have two different types of declarative memory. We have semantic memory and episodic memory. In order for both of these types of memories to be formed, so they're going from working. So working memory to become long-term memory that requires the hippocampus. Without the hippocampus, one or the other, this is not gonna happen. Neither semantic memories nor new episodic memories can be learned. And HM, in fact, learned no new episodic memories. He didn't ever learn a new event. He didn't know who was president. And he also didn't learn semantic events, he didn't learn semantic memories, so he didn't learn new words like psychedelic, he didn't learn about new celebrities, Ronald Reagan remained for him an actor, not a president. And so no new memories can be formed, either semantic or episodic, without the hippocampus. Now once we get to recall, let's say the memory has been formed, in both situations the hippocampus ships that information out to the neocortex. The neocortex is all of this. We've seen many pictures, all that outer rind of the brain, that's all neocortex. Okay, so the memories get shipped out to the neocortex. In the case of semantic memory, that's where it stays. It never has to go back to the hippocampus. It is stored in the neocortex and that's all it takes. So if you lose your hippocampus, you do not lose your old semantic memories. And this was really well illustrated by the account of Jill Bolte Taylor. She is and was a neuroscientist and she had a hemorrhagic stroke in the middle of a post-doctoral fellowship that she was on, and she recovered remarkably. She recovered so much that she was able to actually write this book which tells talks about her amazing journey back from completely being debilitated. And she not only wrote the book, but she narrated the audio book in a delightful way, which is what I listen to. But in it what she tells of is that there was one section of semantic memory that she lost. It was just gone, never to be returned, no glimpse of it again. And that was math. So the interpretation is that her hemorrhagic stroke actually killed a piece of her neocortex, and that piece of the neocortex that was killed was the part that stored all that mathematical memory. Okay, so let's get back. So semantic memory made in hippocampus, stored in neocortex, all we need to get it back is to reaccess that neocortex. As long as that piece of neocortex is alive we're good to go. Episodic memories are different. They're again, made in hippocampus and they're stored in the neocortex, but to access them we actually send them back to the hippocampus for re-consolidation, and then they come back to neocortex. Well, so you might wonder, is the memory. If this memory gets shipped out to neocortex, and then this memory gets shipped back to hippocampus, does it come back to neocortex in exactly the same form? And I think Ulric Neisser's experiments tell us that the answer is no. Every time it goes through this loop the memory can be changed. So there are no absolutes in memory and this reconsolidation, this circuit that episodic memories continually go through, upon recall it gives us a mechanism by which every time we recall it, we can change it a little bit, or not. But this is probably the reason why memories are so labile. Okay, in the next segment we're gonna look at what does a loss of memory mean? What is an amnesic patient? [MUSIC].
